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Eutrophication in Australian rivers_ reservoirs and estuaries C a

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Eutrophication in Australian rivers_ reservoirs and estuaries C a Powered By Docstoc
					Hydrobiologia (2006) 559:23–76                                                                        Ó Springer 2006
DOI 10.1007/s10750-005-4429-2

Review

Eutrophication in Australian rivers, reservoirs and estuaries – a southern
hemisphere perspective on the science and its implications

J. Richard Davis1,3,* & Klaus Koop2
1
  CSIRO Land and Water, ACT 2601, GPO Box 1666, Canberra, Australia
2
  Department of Environment and Conservation, NSW 1232, P.O. Box A290, Sydney South, Australia
3
  Present address: Land & Water Australia, GPO Box 2182, 2601, Canberra, ACT, Australia
(*Author for correspondence: E-mail: richard.davis@lwa.gov.au)

Received 19 December 2005; accepted 18 March 2005


Key words: eutrophication, nutrients, nutrient limitation, phosphorus, nitrogen, fertiliser, diffuse sources, rivers,
estuaries, phytoplankton, akinetes, algal blooms, cyanobacteria, blue green algae, stratification, sediments, sediment
nutrient fluxes, bioavailability, residence time, biomanipulation, Murray-Darling Basin, Australia



Abstract
Australian science has made rapid advances in the last decade in understanding eutrophication processes
in inland waters and estuaries. The freshwater research on which these advances are based was triggered
by well-publicised blooms of cyanobacteria during the 1980s and early 1990s, particularly a 1000 km
long bloom on the Darling River. In estuaries the study which greatly enhanced our understanding but
simultaneously served to stimulate further research into estuarine eutrophication, the Port Phillip Bay
Study, was initially designed to address perceived problems of toxicants in the Bay but provided
profound insights into drivers for, and ecosystem responses to, eutrophication. Subsequent estuarine
research has largely been stimulated by management questions arising from AustraliaÕs increasing
coastal development for residential purposes. The research has shown that some of the beliefs extant at
the time of the blooms were incorrect. For example, it is now clear that stratification and light pen-
etration, not nutrient availability, are the triggers for blooms in the impounded rivers of southeastern
Australia, although nutrient exhaustion limits the biomass of blooms. Again, nitrogen seems to play as
important a role as phosphorus does in controlling the biomass of these freshwater blooms. The
research has also shown that aspects of eutrophication, such as nutrient transport, are dominated by
different processes in different parts of Australia. Many of the biophysical processes involved in
eutrophication have now been quantified sufficiently for models to be developed of such processes as
sediment-nutrient release, stratification, turbidity and algal growth in both freshwater and estuarine
systems. In some cases the models are reliable enough for the knowledge gained in particular water-
bodies to be applied elsewhere. Thus, there is now a firm scientific foundation for managers to rely
upon when managing algal blooms. Whilst these findings have already been presented to managers and
communities throughout Australia, there is still a considerable way to go before they are absorbed into
their modus operandi.


Introduction                                                 and coastal waters are added, including the costs to
                                                             fisheries, aquaculture and tourism, the full cost of
Algal blooms in Australian freshwaters cost the              algal blooms in Australia is much higher. Signifi-
community between AUD180 and 240 million                     cantly, the report by Atech (2000) showed that
every year (Atech, 2000). When blooms in estuaries           these costs are incurred by urban water users,
24

dryland farmers and irrigators in roughly equal         lakes in Victoria), algal problems in large water
proportions. Thus, algal blooms are a problem           storages (e.g., Cannon et al., 1970) including
that affects the whole Australian community.             serious illness following a cyanobacterial out-
    Eutrophication is the process of increasing         break in Solomon Dam (Hawkins et al., 1985)
organic enrichment of an ecosystem, where the           and seasonal blooms in riverine impoundments in
organic enrichment causes changes to the ecosys-        the Murray-Darling Basin (MDB). Algal blooms
tem (Nixon, 1995; see also National Research            received widespread public attention with the
Council, 2000). This has usually been characterised     1991/92 bloom that covered a distance of some
by increased supplies of nutrients to that system       1000 km in the Barwon-Darling River system
but, as we shall argue below, in Australian fresh-      (Bowling & Baker, 1996) and a major cocco-
water systems solar radiation and stratification also    lithophorid bloom in Jervis Bay in late 1992
play important roles. The organic enrichment fre-       (Blackburn & Cresswell, 1993). The Barwon-
quently takes the form of phytoplankton blooms,         Darling bloom caused losses conservatively esti-
both cyanobacterial and eukaryotic blooms,              mated at AUD9m (NSW Blue-Green Algal
although in many Australian eutrophic estuaries it      Taskforce, 1992).
manifest itself as blooms of filamentous macroal-            The 1991 Barwon-Darling River bloom gal-
gae. As emphasised by the National Research             vanised government action. Algal management
Council (2000), it is important to understand           strategies were drawn up for Victoria (Victorian
clearly the cause and effect relationship between        Blue-Green Algal Project Team, 1995), New South
nutrients and their ecological effects as confusion      Wales (NSW Blue-Green Algal Task Force, 1992)
may ‘‘impede mitigation efforts because proposed         and the MDB (Anon, 1994b), and Queensland
changes may not bring about desired effects’’. A         formed a Water Quality Taskforce which reported
comprehensive knowledge of these cause and effect        on the managerial, operational and situational
relationships is vital to effectively target appropri-   aspects of algal blooms. These reports all recog-
ate management actions.                                 nised that there was insufficient understanding of
    In a comprehensive review of eutrophication in      the processes governing the triggering, formation
coastal ecosystems, Cloern (2001) pointed out that      and decline of algal blooms under Australian
the traditional eutrophication model based on           environmental conditions and called for research
nutrient inputs as the signal leading to enhanced       to be carried out to clarify these processes.
phytoplankton production is increasingly being          Two coordinated R&D programs (the CSIRO
replaced by emphasis on the study of system-spe-        Blue-Green Algal Program and the National
cific attributes that act to modulate complex direct     Eutrophication Management Program) were
and indirect ecosystem responses. In Australia          commenced. Further site-specific projects were
eutrophication research and management atten-           initiated to provide answers for management
tion has been focussed on algal blooms, particu-        authorities faced with eutrophication problems in
larly toxic cyanobacterial blooms in fresh waters.      estuarine and fresh waterbodies. These initiatives
Some work has been done on phytoplankton re-            are described in more detail later.
sponses in coastal systems where mainly diatoms             This paper assesses current understanding of
and dinoflagellates are involved. That bias is re-       the eutrophication process in Australian fresh-
flected in this review, although in recent years         waters and estuaries following this research,
there is an increasing move, particularly in studies    without undertaking a comprehensive interna-
of estuaries, to studying more complex system           tional review. Ocean waters are not reviewed.
characteristics and interactions.                       The state of knowledge about eutrophication in
    Blooms of cyanobacteria have been a recog-          the early 1990s is used as a benchmark against
nised phenomenon in Australian fresh waters             which the improvement in knowledge is mea-
since late in the 19th Century (Francis, 1878).         sured. Some assumptions of that time have been
Throughout the 1970s and 80s there were per-            confirmed; some have been overthrown; while
sistent cyanobacterial blooms in the Peel-Harvey        others remain unresolved. Finally, some of the
estuary in Western Australia (EPA, 1988), occa-         broader management implications of this new
sional blooms in other estuaries (e.g., Gippsland       understanding are discussed.
                                                                                                         25

The Australian environment                             material and this is further exacerbated during
                                                       episodic flood events when turbidity may increase
Mainland Australia is an old, highly weathered,        by several orders of magnitude. Rivers along the
and dry continent. About 30% of the landmass is        southeastern seaboard are faster flowing and
covered by desert with an annual rainfall of less      shorter than those draining inland and, conse-
than 200 mm; some 90% receives a median annual         quently, are generally less turbid during non-flood
rainfall of less than 800 mm. Among the inhabited      flows. Coastal rivers along the Queensland coast
continents, Australia has the lowest annual rainfall   are highly episodic, being driven by cyclones dur-
and the highest proportional loss of surface water     ing the monsoon season. During this period, they
through evaporation and transpiration. Variability     are usually extremely turbid, because of erosion in
in rainfall is extreme, with both seasonal variation   their catchments.
and fluctuations between high rainfall years and            Floodplains are crucial for the ecology of
drought. Most of tropical and subtropical eastern      Australian inland rivers, being significant sources
Australia is influenced by el nin˜o and la nin˜a or     of carbon and nutrients for the river ecosystem.
ENSO events with a periodicity of approximately        Lateral transport of sediment and nutrients to the
7 years, causing periods of high flow in rivers to be   floodplain during over-bank flows, followed by the
followed by drought periods (e.g., Harris, 1996).      return transport of organic matter and nutrients to
During droughts flow, even in the largest rivers,       the river channel are major drivers of ecological
ceases altogether and the rivers themselves become     processes in the river-floodplain ecosystem. Input
a series of stagnant pools. In the Murray-Darling      of carbon from the catchment to the main channel
river system in eastern Australia, for example,        is the major energy source to the river ecosystem
river flow, which averages approximately                for driving microbial food webs (Robertson et al.,
14 000 Gl yr)1, may vary between 40 000 Gl yr)1        1999). In these clay-rich systems, phosphorus is
during wet years and zero flow during times of          sediment-bound and little of it is bioavailable. As a
drought (Newman, 1998). Similar, although less         result, internationally accepted limits of phospho-
well-understood, climate cycles also affect the west    rus concentration in waterways are not applicable
coast of Australia (Bureau of Meteorology              to these systems. Nitrogen is delivered via
Research Centre et al., 1999).                         decomposition of floodplain vegetation and from
    Australian inland rivers characteristically have   animal wastes. There is little information on
very low gradients through most of their course as     nitrogen fixation and atmospheric deposition of
a consequence of a long history of erosion pro-        nitrogen as input sources to rivers.
cesses that have created great infilled basins with         Variability in discharge in Australian arid zone
very low reliefs and predominantly clay-rich river     streams is high, while variability in stream dis-
systems. For example, the Murray River in the          charge in high rainfall areas is higher than in
MDB, has a slope of only 0.2 mm km)1 for about         comparable streams in other parts of the world
the last 500 km of its course (Thoms et al., 1998).    (Lake, 1995; Finlayson & McMahon, 1988). The
This has led to the development of extensive           structures of river channels and floodplains, as
floodplains that are temporary storages for alluvial    well as indigenous plants and animals, have
material adjacent to the main river channel, along     evolved to accommodate this variable flow.
with numerous meanders and billabongs                      Probably the most profound anthropogenic
(oxbows). After heavy rain, there is extensive         influence on Australian rivers, however, has been
flooding of these near-river areas. Biodiversity is     through flow regulation by a series of dams and
high on these floodplains because of the diversity      reservoirs in the upper catchments of most of the
of terrestrial and aquatic habitats.                   major rivers and weirs in the lowland regions.
    Australian landscapes are typically highly         These rivers are managed as rural water supply
weathered with high percentages of clay material.      systems for the major irrigation areas that have
Erosion in the upper catchments of most rivers of      been developed within the MDB, parts of
eastern Australia, particularly gully erosion and      Queensland and the northwest of Western Aus-
bank slumping, provides a high sediment load.          tralia. Apart from reducing in-stream flows and
Thus, rivers are turbid with high loads of colloidal   disconnecting river channel and floodplain these
26

new flow regimes tend to be delivered at the time         catchment and ecological processes are driven by
of year that suits the needs of irrigators rather than   material and chemical loads entering the estuary
the in-stream environment. It has been only re-          from the river (see also National Research Council,
cently that river management has begun to con-           2000). Studies suggest that, even in permanently
sider the importance of river flows to aquatic            open estuaries in temperate Australia (such as some
ecosystems and measures are slowly being imple-          of the drowned river valley estuaries in southeast-
mented to address this issue.                            ern Australia) on an annual average only about
    AustraliaÕs coastal regions have also been           30% of the material transported to the estuary
affected by European settlement. Australia has a          reaches the nearshore ocean (SKM, 1997). This is
coastline of almost 37,000 km with 758 major             likely to be different in tropical areas where the
estuaries (Digby et al., 1998). Relatively little work   greatest proportion of loads are delivered in a rel-
has been done on Australian estuaries, compared          atively small number of floods (e.g., Mitchell et al.,
to northern hemisphere estuaries. Only a handful         1996; Eyre, 1998) and in a small number of estu-
of AustraliaÕs estuaries have been studied inten-        aries in Tasmania, where year-round high runoff
sively and those studies have mostly been of             leads to short residence times of the water (e.g.,
estuaries near the larger population centres of          CSIRO Huon Estuary Study Team, 2000).
Brisbane, Hobart, Melbourne, Perth and Sydney.
There are no comprehensive data sets comparable
                                                         Eutrophication in Australia – historical perspective
to those in Chesapeake Bay in the eastern USA
and the Baltic Sea in Europe.
                                                         Phytoplankton blooms including toxic cyano-
    It is difficult to extrapolate from the Northern
                                                         bacterial blooms have occurred in AustraliaÕs
Hemisphere estuarine studies because, apart from
                                                         estuaries, lakes and rivers for at least 100 years.
some estuaries in Tasmania, Australian systems
                                                         In this section, we briefly review four occasions
are dominated by features such as overall low
                                                         when blooms in each of these types of water
freshwater inflow, interspersed with episodic
                                                         body have caused major management difficulties,
events, which deliver the bulk of water. As a first
                                                         and the effectiveness of the management
step towards developing management strategies
                                                         responses.
for these systems the National Land and Water
Resources Audit (NLWRA, 2001) developed a
                                                         Lake Alexandrina
three-part estuarine classification system based on
the interplay of geology, hydrology and ecology:
                                                         Francis (1878) is often quoted as being the first
1.   a relatively small number of estuaries with         scientific paper to describe a toxic algal bloom.
     permanent connections to the sea, mainly            Codd et al. (1994) describe the background to this
     large drowned river valleys,                        bloom in Lake Alexandrina, an estuarine lake at
2.   wave dominated barrier estuaries, most with         the mouth of the Murray River in South Australia,
     trained entrances that are permanently open,        (Fig. 1a) and FrancisÕ subsequent investigations.
     or with untrained entrances that remain par-        The species causing the problem was Nodularia
     tially open, and                                    spumigena. The bloom occurred when water levels
3.   estuaries, saline coastal lagoons and creeks        were low and water temperatures relatively high.
     with entrances that are mostly closed.              Scums formed and floated to the shore of the Lake
                                                         where cattle, horses and sheep died after drinking.
    All estuaries in category 3 as well as the barrier   The post mortem results on sheep that died at the
estuaries in group 2 with untrained entrances that       lakeshore were identical to the results on sheep
close occasionally are termed Intermittently Closed      that had been fed known amounts of fresh scum
and Open Lakes and Lagoons (ICOLLs) (Roy et              (Francis, 1878). There was no sign of the scums in
al., 2000). Because of the episodic flow regime of        the stomachs of the experimental sheep, implying
many of its rivers, Australia has a disproportion-       that the material had been absorbed into the
ately large number of ICOLLs. These estuaries act        bloodstream. Thus, Francis concluded that the
as terminal sinks for material exported from the         Nodularia was responsible for the deaths. Codd
                                                                                                                                                                     27

                       (a)
                                                                  DARWIN




                                                                                                                                       South Johnston River

                                                                                             NT                                        Herbert River



                                                                                                                             QLD


                                              WA


                                                                                                                                                       Moreton Bay
                                                                                                  SA                                                  BRISBANE



                                      Swan and Avon River
                             PERTH                                                                                             N SW
                      Peel Harvey
                          Estuary                                                            ADELAIDE
                                                                                                                                             SYDNEY
                                                                                                                                         CANBERRA A CT
                                        Wilson Inlet                         Lake Alexandrina                           VIC
                                                                                                               MELBOURNE           Lake Mokoan

                                                                                                       Port Phillip Bay
                                                    SCALE
                                        0     200   400   600   800 1000km

                                                                                                                 T A S
                                                                                                                                    HOBART
                                                                                                                                   Huon Estuary




                       (b)


                              Murray-Darling
                                      Basin


                                                                                                                                                         Brisbane
                                                                                     R




                                                                                                              aR
                                                                                 oo




                                                                                                           lgo               MacIntyre R
                                                                             Par




                                                                                                        Cu
                                                                                                                              Gw y d
                                                                                                                                    er R
                                                                                                                      Namoi
                                                                                                                            R
                                                                                      V      ER
                                                                                   RI
                                                                                                                 Ma




                                                                       NG
                                                                                                                    cqu




                                                                     LI                                                  Chaffey Dam
                                                                   AR
                                                                                                                        ar




                                                                  D
                                                                                                                      ie R




                                                                                                       Lac
                                                                                                          h   lan                       Hawkesbury R
                                                                                    Mur                           R
                      Adelaide                                                         rum                                                 Sydney
                                                                                          bidg
                                                                                                         ee R
                                                                     MU
                                                                        RR
                                                                          AY                                             Canberra
                                                                                              RIVER
                                                                                         Broke
                                                                                              nR
                                                                              Goulburn




                                                                                         R

                                                                                Melbourne
                       0     100     200 km



Figure 1. Location of the sites in Australia where the eutrophication research reviewed in this paper has been done (a) and a more
detailed location map of the MDB (b).
28

et al. (1994) describe how an informal water            tion in the Serpentine River feeding the estuary,
quality monitoring program operated through the         with toxic cyanobacteria and nuisance dinofla-
observations of police officers stationed in the          gellates occurring in the 4 years following the
lower Murray area and how community education           opening of the channel (Rose, 1998). Thus, the
had alerted landowners to the dangers to stock          elimination of Nodularia blooms and general
when scums appeared in water storages. There is         improvement in estuarine water quality since
no information on the effectiveness of these man-        1994 is almost certainly due to the opening of the
agement actions.                                        Dawesville Channel. Rose (1998) attributes this,
                                                        at least partly, to the increased salinity of bottom
The Peel-Harvey estuary                                 waters preventing the germination of Nodularia
                                                        akinetes.
The Peel-Harvey estuary, south of Perth (Fig. 1a),          The effect of the nutrient reduction program is
had experienced nuisance macro-algal growths            less certain. A phosphorus reduction program had
from the 1960s to the early 1990s. The potentially      already been instituted in the catchment in 1983.
toxic cyanobacterium Nodularia spumigena was            From 1988 this was strengthened by setting strict
noted in one of the rivers feeding the estuary in the   load targets and through controls over develop-
early 1970s. The first large cyanobacterial bloom,       ments in the catchment, increased education of
attributed to excessive nutrient loads from the         farmers, controls over point source inputs and
surrounding catchments, occurred in the estuary in      development of ameliorative measures such as
1974. The shallowness (average depth 1 m) and           spreading bauxite wastes with high P binding
warm water temperatures of the estuary, the re-         capacities. A 1993 review of water quality data
stricted flushing with the ocean through the nar-        (Bott, 1993; Humphries & Robinson, 1995)
row Mandurah channel and the high nutrient              claimed that total P loads to the estuary had
input provided ideal conditions for plant growth.       decreased significantly since the introduction of
Phosphorus was identified as the nutrient limiting       the phosphorus reduction program. However, a
the growth of the Nodularia.                            1994 review (Anon, 1994a) concluded that reduced
    A wide range of management responses were           river flows prior to 1996 and errors in the original
considered by the Peel-Harvey Study Group               nutrient load estimates made it impossible to show
(1988) with seven of the most promising being           that there had been a statistically valid reduction
reviewed in detail in a subsequent study (Kinhill       in TP loads entering the estuary. Jakowyna (2000)
Engineers, 1988). The latter review showed that a       has also concluded that it is not possible because
combination of increased salinities by construction     of natural variability to conclude whether the
of a second channel to the ocean and a reduction        program has been successful or not.
in phosphorus inputs to the estuary would reduce
the frequency of Nodularia blooms to acceptable         Lake Mokoan
levels. The new entrance to the ocean (Dawesville
Channel) was completed in 1994.                         Lake Mokoan is a shallow water storage
    A statutory review of these measures (EPA,          (362,000Ml when full) covering the site of a natural
1999) stated that the predicted effects of the           swamp in northeastern Victoria (Fig. 1a). When
Dawesville Channel on estuarine water quality           the lake was formed in 1970 as a supplementary
had been achieved. Nodularia blooms, which had          storage for irrigation purposes, it was covered in
occurred 9 years out of 12 prior to the channel,        extensive beds of native ribbon weed (Vallisneria
had not been recorded in the 5 years since the          sp.), water clarity was good (NTU about 10) and
Channel was opened. Harvested macroalgal loads          nutrient levels were low. The Lake was almost fully
had fallen from 60,000 tonnes in 1979 to                drawn down during a drought in 1982/3 and the
5000 tonnes in 1996 and all common water                lakebed, consisting of fine clays, was fully exposed
quality parameters (chlorophyll, Secchi depth,          for an extended period during which the macro-
etc) had improved. Seagrasses have been more            phytes died. The exposed sediments were mixed
abundant since 1994. Although estuarine water           into the water column when the lake refilled over
quality had improved, there had been deteriora-         the next few years. High turbidity (300NTU in
                                                                                                          29

1993) and nutrient levels were recorded and the         coordinating agency across the Basin) mounted
macrophytes were unable to regenerate under these       research programs into the causes of the problem
low light conditions. Cyanobacteria (Microcystis        while simultaneously undertaking management
aeroginosa) were first recorded in bloom concen-         actions based on best available information.
trations in 1989 and have recurred every year since.        A ‘‘run of river’’ study (Donnelly et al., 1992),
This has restricted the use of the water for both       commissioned by MDBC, proposed a model to
irrigation and recreation.                              explain the occurrence of the bloom. This model
    A management program consisting of control-         envisaged the build-up of P-enriched, low carbon
ling the water regime, nutrient load reductions from    sediments under generally oxic conditions between
the catchment and re-establishing macrophyte beds,      major flood events. During extended periods of
was instituted in the early 1990s to restore the Lake   low flow, an influx of sulphate rich, saline
to its previous macrophyte dominated state (Loone       groundwaters into the river caused the fine sedi-
& Lloyd, 1996). A review of the program in 1998         ments of the river to flocculate and allowed light to
(Water ECOsciences, 1998) found that, although          penetrate into the water body. Phytoplankton
turbidity levels had dropped to about 150NTU, the       started growing and, as algal detritus was depos-
water was still too turbid to support macrophytes       ited on the bottom, sulphate reducing bacteria
and that nutrient and chlorophyll concentrations        became active. As oxygen levels decreased in the
had not been reduced significantly. The water            bottom waters, sediment-bound P was released as
management program was refined and expanded as           soluble P into the water column, promoting the
a result of the review (Lloyd, 1998). Additional        growth of the phytoplankton leading to bloom
management options were canvassed during the            conditions. The authors noted that the nutrients in
review (Sandercock, 1998) and re-establishing           the bottom sediments originated from catchment
macrophytes was reaffirmed as crucial to improving        sources and these too needed to be investigated.
the LakeÕs water quality. There had been little             A series of studies were then initiated into the
experience in rehabilitating shallow lakes in Aus-      sources of nutrients within the river, the biogeo-
tralia but overseas experience has shown that a         chemistry of river sediments, algal ecology and
range of measures have to be instituted simulta-        hydrology of the river. The results from these
neously for successful rehabilitation (e.g., Moss       individual studies are discussed later. The overall
et al., 1996a; van Dijk & van Donk, 1991).              conclusion was that the hypothesis advanced by
    The experience of Lake Mokoan has demon-            Donnelly et al. (1992) was supported and that the
strated the difficulty of reducing nutrients and          combination of low flows, saline, sulphate-en-
turbidity sufficiently and for long enough to turn a      riched groundwaters and relatively P-rich clays
high-turbidity, high-nutrient, cyanobacteria-domi-      within the river system, derived from the relatively
nated system back into a microphyte-dominated           large basaltic areas in the catchment, triggered the
system. Both states appear to be stable and it takes    1991/92 bloom.
considerable energy to move between the two. The
Lake remains eutrophic with turbidity still drop-
ping slowly but with little change in nutrient or       State of knowledge in early 1990s
chlorophyll status.
                                                        Various task forces set-up by State governments
The Darling River                                       and the MDBC during the early 1990s (NSW Blue-
                                                        Green Algal Task Force, 1992; Schonfeldt, 1993;
The 1000 km long bloom on the Darling River in          Anon, 1994b; Victorian Blue-Green Algal Project
the summer of 1991/2 caught both national and           Team, 1995) together with two scientific reviews
international attention (Fig. 1a, b). The rapidity of   commissioned by the Land and Water Resources
the bloomÕs development and its extent caught           Research and Development Corporation (Harris,
river managers by surprise. As there had been no        1994) and the MDBC (Donnelly, 1994) provide a
extensive studies of eutrophication in the river        snapshot of the state of knowledge about algal
prior to the bloom, the NSW government and the          blooms in southeastern Australia in the early 1990s
Murray-Darling Basin Commission (MDBC – the             (summarised in Table 1).
                                                                                                                                                                           30




Table 1. Summary of the state of knowledge at the beginning of the 1990s about factors and processes resulting in eutrophication of Australian freshwaters and estuaries

 Issue/process               Summary of knowledge                                                                                References

 Nutrient sources
 Anthropogenic sources       Diffuse sources are a major contributor to nutrient loads in MDB; in dry years point sources,        Gutteridge Haskins &
                             mainly STP, are significant producers of bioavailable nutrients                                      Davey P/L (1992)
                             P transport is closely linked with soil movement and erosion management is seen as essential;       NSW Blue-Green Algal
                             few data on loads of diffuse source P from different sources, particularly at larger scales           Task Force 1992; Schonfeldt (1993)
                             P transport believed to be mainly by surface flows, but some evidence of sub-surface transport
 Phosphate fertilisers       P fertiliser seen as potential source, but magnitude unknown                                        NSW Blue-Green Algal
                                                                                                                                 Task Force (1992)
 Internal nutrient            Simple calculation suggest that P stocks in sediments in the MDB may be two orders of              Schonfeldt (1993)
 sources                      magnitude greater than water column standing stocks; biogeochemical links well documented
                              from European and North American studies, but few Australian data
                              Speculation about importance of nutrient cycling by zooplankton and fish
 In-water physical/chemical processes
 Phosphorus                   P from sewage treatments plants generally bioavailable                                             Simmons & Cheng (1981)
 bioavailability              P in stormwater refractory except when stormwater runoff occurs immediately after fertiliser        Schonfeldt (1993)
                              application in rural areas, though there was little general agreement
 Nutrient limitation          P limits phytoplankton biomass in freshwaters, although there were a few alternate views           Schonfeldt (1993), Anon (1994b)
                              (effect of flows)
 Nitrogen                     N accepted as a factor contributing to phytoplankton blooms in freshwater,                         Forsberg (1975)
                              but not in a controlling way
                              N widely accepted as the limiting nutrient in estuaries; knowledge of biogeochemistry of           Bauld & Millis (1977); Newell (1990);
                              estuaries extremely limited; only a few published papers                                           Skyring et al. (1992)
 Temperature                  Effects of changes in temperature on algal (particularly cyanobacterial) succession well studied;   Nicholas (1980), NSW
                              indirect effects also documented                                                                    Blue-Green Algal Task
                                                                                                                                 Force (1992)
 Turbidity                   Turbidity in lakes and rivers favours cyanobacteria because of their ability to                     Ganf & Oliver (1982),
                             regulate cell buoyancy                                                                              Smith (1986), Geddes (1988),
                                                                                                                                 Shapiro (1990)
                             No process understanding of the role of light in controlling phytoplankton biomass and
                             succession in freshwaters
 River flow                   Correlation between increased river flow and decreased algal numbers observed in                     Hotzel & Croome (1994), Jones (1993);
                                                                                                                                   ¨
                             a few studies; not sufficient information to set flow targets                                          Schonfeldt (1993)
Biological and ecological interactions
Phytoplankton                 Increasing reports of toxic cyanobacterial blooms in inland and toxic dinoflagellate   Bowling & Baker (1996),
dynamics                      blooms in coastal waters                                                              Falconer et al. (1983), May &
                                                                                                                    McBarron (1973),
                                                                                                                    McBarron & May (1966),
                                                                                                                    McBarron et al. (1975),
                                                                                                                    Negri et al. (1995) and
                                                                                                                    Runnegar et al. (1988)
                            Contributing factors are buoyancy regulation and vertical migration to                  Ganf & Oliver (1982),
                            exploit light and nutrient regimes, plus ability of both groups to form                 Fay & Van Baalen (1987)
                            resting stages that can survive adverse conditions in sediments, including
                            during periods when sediments dry out
                            In coastal waters, shipsÕ ballast water identified as a major source                     Hallegraeff & Bolch (1992),
                                                                                                                    Hallegraeff & Fraga (1998)
Macrophyte                  Conflicting evidence of competition between macrophytes and                              van Donk (1991),
competition                 phytoplankton, with few published studies                                               Malthus et al. (1990)
Food web effects             Food web manipulation (biomanipulation) in its infancy in Australia;                    Matveev et al. (1994a)
                            some indication that it might be a useful tool
                                                                                                                                                    31
32

    It is clear that, in the early 1990s, the range of   aquatic environments, it is the dissolved inorganic
factors controlling eutrophication in both fresh-        forms of N and P that are most readily available
waters and estuarine systems was well understood         for assimilation by algae (e.g., Gabric & Bell,
in principle. However, much of this understanding        1993). Whereas analyses of the inorganic species,
arose from research conducted in the Northern            NH4 and NOx, appear to give reliable estimates of
Hemisphere. While the basic processes leading to         bioavailable N, bioavailable P is much more dif-
high algal concentrations are the same throughout        ficult to measure because of its high affinity to
the world, the relative importance of these              particles. Studies by Oliver et al. (1993) and Oliver
processes and how they are influenced by climate          et al. (2000) have shown that the filtrate from
and the particular physical and chemical charac-         0.45 lm filtered water commonly used to estimate
teristics of different landscapes may profoundly          filterable reactive phosphorus (FRP) still contains
influence their relative importance and thus the          some P attached to the fine clay sediments of the
options for managing them. For example, flow and          rivers of the MDB and that filtration through
turbidity were known to be correlated with algal         0.2 lm filters provides better estimates of FRP in
biomass in Australia, although the reason for the        this part of Australia (McKelvie et al., 1995).
correlation was not clear.                                   These filtration techniques will tend to under-
                                                         estimate bioavailable P, however, because they do
                                                         not measure phosphate reversibly bound to parti-
Current state of knowledge                               cles that can become available to phytoplankton as
                                                         the dissolved fraction is taken up in algal growth.
Scientific initiatives                                    Using iron-oxide coated paper (filter strips) is a
                                                         simple technique which extracts both dissolved
Various coordinated eutrophication R&D pro-              and loosely bound orthophosphate from unfiltered
grams were developed in freshwater and estuarine         water samples and thus provides a truer measure
systems in response to the increased concern about       of bioavailable P than filtration techniques. Oliver
eutrophication. These included the CSIRO Blue-           et al. (1993) have shown good correspondence
Green Algal Program (Davis, 1997); the National          between this ‘‘desorbable’’ P (DP) and the P
Eutrophication Management Program (1995–                 requirements of phytoplankton calculated from
2000); the Port Phillip Bay Study (Harris et al.,        growth bioassays. Field studies have shown that
1996); the Moreton Bay Study (Dennison & Abal,           this DP may make up anything between 0 and
1999); the Huon Estuary Study (CSIRO Huon                80% of the total P in rivers of the MDB, high-
Estuary Study Team, 2000); the Swan Estuary              lighting the inaccuracies arising from continued
Study (Hamilton & Turner, 2001); and the Wilson          usage of TP as a measure of bioavailable phos-
Inlet Study (Thompson & Twomey, 2000).                   phorus. Gel probes (Davison & Zhang, 1994) are a
The Darling River Study (Oliver et al., 2000), the       relatively new technique with possibilities for wide
Chaffey Dam Study (Sherman et al., 2001) and the          application, including measurements of bioavail-
Johnstone Study (Hunter & Walton, 1997) have             able phosphorus in situ.
been the major site-specific freshwater manage-               To target management efforts efficiently, it is
ment studies conducted in recent years (Fig. 1a, b).     important to know the bioavailability of phos-
   These coordinated studies, together with other        phorus from different sources. Thus, Gerdes &
individual research projects conducted by Uni-           Kunst (1998) have shown that, while 72% of the
versities, Cooperative Research Centres, the             total P in effluent from sewage treatment plants
CSIRO and government agencies have greatly               (STP) was bioavailable, only 30% of the total P in
expanded our knowledge base on eutrophication.           eroded material entering the river Ilmenau in
                                                         Germany was bioavailable. They also showed that
Nutrient delivery to waterways                           this proportion increased to 59% when the soils
                                                         from which the material was sourced were ferti-
Bioavailability                                          lised, suggesting that fertiliser introduced sig-
Although the plant nutrients nitrogen and phos-          nificant amounts of bioavailable material into the
phorus occur in many different chemical forms in          runoff. Sherman et al. (2001) also found that FRP
                                                                                                         33

from the Peel River in northwestern NSW made                It is difficult to interpret the ecological impor-
up about 30% of the total P load entering Chaffey        tance of point and diffuse source nutrient loads
Dam (Fig. 1). The source of this dissolved P is         simply from their relative size (Eyre et al., 1997).
unknown because the tracer techniques used in           Loads are delivered from point and diffuse sources
these studies consider particulate matter only. The     in quite different ways: point sources such as STP
authors stressed the importance of tracing FRP          deliver a constant discharge all year round to a
rather than total P if good management decisions        particular location, while most diffuse sources
are to be informed by these studies.                    operate primarily during episodic storm events
    In a study of the Goulburn River in Victoria,       over a wide area of the waterway. Eyre et al. (1997)
Oliver & Webster (2001) found that 53, 32 and 18%       compared the ecosystem responses of two rivers in
of the TP coming from an irrigation drain, a dryland    northern NSW that have different mixes of nutri-
sub-catchment and a STP, respectively, were readily     ent sources. They found that in the Richmond
available to algae. The majority of this available      River, which has only diffuse nutrient sources, P
phosphorus occurred in dissolved form. Not              and N concentrations in water and sediments did
surprisingly, the particles from the STP were largely   not change significantly over time and chlorophyll
organic while those from the other two sources were     a concentrations remained low even though
predominantly inorganic. Despite these differences       phosphorus and nitrogen loads had increased by
in particle types, the percentage of the phosphorus     factors of 2.5 and 3 respectively over the last 50
associated with particle surfaces but rapidly           years. On the other hand, the Brunswick River
exchangeable was, on average, similar for each          received about 10% of its nutrient load from
source (23, 25 and 29% respectively).                   sewage effluent and its estuary was subject to fre-
                                                        quent phytoplankton blooms.
Diffuse vs. point sources                                    Thus, it is not only the quantity of nutrient
Diffuse sources of nutrients usually dominate the        loading to rivers that is important, but also the
input loads to rivers and estuaries. For example, in    timing, location and nature of the loading. In dry
the Chesapeake Bay system in the US, Boynton            periods, residence times are long, water velocity is
et al. (1995) found that diffuse loads contributed       low and particles settle out of the water. During
approximately 60% of the total nitrogen and total       these periods there is likely to be greater light
phosphorus loads to the Bay, whereas point source       penetration into the surface waters, and hence
inputs contributed 28 and 35% of the TN and TP          nutrients from point sources will often lead to the
load respectively. Atmospheric deposition made          development of algal blooms. Kerr (1994) found
up the remainder. Diffuse sources are even more          that blue green algal blooms in the Hawkesbury-
dominant in Australia. In one of AustraliaÕs most       Nepean River near Sydney, Australia, were
urbanised catchment, the Hawkesbury-Nepean              strongly associated with low flow and point source
river system near Sydney, Davis et al. (1998a)          discharges from STP. In a period of low river flow
estimated that diffuse sources still contributed         in 1994, which led up to the development of a large
approximately 70–80% of the TN and TP loads             summer algal bloom, he calculated that 75% of the
with point sources contributing the remainder. In       phosphorus and 84% of the nitrogen delivered to
less populated areas, the diffuse source contribu-       the river was from STPs. Similarly, Eyre et al.
tion can be as high as 96–98% of the total load         (1997) found that the majority of the low flow
(McKee et al. 2000a).                                   nutrient load in the Brunswick River estuary was
    Meyer et al. (2001) have summarised the small       from a STP and that phytoplankton blooms were
amount of information available on atmospheric          common during this period when the upper estu-
wet and dry deposition of N in Australia. They          ary was poorly flushed.
report annual mean deposition rates ranging from
0.3 kg ha)1 in oceanic areas to approximately           Nutrient limitation
10 kg ha)1 in urban centres, values which are of the    Following Howarth (1988) a nutrient is defined to
same order as the average annual Australia-wide         be limiting if it causes an increase in the rate of
emission of about 3 kg ha)1. These values are rel-      net primary production when added to a system.
atively small compared with catchment loads of N.       This may lead to changes in the structure of the
34

community. It is thus the effect on primary                   Phosphorus limitation has been observed in
production or growth rate that is important and          Australian estuaries. For example, nutrient limi-
not the effect on the final biomass achieved. The          tation occurred from late spring to autumn in the
latter is affected by a range of factors unre-            Swan River estuary (Thompson & Hosja, 1996)
lated to limitation such as self-shading in              with nitrogen being the nutrient most likely to
phytoplankton.                                           limit algal biomass during this period. However,
    It has become clear in the literature of the last    these authors observed a low degree of phosphorus
decade that the earlier generalisation that              limitation, supporting an earlier study (Jack, 1987)
phosphorus is limiting in freshwater systems (e.g.,      that had found a correlation between phosphorus
Vollenweider, 1976; Schindler, 1977, 1978; Harris,       loading and chlorophyll a concentration.
1994) whereas nitrogen limits primary production             In Australia other factors, particularly light,
in marine and estuarine systems (Ryther & Dun-           also exert influence on plant growth. In addition,
stan, 1971; Howarth, 1988) is an oversimplification.      changes to the light spectrum at depth due to the
    Elser et al. (1990) reviewed studies of 77 North     presence of light-absorbing substances in the water
American lakes and concluded that ‘‘Little support       (gelbstoff), may further reduce phytoplankton
can be found .… for the conventional wisdom that         production (Kirk, 1994; Koop, Larkum & Drew,
P is the predominant primary limiting nutrient in        unpubl. data). It appears that plant growth (in-
freshwater’’. They found that, apart from serious        cluding phytoplankton growth) in many turbid
deficiencies in experimental design of a large            Australian rivers is primarily light limited but once
number of the studies, there was no statistical          the water enters weir pools or dams, where flows
difference between the responses to P and N               are reduced and particulates in the water can settle
enrichments. Positive growth responses to both N         out, this limitation is removed and phytoplankton
and P enrichment had been recorded in 86% of the         blooms occur.
studies; 47% had shown increased growth when P               In blackwater streams and estuaries carrying
alone was added and 40% when N alone was                 high loads of coloured dissolved organic matter
added. Moss et al. (1996b) have also pointed out         (CDOM) light penetration is significantly reduced.
that, although it is theoretically true that nitrogen-   Spectral composition can also be influenced. This
fixing cyanobacteria in lakes can take advantage of       has been found in TasmaniaÕs Huon estuary,
high levels of dissolved phosphorus and atmo-            which carries high loads of humic substances
spheric nitrogen for growth, in practice ‘‘the           (CSIRO Huon Estuary Study Team, 2000).
development of blue-green algal nitrogen fixers in
lakes is slow and the phytoplankton as a whole           External nutrient sources and transport
may be strongly nitrogen-limited during mid-
summer’’.                                                Point source pollution to coastal rivers and estuaries
    Similar results have been found in Australian        As has been found elsewhere in the world, sewage
freshwaters (Robertson, 1999). Studies by Grace          pollution of coastal rivers, estuaries, and nearshore
et al. (1997) and Oliver & Webster (2001) in the         waters in Australia has been implicated in eutro-
Darling River and in the Murrumbidgee and                phication. Most of the sewage produced in coastal
Murray Rivers (Oliver, reported in Robertson             areas of Australia is discharged to estuaries and
1999) have shown that, when nutrient limitation          coastal waters following treatment, although the
was detected, nitrogen was most frequently the           extent of treatment varies. Environmental effects
nutrient limiting phytoplankton growth. Limita-          from major sewage outfalls to coastal waters,
tion was related to the time of year with nutrients      especially through deepwater outfalls, have been
controlling primary production during late spring        difficult to detect because of the highly dynamic
and early summer. Both nitrogen and phosphorus           nature of the coastal environment (Koop &
can limit phytoplankton growth in deep storages          Hutchings, 1996). The much smaller outfalls dis-
such as Lakes Dartmouth and Hume (Matveev &              charging directly to the coastline in New South
Matveeva, 1997) and Chaffey Dam (Sherman                  Wales have been shown to have only local effects
et al., 2001).                                           (e.g., Krogh, 2000).
                                                                                                            35

    Discharges to coastal rivers and estuaries,         atmospheric N onto agricultural lands and directly
however, have contributed to significant                 into water bodies has also risen, so that Smith
eutrophication problems in many areas. Thus,            et al. (1999) estimate that Northern Hemisphere
chronic cyanobacterial blooms in the Hawkesbury         inputs are approx 40–90 kg ha)1 yr)1 in heavily
River near Sydney (Fig. 1b) have been linked with       industrialised areas.
discharges of bioavailable N and P from STPs                 These excess nutrients can potentially be
upstream (Kerr et al., 1996). A vigorous scientific      transported to waterbodies, particularly in areas
debate about the relative importance of point and       where the flow pathway to streams has become
diffuse sources of nutrients for eutrophication and      saturated with phosphorus (Johnes & Hodgkin-
the discussion about the relative significance of N      son, 1998). Although there has not been a com-
and P limitation (see above) have made it difficult       prehensive study of eutrophication across Europe,
for environmental managers to decide on nutrient        there are clear indications from more restricted
reduction strategies, particularly given the high       studies of major increases of nutrients in water-
cost implications.                                      ways over the last 30 years. For example, a third
    Spatial statistical analysis techniques developed   of 200 lakes in the UK have been found to have
in the Port Phillip Bay Study (Harris et al., 1996)     had more than 500% increase in nutrient loads
and further developed in the study of Moreton           over the last 50 years (Moss et al., 1996a) and a
Bay (Dennison & Abal, 1999) are now available to        study of four European countries found that
map the extent of sewage signals around sewage          agriculture contributed more than 60% of the TN
outfalls. The latter study developed a novel            loads and 40% of the TP loads (Kristensen & Ole
method of tracing sewage plumes using measure-          Hanson, 1994). Thus, it is not surprising that
ments of stable isotopes of N in sediments and          Carpenter et al. (1998) claim that eutrophication is
naturally occurring aquatic plants and in cultured      the most widespread water quality problem in the
algae (Dennison & Abal, 1999). In that way the          US and many other Northern Hemisphere nations.
effects of STP located on separate rivers could be            Although there had been considerable research
distinguished in Moreton Bay (Fig. 2a and b).           into sources and pathways for nitrogen, particu-
Rush (2005) has further developed this technique        larly nitrate, from agricultural sources, relatively
using an algal bioindicator. She was clearly able to    little attention had been paid to diffuse sources of
show a sewage-nitrogen signal in the algae              phosphorus. As a generalisation, managers had
throughout the Hawkesbury-Nepean river system.          focussed on point sources of phosphorus, partly
                                                        because a significant fraction of the phosphorus is
Diffuse source pollutants – phosphorus                   discharged from point sources, partly because point
In Europe there has been an intensification of           sources are easier to identify and manage, and
agriculture, particularly in the post WWII period       partly because phosphorus was believed to be
as countries that suffered food shortages attempt        strongly bound to soils and therefore relatively
to gain food security (Johnes, 1999). Over 40% of       immobile (Oenema & Roest, 1997). Fertilisers and
EC lands are now used for agriculture. There have       animal manures are believed to have been applied at
been a number of studies that quantify the              such rates and for so long that in many areas soil P
resulting nutrient surpluses in agricultural lands      has built up to levels well in excess of crop needs and
(Foy & Withers 1995; Vitousek et al., 1997; Car-        has the potential to significantly increase phos-
penter et al., 1998). Heathwaite (1999) quotes an       phorus levels in waterways (Sharpley et al., 1999).
annual phosphorus surplus of 10 kg/ha in the UK         This pool of phosphorus is mobilised through both
(Sharpley & Withers 1994) and 26 kg/ha in the           surface and subsurface runoff as well as through soil
US, and Iserman (1990) quotes between 55 and            erosion. In Australia, the importance of diffuse
88 kg/ha for parts of Europe. Vitousek et al.           phosphorus sources has been accepted since the
(1997) estimate that the global agricultural N          early 1990s (Gutteridge Haskin & Davey, 1992).
fertiliser usage has increased from <10 million              The Australian environment and Australian
tonnes to 80 million tonnes over the last 50 years      land and waterway management practices differ
and that the spreading of farm animal wastes has        considerably from those in the Northern Hemi-
increased even more sharply. Deposition of              sphere. Animal densities and crop densities are
36

     (a)



             N

                                         12.2
                                 Caboolture R
                                                                   3. 8
                                                                                                                             Moreton
                                                                  3.9       Deception Bay                                     Island



                 Lake                                                                     6.0
            Samsonvale                                                          11. 1       M O R E T O N

                                            Nth                     11. 3                            B A Y
                                                  Pin
                                                        eR
                                                                            Bramble Bay                                       2.2
                                                                                                    5.1

                   LEGEND                                                        9.1                                           1.6
            4.3     Mangroves
           11. 3    Algae                                                                                                      2.5
                                                                                          8.7
            5.4     Sediment
            8.7     Seagrass                                r                                    Waterloo Bay                      2.4
                                                          ve
                                                        Ri                8.7              5.4                                     2.5
                                                                                                 6.7
                                                                                                                             2. 0       North
                                                            9.9                            9.7                                       Stradbroke
                                         ne                                                                                  2.5        Island
                                    is ba                  9. 3
                                 Br                     4.3
                                              6.2
                                 5.5
                                                                                                                4.0           3.9
                                                                                                                              4.3

                                                                                                                      5. 0
                                                                                                 9.8 Lo               3.6
                                                                                                        gan R
                         SCALE                                                                                        4.2
              0             10                20km




Figure 2. d15N Sewage signatures in naturally occurring material (a) and sewage plumes based on d15N signals in indicator algae (b) in
Moreton Bay, subtropical eastern Australia. Ambient d15N signatures of sediments and primary producers near sewage outfalls were
characterised by high d15N signatures, while remote sites had low values (a). Note that use of indicator algae allowed discrimination of
sewage signals from sources relatively close to one another (b). (Redrawn from Dennison & Abal (1999)).


relatively low (except in designated irrigation                                        is high and increasing. Between 1983/4 and 1996/7
areas) and, for most dryland agricultural areas,                                       water use increased by 65% (AATSE/IEA, 1999),
fertiliser usage rates are much lower than in Eur-                                     with most of the increase arising from increased
ope and the US. On the other hand, river water use                                     use for irrigated agriculture.
                                                                                                                   37

  (b)



         N


                           Caboolture R
                                                                                                 Moreton
                                                            Deception Bay                         Island



             Lake                                                             M O R E T O N
        Samsonvale
                                                                                     B A Y

                                      Nth
                                            Pin                       Bramble Bay
                                                  eR
             LEGEND
                      15
             0-3       N (ppt)

             3-4

             4-5

             5-7                                      r
                                                    ve                            Waterloo Bay
                                                  Ri
             >7                                                                                          North
                                                                                                      Stradbroke
             STP                   ne                                                                    Island
                              is ba
                           Br




                                                                                   Loga
                                                                                       nR
                     SCALE
         0            10                20km




                                                          Figure 2. (Continued)



   With the exception of specific rivers near ur-                       phosphorus and 29% of total nitrogen in dry
ban areas or draining irrigation areas, simple                         periods and 83% of total phosphorus and 84%
mass budgets readily demonstrate the dominance                         of total nitrogen in wet years in the MDB.
of diffuse inputs in Australia catchments.                              Compared to dry years, the total loads were 3–4
Gutteridge Haskin & Davey (1992) estimated                             times greater in average years and 20 times
that diffuse sources accounted for 28% of total                         greater in wet years.
38

    Thus, from the early 1990s, a prime issue for        as very sensitive tracers of the geologic origins of
Australian natural resource managers and scien-          the sediment material (Fig. 3). Different rock types
tists was identifying the sources of diffuse nutrients.   contain distinctive isotopic ratios of these elements
It was critical to know where the nutrients were         and the source rock types can be calculated from
originating and what pathways they were taking           the mix of trace elements and isotopes in the river
from sources to waterbodies, so that limited funds       sediments. In particular, inorganic fertiliser
could be targeted at the major contributing areas.       obtained from phosphatic rock carries with it the
Possible agricultural sources included inorganic         isotopic signature of the rock from which it was
fertiliser, animal wastes, natural soil stores or the    mined. Thirdly, mineral magnetics can also indi-
breakdown of plant materials. Bek & Robinson             cate the origin of sediments. The combination of
(1991) identified diffuse anthropogenic sources            different magnetic minerals in river sediments
such as fertilisers, and point sources such as STP       depends on the parent rock from which the
to be the most likely sources of phosphorus.             material was originally eroded.
Phosphorus was assumed to travel via surface                 Caitcheon et al. (1995) found that approxi-
pathways and nitrogen was assumed to travel              mately 90% of the sediments in Chaffey reservoir –
primarily in dissolved form via both surface and         a reservoir with a catchment of approximately
sub-surface flows.                                        42,000 ha, a surface area of 542 ha and a capacity
    Nutrient losses can be linked to catchment           of 62,000 Ml in north-western New South Wales
characteristics. Thus, yields of TP and TN have          (Fig. 1) – were derived from basaltic material. The
been related to land use, rainfall, land cover, slope    concentration and ratios of Cs137 and Pb210
and soil type in different parts of Australia (Young      showed that about half the sediments came from
et al., 1996). These empirical results have been         surface sources and the other half came from
codified into a simple annual average nutrient            subsurface sources. The analysis showed that the
export model (Davis & Farley, 1997) which has            surface sources were very limited in areal extent;
been used to identify likely sources of TN and TP        approximately 2 km2 or 1% of the catchment area.
within catchments in New South Wales and parts           Thus, this surface erosion could be identified as
of Queensland and Victoria. While this empirical         most probably originating from a small amount of
approach is simple to implement and provides             cropping that occurs on basaltic soils in the upper
managers with guidance about likely sources of           catchment. The subsurface component is most
nutrients, it is indirect, does not take account of      probably derived from streambank erosion as the
local conditions and is generally not sophisticated      river channel passes through floodplains alluvium
enough to account for different forms of nutrients        derived from upland basaltic soils.
or different size flow events.                                 The dO18 deviation (2.1 ppt) of the ortho-
    A number of experimental techniques have             phosphate was strongly characteristic of basaltic
now been developed to determine the origins of           phosphate (0.3 ppt) and showed no significant
the sediment-attached phosphorus in the rivers           contribution from fertiliser phosphate (23 ppt).
across Australia. Source and transport processes         Thus, the tracer results indicated that the soluble
differ in different regions of Australia (Davis            orthophosphate in the storage came from subsur-
et al., 1998b).                                          face drainage of basalt soils which are naturally
                                                         rich in phosphorus rather than from fertilisers.
Murray-Darling Basin                                     Further support for this conclusion came from
Three of these techniques have been used in the          Martin & McCulloch (1999) who, in a later study
MDB to determine the sources of sediment-at-             using rare earth isotopic techniques, confirmed
tached phosphorus. First, the different penetration       that most of the reservoir sediments came from
depths of Cs137 – an episodic nuclear bomb fallout       actively eroding areas in the Peel River head-
product during the 1950s – and Pb210 – a contin-         waters. They concluded that the contribution of
uous fallout product – can be used to distinguish        fertiliser phosphorus to the sediment phosphorus
river sediments originating from surficial and            was less than 0.2% in this catchment (Fig. 3).
deeper erosion events. Secondly, trace element               Martin & McCulloch (1999) also applied their
isotopes (Sr and the rare earths) can also be used       techniques further downstream to suspended
                                                                                                                                          39


                                               basalt                                      metased soil              10
                                               metasedimentary rock                    C   metased soil comp
                                               basaltic soil                               reservoir sediment
                                         C     basaltic soil comp                          fertilizer
                        0.5130
                                                             er                   10       1
                                                          liz          50
                                                       rti                                                           5
                                                    fe
                                               t%
                                              w
                                                  90
                                                                                               C
                 Nd




                                                                                                                          e
                                                                                                                     0        Nd
                144




                        0.5126
                 Nd/
                143




                                                                                                   C
                                         99


                                                                                                                     -5




                        0.5122

                                                                                                                     -10

                                        0.1                       1          10             100         1000
                                                                      Nd (ppm) / P (wt%)
Figure 3. Plots of 143Nd/144Nd vs. Nd/P for sediment from Chaffey Dam and fertiliser. Bulk mixing relations between average fertiliser
and basaltic soils as well as mixing proportions are shown. Nd/P ratios of rocks, soils and dam sediments are at least a factor of
20 lower than fertiliser, so as little as 1% addition of fertiliser can be readily distinguished. The data suggests there is little fertiliser
incorporation into the soils and sediments. Nd – neodymium; P – phosphorus. (Redrawn from Martin et al. (1999)).



sediments from the Namoi River near its junction                                 Wallbrink et al. (1998) used Cs137 and Pb210
with the Barwon River. At this point, the Namoi                               tracing techniques to show that about 85% of the
drains a catchment of 43 000 km2. There was                                   sediment in the Murrumbidgee River was derived
very little evidence of sediments derived from the                            from sub-surface sources, 13–16% came from
basaltic soils of the Chaffey catchment at this                                cultivated land and a small percent (<2%) came
location. The phosphorus concentration was 2.5                                from uncultivated land (Fig. 4). The sub-surface
times lower than in the suspended sediments de-                               sources could be either collapse of streambanks or
rived from soils of the basaltic uplands. Again,                              gully erosion. This result supports Neil & Fog-
there was strong evidence of sub-soil derived                                 ertyÕs (1991) earlier conclusion that channel sedi-
material (Davis et al., 1998b). There was no                                  ments were the major sources of sediments in the
evidence of fertiliser-derived phosphorus in these                            first order streams of this part of the MDB.
sediments although, because of the admixtures                                 Although Wallbrink et al. (1998) did not trace the
of geologies in this catchment, they could not                                origins of the phosphorus attached to these sedi-
rule out the possibility of a small fertiliser                                ments, they showed that the phosphorus concen-
contribution.                                                                 tration on the suspended sediments in both the
40

                                                      400

                                                                  Murrumbidgee uncultivated
                                                      350                       surface soil

                                                      300
                        -1
                         Pbex concentration (Bq kg)




                                                      250

                                                      200
                                                                   Murrumbidgee cultivated
                                                      150          surface soil


                                                      100
                        210




                                                                  Murrumbidgee, & tributaries,
                                                       50         suspended sediments

                                                        0
                                                                Murrumbidgee, headwater channel/gully sources
                                                      -50
                                                            0             10            20             30        40
                                                                    137                           -1
                                                                       Cs concentration (Bq kg )
Figure 4. Plots of lead vs. caesium concentrations in different source material in the Murrumbidgee River catchment and in river
sediments. The close correspondence of the signals from headwater channel and gully sources and the sediment suspended in the river
suggest that most of the river sediment is from sub-surface sources with only a small proportion derived from agricultural and
uncultivated land. (Redrawn from Wallbrink (1996)).


tributaries and the main channel of the Mur-                                        ment. That is, the sediments come from reworking
rumbidgee River (Fig. 1) was consistent with that                                   of the floodplains rather than from fresh erosion of
on the fine particle fraction (<2 lm) in the                                         the upper catchment. The phosphorus concentra-
catchment soils. From this they concluded that                                      tion in the river sediments is less than that on the
the phosphorus on suspended (and presumably                                         natural soils from which the material is derived
also bottom) sediments could be adequately ac-                                      and has not changed significantly over the last
counted for by the native phosphorus content of                                     280 years. Presumably, the in-water P concentra-
the eroded soils. Although they could not rule out                                  tions have also remained relatively constant
a contribution from fertilisers, there was no need                                  throughout the period of European settlement.
to invoke any anthropogenic source of diffuse                                        Thus there was no evidence for any anthropogenic
phosphorus.                                                                         increases in P concentration in this river.
    The      Darling-Barwon        River    drains                                       These studies in three major and one minor
650,000 km2, including the fertile and erodible                                     inland river of the MDB all show no trace of fer-
basaltic soils of the Darling Downs. An analysis                                    tiliser-derived phosphorus although it could not be
(Caitcheon et al., 1999) of the major element                                       conclusively ruled out in two of the studies. There
chemistry of the river sediments showed that the                                    was strong evidence of natural phosphorus enter-
material came originally from sedimentary-gra-                                      ing the river from accelerated erosion of subsoils
nitic rocks and not from the basaltic areas of the                                  and streambanks. Surface erosion can dominate at
catchment. The weathering of the particles showed                                   paddock scale but this source is overwhelmed by
that they had probably been derived from the                                        greater subsoil losses from eroding gullies and
lowland areas rather than from the upper catch-                                     streambanks.
                                                                                                          41

Tropical Queensland                                     soluble organic N and NH4 concentrations were
In 1993, a major desktop study of coastal Queens-       relatively constant over time despite large changes
land catchments (Moss et al., 1993) concluded that      in flow. The nitrate-N concentrations were clearly
sediment and nutrient loads had increased in coastal    driven by base flows although these concentrations
streams, wetlands, estuaries and the GBR lagoon         did rise if there was surface flow and interflow. In
over the last 100 years. Urban sources were believed    common with the previous studies, they found that
to be the most significant point source contributors     horticultural crops (bananas and sugarcane) con-
of nutrients and agriculture was the major diffuse       tributed significantly greater yields of TP (about
contributor. Point sources were largely insignificant    7 kg ha)1 yr)1) than pastures and rainforest
in terms of total load, but in many instances could     (about 2 kg TP ha)1 yr)1).
have local significance.                                     From national soil testing data, Reuter (2001)
    The studies that have been undertaken since         has shown high levels of phosphorus in the soils
then have been driven by a concern for the integrity    underlying the cultivated crops areas of north
of the Great Barrier Reef and its lagoon and so         Queensland, compared with other Australian
analyses have concentrated on delivery from estu-       States. Concentrations were commonly in the
aries rather than on the upstream processes             range 40–150 mg P kg)1 using the Colwell phos-
(e.g., Hunter et al., 1996; Eyre 1993). These studies   phorus test. Although the data do not identify the
have typically derived the total and areal loads of     origins of the phosphorus, it is extremely likely
sediment, nitrogen and total phosphorus (e.g.,          that the nutrient comes from fertiliser additions to
Moss et al., 1993; Mitchell et al., 1996) and,          the crops.
sometimes, attributed these nutrient loads to land          Direct tracing of sediments and attached
use types.                                              nutrients in rivers has only been carried out in
    Thus, Bramley & Johnson (1996) found that           Queensland in the Herbert catchment and a
nutrient export rates from intensive agriculture        smaller study of Berner Creek in the Johnstone
were greater than those from other land uses in the     River catchment. While the results of the Herbert
Herbert catchment irrespective of the type of           study are unavailable, those from Berner Creek
production being undertaken. Mitchell et al.            show the different processes that operate in these
(1996) conclude that fertiliser accounts for sig-       tropical areas compared to the MDB (Wallbrink
nificant nutrient input to the lower Herbert river,      et al., 2001). The majority (70%) of the sediment-
principally from sugarcane areas. Large nutrient        bound phosphorus comes from cultivated lands
loads are moved down rivers during tropical             (bananas in the steeper uplands and sugar cane on
storms. They estimated that 600 tonnes of nitrogen      the lower, flatter areas). Surface erosion is the
and 65 tonnes of phosphorus were exported from          major pathway for the transport of sediment to
the lower catchment in a single large storm event       rivers from these cropping systems, with only 11%
that was monitored in 1994. In a major study of         of the phosphorus originating from sub-soil sour-
the Johnstone River, Hunter et al. (1996) also          ces. Overall, 42% of the phosphorus exported
found that large sediment loads (67 000 tonnes),        from the catchment originated from fertiliser; the
total nitrogen (343 tonnes) and total phosphorus        majority of this fertiliser phosphorus originates
(104 tonnes) were moved during a cyclonic storm         from the cultivated lands.
in the Johnstone River in 1994. As part of the              Eyre (1993) has carried out one of the few
same study, Walton & Hunter (1997) found that           studies in tropical Queensland that attempted to
horticultural crops produced significantly greater       uncover the processes controlling nutrient trans-
yields of suspended sediments, total phosphorus         port within the waterways. In his study of the river
and total nitrogen than did pastures and rain-          and estuary of the Moresby River, he concluded
forest. These three contaminants were delivered         that particle-attached phosphorus is eroded from
quite differently. The suspended sediment and            canelands during the wet season and deposited
particulate phosphorus concentrations increased         into drainage lines and the upper estuary. It then
rapidly with flow, so that the majority of the           moves slowly down the estuary during the dry
annual loads were carried during large flow events.      season under the influence of tidal movement. This
However, dissolved phosphorus (PO4 and DOP),            result is not necessarily contrary to the conclusions
42

of Hunter et al. (1996) about the dominance of         terised by deep duplex and lateritic soils with high
storm flows, since Eyre analysed estuarine sedi-        phosphorus retention abilities. Most of the natural
ments and did not monitor flow events. It is likely     vegetation of the catchment has been cleared for
that both results are true, depending on whether       agriculture since European settlement in 1829
cyclonic storms occur during the period of interest.   (Avon River System Management Committee,
                                                       1993). The introduction of seasonal, shallow-roo-
Southwest Western Australia                            ted crops has led to increased recharge and
Nutrient dynamics have been studied for many           increased surface runoff.
years in Western Australia because of eutrophi-            The hydrologic changes brought about by these
cation of estuaries in the southwest of the State.     changes coupled with increased potential for soil
Deeley et al. (1999) showed that sandy coastal soils   erosion and bank degradation and the widespread
with low phosphorus binding capabilities,              use of fertilisers have resulted in considerable
replacement of native perennial vegetation with        increases in nutrient export from the catchment
annual grasses, the rise in groundwater levels and     since European settlement (Viney & Sivapalan,
the regionÕs marked Mediterranean climate all          2001). These authors modelled pre-European dis-
make this area especially susceptible to eutrophi-     charges and particulate phosphorus yields and
cation. Fertilisers are the main sources of nutri-     concluded that, prior to clearing, discharge was
ents, even at moderate application rates, (Hodgkin     only about 20% of current levels and the TP yield
& Lenanton, 1981; Hodgkin & Hamilton, 1993;            was just over 6% of the current yield. They esti-
Deeley et al., 1999) although point source inputs      mated that flow-weighted average concentrations
are known also to be important in at least one of      for the entire catchment have increased by a factor
the catchments of the Peel-Harvey estuary              of 3.3 for TP and 1.9 for soluble phosphorus since
(Fig. 1a).                                             European settlement. In one catchment on the
   The Swan/Avon River drains a large catchment        coastal plain, Ellen Brook, the current concentra-
(121,000 km2), exiting to the Indian Ocean             tion of total phosphorus is 5.6 times the estimated
through the city of Perth (Fig. 1a). The river has     pre-European concentration. These predicted
experienced regular phytoplankton blooms and, in       changes in phosphorus concentrations are similar
early 2000, a major cyanobacterial bloom               to the concentration changes detected over the
(Hamilton, 2000). Because of the aesthetic and         same period by Gerritse et al. (1998) in sediment
recreational importance of this waterway, there        cores taken below the Ellen Brook confluence.
was an intensive catchment, river and estuary              Peters & Donohue (2001) analysed nutrient
study (Hamilton & Turner, 2001) to understand          concentrations and fluxes in 15 tributaries to the
the causes of these blooms and develop manage-         Swan-Canning Estuary for the period 1986–1996
ment options. Phytoplankton are known to be            and 1986–1992, respectively. The Avon River
predominantly nitrogen limited during the spring       contributed 65% of the nitrogen load but only
and summer months when the major blooms ap-            32% of the phosphorus load. Most of the phos-
pear (Thompson & Hosja, 1996; Thompson et al.,         phorus load originates from the catchments of the
1997) and so the study included a component to         coastal plain through rapid leaching of phospho-
identify the sources of both nitrogen and              rus from fertiliser applied to the infertile sandy
phosphorus.                                            soils (Gerritse et al., 1990). This is consistent with
   The Swan River catchment includes a large           Pionke et al.Õs (1990) conclusion that nutrients are
area of the Darling Plateau (drained by the Avon       being added to some horticultural activities con-
River), a series of catchments on the coastal plain,   siderably in excess to crop uptake (5· for N and
and the urban area of Perth. Perth and the coastal     10· for P) throughout the Swan coastal region.
catchments experience a Mediterranean climate          Most of the added phosphorus is in soluble form
with most rain falling from autumn through to          (orthophosphate and organic complexes), whereas
spring and a pronounced dry period from                the remainder of the Swan catchment provides
November to May. The soils of the coastal plain        primarily particulate phosphorus.
are predominantly sandy with poor nutrient                 The Ellen Brook catchment (664 km2 or 0.6%
retention capabilities, while the plateau is charac-   of the total Swan catchment area) alone contrib-
                                                                                                           43

uted a remarkable 62% of the FRP and 42% of              soils with higher Fe content can accumulate
the annual TP flux to the estuary. Although the           phosphorus for a number of decades. Average
superphosphate application rates are relatively low      phosphorus storage in the surface soils has in-
in this catchment (Gerritse & Adeney, 1992) there        creased from about 930 to 3200 kg ha)1 with river
is a high (25–50%) and rapid loss of the applied         sediments showing elevated phosphorus concen-
phosphorus to the stream. In recent years, the flow       trations within the upper few centimetres (Weaver,
adjusted TP concentrations have declined in Ellen        quoted in Davis et al., 1998b).
Brook (Donohue et al., 2001) although still                  Transport of both particulate-attached phos-
remaining very high (median TP of 0.54 mg l)1            phorus and dissolved phosphorus via surface
from 1987 to 1994 declining to 0.37 mg l)1 by            runoff and leaching through the soil profile are
1998). The reasons for this decline are unclear.         known to occur, although the contributions of
     Ellen Brook is also a major N source along          surface erosion, gully erosion and streambank
with some vegetated areas on the Darling plateau.        collapse to the particulate fraction are not well
Thus, Ellen Brook has the highest annual average         known. However, McKergow et al. (2001) in a
organic N (1.8 mg l)1) and total N (2.1 mg l)1)          study of a small catchment on the south coast
concentrations of any of the Swan river tributaries      found that streambank erosion contributes little to
(Peters & Donoghue, 2001). The N/P ratio of the          the TP load. Weaver & Reed (1998) conclude that
annual loads from this tributary is about 3 (the         significant areas with sandy soils of low PRI and
bioavailable ratio is even lower), thus biasing          histories of fertiliser application are already
downstream waters towards nitrogen fixing cya-            showing signs of phosphorus saturation with the
nobacteria. Groundwater nutrient concentrations          potential for major releases of phosphorus to
are exceptionally high at 5 mg l)1 TP and                rivers if fertiliser applications continue.
12 mg l)1 TN under horticulture on the sands
within the Ellen Brook sub-catchment. Sharma             Subsoil movement of phosphorus
et al. (1996) have forecast that the P concentra-        The standard transport paradigm states that
tions in Ellen Brook baseflows will rise steeply          phosphorus is transported in surface runoff, par-
over the next few years as saturated plumes from         ticularly attached to soil particles, except in areas
the sandy soils of the catchment reach the Brook.        where there are sandy soils with low phosphorus
     Nutrient sourcing and transport processes in        sorption capacities. However, studies have now
the southern coastal catchments of WA have also          shown that macropores have the potential to
been studied for a number of years because of            transport phosphorus through heavy, duplex soils
actual or suspected eutrophication of estuaries          via sub-surface pathways.
along this coast. The soils of these catchments are          Kirkby et al. (1997) report a study of phos-
commonly duplex with siliceous sands overlying           phorus transport processes in a duplex soil in
laterite and clay in the higher landscape and sands      South Australia. Phosphorus, from fertiliser
and gravels at lower elevations (Davis et al.,           applications, was transported in dissolved form
1998b). The sandy soils have low phosphorus              through macropores into the B horizon and pos-
retention indices (PRI). Much of the area has been       sibly into the watertable. The fate of this phos-
converted from native bush to agriculture in the         phorus was not known but it was likely to have
last 40 years (Weaver & Reed, 1998). There is a          travelled along the A/B or B/C soil horizons to
monotonic increase in phosphorus concentration           either enter streams directly or indirectly from
with time since clearing from 3 mg kg)1 (uncle-          return flow lower down the landscape. Kirkby
ared) to 29 mg kg)1 (>30 years) which Weaver             et al. (1998) extended this work to 10 soil types,
and Reed attribute to the addition of phosphatic         ranging from sands to heavy clays, taken as large
fertilisers. However, some of this difference may         (1 m diameter) cores from field sites in South
also be attributable to the increased clearing of less   Australia. Under laboratory conditions simulating
productive, sandier soils over the period. The           natural storm events, they found that phosphorus
sandy soils of this region become saturated with         was flushed rapidly through heavier soils when
phosphorus within about 10 years at typical fer-         macropores were present, even when the soils had
tiliser application rates (13 kg P ha)1), while the      high P-adsorption characteristics. The macropores
44

prevented the necessary contact time between the        Residence time and effective load
applied phosphorus and the soil particles.              Depositional environments such as lakes and some
Macropores need occupy only 5% of the soil              estuaries act as sinks for nutrients. In coastal rivers
volume to completely alter the phosphorus trans-        without major estuaries, there is greater through-
port pathways. The prevalence of macropores in          put, with much of the load being delivered in short
the field is not known, but macropores were              storm runoff events (Cosser, 1989; Cullen, 1991;
present to some extent in all 10 cores from this        Harris, 1996; McKee et al., 2000b). Sediments and
region and other studies have shown them to be          nutrients are transported downstream in flood-
common soil features.                                   waters through the estuary and out onto the con-
    This work was extended into the field in paired      tinental shelf. Similarly, in estuaries with constant
catchments at two sites (Stevens et al., 1999). One     high flows, such as those in Tasmania where water
site (Myponga) had loamy sands over heavy clays         residence times are short, the bulk of material
while the other (Mt Bold) had silty clay loams          transported by the river may be exported to
over medium heavy clays. Surface, A/B and B/C           coastal waters. Although some nutrients are
horizon flows were intercepted after flow dis-            deposited in rivers during these storm events, the
tances of between 100 m (Myponga) and 300 m             residence time of most of the catchment-derived
(Mt Bold). Phosphatic fertilisers were applied at       nutrients may only be a few days or at most a few
both sites at a typical agricultural rate of            weeks. Once these nutrients are exported to the
15 kg P ha)1. Over 2 years of monitoring at the         lower estuary and inshore coastal waters, pro-
Myponga site, over 95% of the flow and the total         cesses of dilution, dispersion, sedimentation and
phosphorus was transported via macropore flow            denitrification will tend to minimise any potential
along the A/B soil horizon. Even at the Mt Bold         these nutrients have to contribute to eutrophica-
site with its heavier textured surface soils, 45% of    tion, at least in the short-term.
the water over the 2 years moved via subsurface             In the sub-tropical Richmond River catchment
flows, although the proportion of TP that moved          in northern NSW, for example, McKee et al.
via this pathway is not reported. The macropores        (2000b) found that over 97% and 94% respectively
were capable of transporting both dissolved and         of the diffuse phosphorus and nitrogen loads were
particulate phosphorus. Thus, in the first year at       exported from the catchment to the ocean during a
the Myponga site between 90 and 100% of the             storm. Nutrient exports to the ocean were found to
phosphorus was in the particulate form, whereas         be proportional to flushing time; the larger floods
the following year between 90 and 100% was              with rapid flushing times exported larger propor-
dissolved. The reasons for this change in form are      tions of the diffuse load to the ocean than smaller
not well understood.                                    floods with longer flushing times. Temperate rivers
    While these experiments have shown that for         might be expected to export a smaller fraction of
different soil types, subsurface processes are           the diffuse load to the ocean due to their generally
important (and often dominant) transporters of          smaller flooding regime (Eyre, 1998). In a pre-
phosphorus in both dissolved and particulate form       liminary assessment of nutrient exports to the
over distances of hundreds of meters, it is yet to be   ocean from several temperate NSW estuaries,
shown whether significant loads of nutrients can         SKM (1997) estimated that, on average, approxi-
be transported to streams via this mechanism. The       mately 30% of the wet weather diffuse load into
management significance of these studies is that (1)     these systems would be immediately exported to
management activities such as grassy swales and         the ocean. Thus, only a fraction of the diffuse load
buffers that are designed to trap particulate-borne      that enters a coastal river system during storm
phosphorus in surface flows may be ineffective if         events contributes to eutrophication problems in
large proportions of the nutrients are moving           the river and estuary.
beneath the surface, and (2) even though there is           On the other hand, inland flowing rivers such
no evidence of surface erosion from the tracing         as those in the Murray Daring Basin have long
techniques that have been employed in the MDB,          residence times for most of the year due to the very
fertiliser phosphorus may be still transported into     low slope of their lower reaches and the con-
waterways via macropores.                               struction of impoundments along their length for
                                                                                                                             45

irrigation purposes. Even in times of flood these                          10 cm of the sediments in this river reach. The fine
inland rivers have residence times ranging from                           clays (which contained over half this phosphorus
weeks to months. The implications of these long                           load) had a 10% organic content and so there
residence times are discussed below.                                      existed the potential for large phosphorus releases
                                                                          under anoxic conditions, depending on the avail-
Internal nutrient sources                                                 ability of the carbon and phosphorus, the redox
                                                                          conditions within the river and transport mecha-
Sediment-water interactions                                               nisms from the sediments to the water column.
Bottom sediments in shallow aquatic ecosystems,                           They calculated that even a 1% release of this
as well as acting as sinks for nutrients in settling                      available phosphorus into the water column would
particulate matter, may also be important sources                         increase the dissolved phosphate concentration
of regenerated nutrients. To date, freshwater                             from 10 to 70 lg/l. Sherman et al. (1998) in their
studies have concentrated primarily on the role of                        study of the same region of the Murrumbidgee
sediments in phosphorus cycling (e.g., Oliver et al.,                     River found no evidence of anoxia over 2 summer–
2000) although, recently, nitrogen recycling has                          autumn periods, largely because of regular inflows
also begun to receive attention.                                          of colder, oxygenated waters from upstream dam
    Chambers et al. (1997) estimated the phospho-                         releases. Thus, even though the sediments
rus load contained in the sediments of a 3.5 km                           contained a large phosphorus store, there was little
stretch of the Murrumbidgee River, southern NSW                           opportunity for it to be released into the water
(Fig. 1). They found that none of the phosphorus                          column while rivers such as the Murrumbidgee have
was charge-bound to the sediments – the weakest                           regular inflows for irrigation purposes.
form of binding – implying that all such phosphorus                           It has been widely believed that these sediment
had already been released into the water column                           stores of phosphorus would be released into the
prior to the cores being extracted. They estimated                        water column under anoxic conditions. However,
that the potential load of weakly bound phosphorus                        Bourgues et al. (2000), using sediments from the
was approximately 1 tonne per km from the top                             Yarra River (Fig. 5), showed that phosphorus


                                                         Yarra River Sediment (25°C)
                                             7                                                           3.5

                                             6                                                           3.0
                Dissolved Oxygen (mg O2/L)




                                                                                          DO
                                             5                                           FRP             2.5
                                                                                                               FRP (µM)




                                             4                                                           2.0

                                             3                                                           1.5

                                             2                                                           1.0

                                             1                                                           0.5

                                             0                                                           0.0
                                                 0   1   2    3       4         5    6         7     8
                                                                  Time (Days)
Figure 5. Results from Yarra River sediment incubation experiments. Note that phosphorus fluxes from these sediments remained low
                                                              `
even after extended periods of anoxia. (Redrawn from Bourgues et al. (2000)).
46

fluxes did not necessarily increase even when sed-     to 150 lmol N m)2 h)1 at 19 sites in the Brisbane
iments remained anoxic for some days. Similar         River in southeastern Queensland, and Bourgues
results have been obtained by Bourgues et al.         et al. (unpublished) measured rates ranging from
(1998) in sediments from the Hawkesbury River         150 to 380 lmol N m)2 h)1 in the Goulburn and
near Sydney and by Douglas et al. (1997). A           Yarra rivers in Victoria. Thus, these first direct
nutrient diagenesis model developed by Harper         measurements of denitrification rates in Austra-
(pers comm.) accounts for these effects. The model     lian freshwater systems imply that denitrification
incorporates the adsorption of phosphorus by          is not the major loss pathway for nitrogen from
both Fe(OH)3 and non-iron compounds as well as        Australian rivers.
NH3 sorption. It predicts a very wide range of PO4        Denitrification rates similar to those reported
flux responses, depending on the depth of the          above were recorded for inorganic nitrogen fluxes
Fe(OH)3 layer, the adsorptive properties of the Fe    from sediments in Port Phillip Bay (Nicholson
and non-Fe sediment phases, bioturbation and          et al., 1999). Using mass balance calculations and
other mixing processes, and the mineralisation rate   direct measurements of N2 generation, it was also
of particulate organic matter. These responses        found that inorganic N fluxes were much lower
range from a pulse of P into the water column         than expected from Redfield stoichiometry, sug-
within days of anoxia involving only a small          gesting that a significant proportion of the organic
release of P, to a slow buildup and long attenua-     N reaching the sediments was denitrified. Direct
tion of sediment-water P fluxes involving large        measurements using N2/Ar ratios confirmed high
loads of P over many months. These different           denitrification rates (Harris et al., 1996; Heggie
sediment response types have important implica-       et al., 1999) in many sediment facies in the Bay.
tions for management of waterbodies. For exam-        These studies by the Port Phillip Bay team were
ple, if waterbodies contain ‘slow responseÕ           seminal in establishing the hypothesis that Aus-
sediments there may be little release of phosphorus   tralian estuaries have high rates of denitrification
into the water during short-term anoxic events.       and high denitrification efficiencies, thus suggest-
    Nitrogen fluxes have been measured from            ing that they would be able to process relatively
sediments in a small number of Australian rivers.     high inputs of nitrogen. The studies also found,
Studies by Bourgues et al. (2000) in the Goulburn     however, that where sediment respiration rates
River, show that sediment-water nutrient              were high (such as at the mouth of the Yarra River
fluxes were variable with maximum rates of             entering Port Phillip Bay) denitrification efficiency
5 mmol m)2 d)1 of ammonium in summer and              was close to zero and most of the organic N
undetectable phosphorus fluxes. NOx fluxes were         deposited in the sediments was returned to the
always into the sediment suggesting denitrification    water as ammonium.
was actively occurring, driven by water column            A later study in Moreton Bay off Brisbane,
nitrate (up to –1.7 mmol NOx-N m)2 d)1).              southern Queensland, found that muddy sedi-
Bourgues et al. (unpublished) showed low rates of     ments were ‘‘sensitively poised’’ in respect of
denitrification with 50% of samples below detec-       denitrification efficiency (Dennison & Abal, 1999).
tion and maximum rates of 20 lmol N m)2 h)1 in        When sediment oxygen concentrations were
river sediment samples. These rates are similar to    reduced by about 50% of ambient, ammonium
rates measured by Hanington and co-workers            fluxes increased by a factor of 5 and nitrification
(unpubl.) in a tributary of the Hawkesbury River      was suppressed. This phenomenon has not previ-
in Sydney. They found that denitrification could       ously been recorded from coastal systems, but it
remove less than 1% of the total NOx load dis-        suggests that the muddy sediments in subtropical
charged from a sewerage treatment plant into the      Moreton Bay may be particularly sensitive to
river over a distance of 15 km. Nielsen (1992)        changes in oxygen concentrations or organic
measured rates between 34 and 51 lmol N m)2 h)1       loading. Estimates of N input and N export to the
in river sites and a single rate of 122 lmol N        ocean suggest that the difference between those
m)2 h)1 immediately downstream of a STP               two terms is small and N loss through denitrifi-
releasing NOx in its effluent. Udy (2001) found         cation correspondingly small. However, denitrifi-
similar denitrification rates ranging from about 5     cation efficiency for the Bay calculated from the
                                                                                                            47

relationship between the percentage of nitrogen         results were in line with earlier studies of both soils
load removed through denitrification and log res-        (Sparling et al., 1985) and sediments (Fabre, 1988)
idence time (Nixon et al., 1996) suggests that some     where the phosphorus release on rewetting was
25% of the N inputs may be denitrified by Mor-           attributed to decomposition of killed bacteria.
eton Bay sediments. Thus, there is considerable         Qui & McComb (1995) found support for this
uncertainty in the estimates of N loss through          conclusion and estimated that air-drying killed
denitrification from the estuary.                        about 76% of the microbial biomass. Possible
    Sediment flux studies in Wilson Inlet, an inter-     mechanisms for phosphorus release on rewetting
mittently closed estuary in southwest Western           included (1) autolysis and drying-induced
Australia (Fig. 1a), confirmed the importance of         destruction of micro-organisms, (2) stimulation of
denitrification in regulating N fluxes in Australian      decomposition of organic matter during drying
estuaries. Fredericks & Heggie (2000) found N           so that phosphate was ready for release after
fluxes of 2–3 mmol m)2 d)1 (as ammonium) in              re-wetting, (3) crystalline structural changes lead-
spring and summer, declining to around                  ing to inactivation of binding sites for phosphate.
1.5 mmol m)2 d)1 in winter. Denitrification effi-          They claimed that the first process was dominant
ciencies ranged from around 50% in spring to be-        when the P concentrations in the overlying waters
tween 70 and 80% in late summer and autumn.             were relatively low, as occurred in their experi-
Contrary to findings in Port Phillip Bay there was       ments. Baldwin (1996) measured the phosphate
no relationship between denitrification efficiency         adsorption isotherms for sediments from a single
and sediment carbon load. Clearly these three de-       site at a northern NSW reservoir. These sediments
tailed studies of denitrification in estuarine systems   were divided into three groups (a) never dried, (b)
in southern Australia have shown that the model of      oxidised but remained moist and (c) desiccated and
high denitrification efficiency suggested by the Port      oxidised. The results implied that it was the oxi-
Phillip Bay study does not apply to all estuaries.      dation, rather than the desiccation, which changed
                                                        the P absorption characteristics of the sediments.
Nutrient release from sediments after drying                Conversely, Mitchell & Baldwin (1998) found
Drawdown of reservoirs is used as a deliberate          that phosphorus release from dried lake sediments
management technique to control nuisance plant          was almost completely suppressed after rewetting,
growth in Europe and, to a lesser extent, in            irrespective of whether the rewetted sediments
Australia. More commonly, given AustraliaÕs             were held under oxic or anoxic conditions. Phos-
variable climate, wetlands, storage reservoirs and      phate concentrations in water overlying control
rivers dry out following periods of low rainfall.       sediments, that were never dried out, reached
Major ecological changes have been observed in          2000 lg l)1 within 8 days of the onset of anoxia,
some water bodies after refilling. For example,          whereas there was no detectable phosphorus con-
Lake Mokoan shifted from a macrophyte domi-             centration after 30 days in waters above rewetted
nated to an algal dominated system after refilling       sediments. Most importantly, this study added
following the 1982 drought (Water ECOsciences,          strong evidence to earlier work (e.g., Gachter &
                                                                                                      ¨
1996). A number of processes may be responsible         Meyer, 1993) that showed that P release from
for these shifts including changes in nutrient          sediments is a biologically mediated process. Thus,
release from sediments after re-wetting. Baldwin        the addition of formaldehyde completely sup-
et al. (2002) provide a review of the literature on     pressed all P release from anoxic sediments that
this topic.                                             had not been desiccated, the addition of acetate
    Qui & McComb (1995) took cores from an              enhanced the release of P above that seen in the
urban wetland and subjected them to a prolonged         controls and the addition of glucose slightly sup-
(40 day) period of air-drying followed by refloo-        pressed P release. Both carbon sources even pro-
ding under both aerobic and anaerobic conditions.       moted P release from the desiccated sediments,
They found an enhanced P release from the               albeit at a much slower rate than from the control
reflooded sediments compared to controls that            sediments. They interpret these results as showing
were never dried out. The P release was greater in      that bacteria play a major role in the release of
the anaerobic than the aerobic sediments. These         phosphorus and that the bacteria were carbon
48

limited after desiccation. The apparently contrary      in New South Wales, had found that the ‘‘phy-
finding by Qui & McComb (1995) of large P                toplankton population of the Darling River at
releases after rewetting of desiccated sediments        Burtundy appears to be most strongly influenced
arises because of the large size of their samples and   by the high variability in flow and changes in
the consequent load of phosphorus available from        turbidity rather than by the seasonal temperature
the dead bacteria after desiccation (Baldwin, pers.     pattern or availability of nutrients’’. Previously,
comm.).                                                 Sullivan et al. (1988) had found that phyto-
    Given the extent of bacterial mortality follow-     plankton abundance was regulated by flow in the
ing desiccation, we would expect that nitrogen          Murray River and Jones (1993) had come to a
processes in sediments would also be affected by         similar conclusion for the Murrumbidgee River.
desiccation and rewetting. Thus, Qui & McComb               Jones et al. (1997) extended this work to the
(1995) found that wetland sediments exhibited           rivers on the east coast of Australia and showed
increased N mineralisation, nitrification and            that the major rivers fell into one of three cate-
denitrification after desiccation and rewetting, and     gories. In Type I rivers there was a linear, inverse
Boon et al. (1997) reported reduced methanogen-         relationship between cyanobacterial population
esis after rewetting dried sediments. However,          and river flow; in Type II rivers populations
Mitchell & Baldwin (1999) found little evidence of      developed under prolonged low flows, but not
nitrification in either the control (always wet) or      always; and in Type III rivers significant cyano-
desiccated sediments and they concluded that, at        bacterial populations did not develop under either
least for their samples from a water storage in         high or low flows. The duration of low flows, the
southern NSW, nitrification could not act as the         nutrient status and the turbidity of the water body
source of NO3 for subsequent denitrification pro-        determined the size of the cyanobacterial popula-
cesses. They found that denitrifying bacteria were      tion, if one developed. They concluded that flow
nitrate and probably carbon limited. N2 produc-         was the primary determinant of the onset of
tion was similar in sediments from the control sites    cyanobacterial populations, not only in the Dar-
and the desiccated sites under both unaltered           ling and Murrumbidgee Rivers, but also in a wider
conditions and with the addition of nitrate and         range of Australian rivers.
carbon sources to overcome input limitations.               However, the mechanisms underlying the
Unlike the findings of Qui & McComb (1995), they         cyanobacteria-flow relationship were not clear.
did not observe any release of mineral N when the       When the link between stable water and cyano-
desiccated sediments were re-wetted. However,           bacterial populations in rivers had been noted
like Boon et al. (1997), they did find that methane      previously (e.g., Reynolds, 1984), flow had been
production was suppressed when the desiccated           thought to play a stimulating or auxiliary role. The
sediments were re-wetted compared with the con-         above Australian studies suggested that flow was
trol sediments. Addition of carbon sources stimu-       more important than either nutrient availability or
lated methane production more in the control            temperature in controlling algal growth in rivers.
sediments than in the dried sediments, as would be      Increasingly, the importance of residence times
expected if the bacterial population of the latter      (i.e., of flow regimes) on river system behaviour is
had been reduced due to the desiccation.                being recognised as a key parameter in eutrophi-
    Thus the effect of desiccation on N processes is     cation in Australia. As pointed out by Harris
less clear than is its effects on P release. Some N      (1996), widely fluctuating residence times in Aus-
processes are suppressed (e.g., methanogenesis),        tralian rivers profoundly influence system biogeo-
others appear to be unaltered (denitrification)          chemistry and thus the development and
while the results for mineralisation and nitrifica-      sustainability of algal blooms. During times of
tion differ with different studies.                       drought, the residence time of water in rivers is
                                                        increased and eutrophication presents a problem.
Stratification, turbidity and light                      This is exacerbated in Australia by extensive river
                                                        regulation and water abstraction, which further
Hotzel & Croome (1994), in a study of long-term
  ¨                                                     reduces flows in rivers in times of low precipitation
monitoring data from a site on the Darling River        and runoff.
                                                                                                        49

    Webster et al. (1997) and Sherman et al. (1998),   not nutrients was limiting cyanobacterial growth.
in a study of Maude weir pool on the Murrum-           Turbidity dropped below 100NTU – the critical
bidgee River, first explained the mechanisms            value for light limitation of algal growth – when
behind this flow-cyanobacterial relationship in         salinity increased above 300 lS cm)1. These saline
Australian rivers. Australian inland rivers tend to    inflows came from groundwater intrusions at
be highly turbid and so heat is readily absorbed in    specific locations along the river, one such plume
the surface waters during periods of low flow,          having a conductivity of 25,300 lS cm)1 (Wil-
leading to stratification of stationary water in weir   liams, 1993). Thus, in this river, these intrusions
pools and other impoundments. Sherman et al.           trigger cyanobacterial blooms by penetrating the
(1988) showed that at discharge rates below            water column when flows are low and the pressure
1000 Ml d)1 the Maude weir pool was persistently       in the surrounding aquifer drops. The increased
stratified. Once stratification set in, the common       salinity causes the fine clays in the river to floc-
diatom (Aulacoseira sp.) sank out of the photic        culate, light becomes available to phytoplankton
zone and ceased to grow because of light limita-       and, providing physical and biological conditions
tion, while the buoyant Anabaena sp. dominated         are suitable, cyanobacterial blooms develop.
the photic zone. Anabaena populations peaked               In a subsequent study of the lower Murray
after approximately 14 days if the stratification       River (Bormans et al., 1997; Burch & Ganf, 1998),
persisted that long (Fig. 6). It appears that the      it was found that stratification occurred only
population had depleted the available phosphorus.      briefly during periods of low wind. Bormans &
When discharge rates increased above 1000 Ml d)1       Webster (1997) used a simple measure of vertical
the weir pool was diurnally stratified, i.e., total     mixing to predict the duration of stratification
mixing of the water column occurred daily. Under       over a 6-year period, based on the meteorological
these conditions the diatom had a competitive          record, and concluded that stratification never
advantage.                                             persisted for more than a few days at a time.
    This understanding opened up the possibility of    Consequently, cyanobacterial blooms should not
management strategies for river impoundments           be able to develop to nuisance sizes in the flowing
based on breaking stratification through flow            parts of the river. In contrast to the Murrumbidgee
manipulation (Webster et al., 2000). Although the      River, where flow dominates the control of strati-
only times when cyanobacterial blooms were             fication, wind and flow are both important for
observed was when flow was low and stratification        controlling stratification in the lower Murray
was established, there were two occasions during       River.
the period of study when physical conditions were          More recently this work has been extended to
suitable but cyanobacterial populations did not        combine the interactions between light availability,
increase as predicted in the surface layers. The       nutrients, and flow and stratification in dams and
authors speculate that low nutrient concentra-         weir pools in a study of the tropical Fitzroy River
tions, zooplankton predation or some other             in Queensland. This river has highly episodic flows
unknown factor must have limited phytoplankton         driven by summer rainfall followed by many
numbers at these times. Thus, stratification is a       months of low or no flow. Bormans et al. (2004),
necessary but not sufficient condition for cyano-        studying a section of the river upstream of the
bacterial blooms in these impounded river systems      Rockhampton Barrage, found a pattern of
(Sherman et al., 1998).                                decreasing flow after tropical storms, leading to
    In a subsequent study in a weir pool on the        increased sedimentation after the flow had ceased
Darling River, Oliver et al. (2000) came to a sim-     altogether. This sedimentation sequestered a large
ilar conclusion. Cyanobacterial concentrations         proportion of the nutrients, both P and N, from
were found to be inversely related to discharge,       the water column. During the low flow period, the
with large blooms only occurring when discharge        barrage pool stratified leading to decreases in
was below 500 Ml d)1, but not on all occasions (a      oxygen in the hypolimnion. Rates of oxygen
Type II river in JonesÕs scheme). Light penetration    consumption were about 0.5 mg l)1 d)1 (Bormans
was critical to the development of blooms; for at      et al., 2004) and all hypolimnetic oxygen was
least half of the 44 months of the study, light and    consumed within 5 months. During this time water
50

       (a)                                     6000                                                            5

                                               5000       Discharge
                                                                                                               4
         Discharge (MLd )




                                                          ∆Tmin
        -1




                                               4000                                                            3

                                               3000                                                            2 (°C)

                                               2000                                                            1

                                               1000                                                            0



       (b)                                      110

                                                100                                            0.01 m
                 Turbidity (arbitrary units)




                                                                                               1.50 m
                                                 90                                            2.50 m
                                                                                               7.00 m
                                                 80

                                                 70

                                                 60

                                                 50

                                                 40

       (c)                                       50                                                            30000


                                                 40       F2.5
                                                                                                                        Abundance (cells mL )
         Chlorophyll (mg m )




                                                          Anabaena
        -3




                                                                                                               20000
                                                 30       Aulacoseira


                                                 20
                                                                                                               10000

                                                 10
                                                                                                                                      -1




                                                 0                                                             0
                                                26-Dec   5-Jan          15-Jan       25-Jan   4-Feb      14-Feb
                                                                             1993/1994
Figure 6. Transition between diatoms (Aulacoseira spp.) and cyanobacteria (Anabaena spp.) related to flow and turbidity in a turbid-
river weir pool, Maude Weir pool, on the Murrumbidgee River in southeastern Australia. (a) Discharge and difference in temperature
between top and bottom water (DT); (b) turbidity at 4 depths in the weir pool measured by beam attenuation at 880 nm and
(c) Anabaena and Aulacoseira abundance (5 m integrated sample) and in situ chlorophyll a fluorescence (F2.5). Note the switch from
diatoms to cyanobacteria when flow was reduced and stratification persisted for more than about 2 weeks. (composed from Sherman
et al. (1998)).
                                                                                                                                             51

column nitrate was taken up by the sediments and                                            enough for nutrients to be released from sediments
denitrified. Significant growth of phytoplankton                                              into the hypolimnion; in others, the stratification
was not recorded before the ratio of euphotic                                               persists long enough for anoxia to set in and for
depth to mixing depth approached unity (Fig. 7).                                            large nutrient loads to be released from sediments.
At that stage the hypolimnion was anoxic, nitrate                                           In some rivers, residence times are long enough for
had been completely consumed by the sediments                                               sedimentation of particles, leading to greater light
and anoxic conditions had led to release of phos-                                           penetration of surface water; in others, the parti-
phorus and ammonia from the sediments. The                                                  cles are fine enough to remain in suspension
algal community in surface waters consisted of                                              between flow events.
a mixture of nitrogen fixing and non-fixing                                                       It had been widely accepted that cyanobacteria
cyanobacteria. The study showed that nutrient                                               could use their buoyancy and ability to sequester
concentrations in the surface waters were not                                               phosphorus to migrate diurnally between well-lit
sufficient to support this phytoplankton growth.                                              surface waters during the day and nutrient-rich
However, nutrients were entrained from the                                                  bottom waters at night (Fogg & Walsby, 1971;
hypolimnion through diffusive flux and other                                                  Ganf & Oliver, 1982). This gave cyanobacteria a
processes such as wind-induced seiching.                                                    strong competitive advantage over other algae.
    This body of work has firmly established the                                                 In a recent analysis, Bormans et al. (1999)
importance of stratification in controlling eutro-                                           could find no evidence that vertically migrating
phication in impoundments on Australian rivers                                              cells reach sufficient depth in stratified systems to
although the effects of, and the control over,                                               enable them to take advantage of the nutrient-rich
stratification vary as a consequence of the char-                                            bottom waters. Rather, they found that physical
acteristics of the river section. In some rivers,                                           mechanisms, such as periodic deepening of the
stratification is controlled by flow; in others, wind                                         epilimnion relative to the hypolimnion, were suf-
is important as well. In some rivers, the stratifi-                                          ficient to explain replenishment of the nutrient-
cation persists long enough to allow cyanobacteria                                          depleted epilimnion to satisfy the primary
to dominate the surface waters but not long                                                 producer requirements. The most important


                                                                                        Fitzroy River


                                               400,000


                                                                                                        Zeu / Zm ~ 1
                    Concentration (cells/mL)




                                               300,000

                                                                    Zeu / Zm << 1

                                               200,000
                                                                                                                                Weirpool
                                                                                                                                remixed


                                               100,000
                                                            Last day well mixed


                                                    0
                                                   10 Jul   9 Aug     8 Sep 8 Oct         7 Nov      7 Dec      6 Jan   5 Feb      7 Mar
                                                                          1997                                          1998

                                                                                    Persistent stratification
Figure 7. Relationship between euphotic depth, mixing depth and algal production in a weirpool on the Fitzroy river in subtropical
eastern Australia. Note that significant algal production was recorded only once the ratio of euphotic depth to mixing depth reached
unity (Redrawn from Bormans et al. (2004)).
52

consequence of the overthrow of the migration                       Nutrients can be transported directly from catch-
hypothesis is that, during a stratification event,                   ment inflows to phytoplankton, although the effi-
cyanobacterial growth in surface waters will be                     ciency of this mechanism depends on a number of
limited by the nutrients that leak across the ther-                 factors, including the temperature difference
mocline. There are a number of mechanisms con-                      between the inflow and the storage. In Burrinjuck
trolling this leakage.                                              Reservoir during summer, inflows are relatively
    Deep reservoirs usually start to stratify in spring             warm and are captured by the surface layer of the
as the surface waters warm. As soon as persistent                   storage with the consequent direct supply of fresh
stratification is established, the oxygen concentra-                 nutrients into the photic zone (Lawrence et al.,
tion declines rapidly in the hypolimnion and                        2000). However, the load of nutrients delivered
enhanced release of nutrients from the sediments                    through this process was calculated to be very
commences. The released phosphorus and nitro-                       small (less than 0.5% of the annual FRP load). On
gen, especially ammonia, remain in the hypolim-                     the other hand, the inflows studied in Chaffey
nion until the storage mixes the following May or                   Dam mixed with reservoir waters in the shallow
June because of seasonal cooling of the epilimnion                  inlet zones before dropping to the level where
coupled with sufficient wind to provide the energy                    their density matched that of the water column
for mixing. This brings a large load of nutrients                   (Sherman et al., 2001). These sediment and nutri-
into the photic zone where they are available to                    ent enriched waters then spread out into the res-
phytoplankton. Thus, Jones & Poplawski (1998)                       ervoir at this level (Fig. 9). Larger sediment
found in Barron Pocket Reservoir (61,000 Ml,                        particles dropped out during this process and the
49 m deep) in southeastern Queensland that both                     band of nutrient enriched water (100 lgl)1 TP,
TKN and TP concentrations increased roughly                         mainly FRP, in the event shown) remained at
linearly in the bottom waters during the period of                  4–11 m depth for some weeks until it was gradu-
stratification until there was up to 1.6 mg l)1 TKN                  ally mixed into surface waters as the surface layer
concentration differential and 0.2 mg l)1 TP dif-                    cooled, fuelling a phytoplankton bloom many
ferential between bottom and top waters at the time                 weeks after the inflow occurred.
of overturn (Fig. 8).                                                   There was a clear relationship between hypo-
    Other mechanisms, apart from annual over-                       limnetic phosphorus concentrations and the mag-
turn, can transport nutrients into the photic zone.                 nitude of the algal response in Chaffey Dam in the


                2.0                                        Total Kjeldahl Nitrogen (TKN)                                  0.25
                                                           Total Phosphorus (TP)
                                                                                                                          0.20
                1.5
  TKN (mg L )
 -1




                                                                                                                                 1




                                                                                                                          0.15
                1.0
                                                                                                                          0.10
                0.5
                                                                                                                          0.05

                 0                                                                                                      0
                  Aug    Dec   Apr    Aug            Dec        Apr   Aug            Dec        Apr       Aug         Dec
                    1993          1994                            1995                                1996
Figure 8. Phosphorus (Total P) and Nitrogen (Total Kjeldahl N) data from Barron Pocket Reservoir in subtropical eastern Australia.
Data are differences in concentrations between surface and 20 m depth. Increases in concentrations are associated with periods of
stratification of the water column in the reservoir. (Redrawn from Jones & Poplawski (1998)).
                                                                                                                                                                                      53

                    1 February 97          21 February 97            4 March 97                                                1 March 97               25 March 97     1 April 97
             0



             5
Depth (m)




            10



            15



            20



            25
             0.30   0.32    0.34    0.28      0.30     0.32   0.28   0.30    0.32                                  0.34 0.28      0.30      0.32 0.28     0.30   0.32   0.30         0.32
                                                                                                              -1
                                                                 Conductivity (dS m )
Figure 9. Conductivity profiles in Chaffey Dam, eastern Australia before and following an inflow event in February 1997. The
horizontal line indicates the trajectory of the surface layer maximum depth. (Redrawn from Sherman et al. (2001)).


following year (Fig. 10). Maximum hypolimnetic                              gen in the hypolimnion at this point and, presum-
P concentration before the breakdown of stratifi-                            ably, providing a mechanism for nutrients to mix
cation in late autumn explained 88% of the vari-                            up into the epilimnion. Bormans et al. (2004) also
ation in algal abundance the following year.                                noted a marked diurnal tilting of the thermocline in
Similar patterns were observed in Prospect Res-                             the shallow Rockhampton Barrage during pro-
ervoir (Schladow & Hamilton, 1995) and Burrin-                              longed wind events.
juck Reservoir (Lawrence et al., 2000). In these                               Differential cooling was found to be another
deep storages, nutrients that accumulate in the                             important transport mechanism in Chaffey reser-
hypolimnion over one spring and summer largely                              voir (Sherman et al., 2001). This process occurs
determine the following seasonÕs mean algal
biomass.
    Tilting of the thermocline provides another                                                               30
vertical transport mechanism. Romero & Imberger                                                                                                   2
                                                                              Mean annual biovolume (mm L )




                                                                                                                       y = -3.21 + 35.1x r = 0.88
                                                                             -1




(1999) observed a downward deflection of the                                                                               destratifier operated
                                                                             3




thermocline in Warragamba Dam, with warmer
water from the surface being pushed downwards                                                                 20
and a deepening of the mixed layer following wind
events (Fig. 11). Subsequently the thermocline was
re-established, potentially transporting nutrients
into the photic zone. Hamilton et al. (1995) mod-
elled how plunging inflows into Prospect Reser-                                                                10
voir, downstream of Warragamba Dam,
introduced oxygen into the hypolimnion, thus
reducing the potential for nutrient and metal
release from sediments under anoxia. They also                                                                 0
observed the thermocline tilting due to a prolonged                                                                0           0.2     0.4      0.6      0.8                         1.0
                                                                                                                                                      -1
wind event, followed by a series of daily oscillations                                                                           Hypolimnetic TP (mg L )
of the thermocline after the wind eased and the                             Figure 10. Mean annual algal biovolume vs. hypolimnetic total
waterbody relaxed. These oscillations (‘internal                            Phosphorus concentration in Chaffey Dam just prior to water
                                                                            column turnover at the start of the algal growing season. Open
wavesÕ) were believed to induce a turbulent mixing                          circles are measurements in years when a destratifier was
zone when they broke on the wall of Warragamba                              operating (note the lower algal biovolume). (Redrawn from
Dam, accounting for an observed increase in oxy-                            Sherman et al. (2001)).
54




Figure 11. Wind speed and temperature profile in Warragamba Dam near Sydney, eastern Australia. Note the increase in temperature
at the 25 m depth following a moderate wind event on 7–8 May and a much more pronounced deepening of the surface mixed layer
after prolonged winds between 9 and 11 May. Here the deepening of the thermocline lasted about 2 days. This may be a mechanism for
pumping nutrients to the surface from deeper water while at the same time oxygenating deeper water. (Redrawn from Romero &
Imberger (1998)).

when water cools more quickly along the margins                     the photic zone during rapid drawdown of a res-
of the reservoir than in its middle, primarily                      ervoir when there are large water releases from the
during winter but also on a small number of                         dam. The effects on nutrient release of drying
occasions during spring and summer. The cooler,                     bottom sediments when these storages are drawn
denser water from the margins of the storage                        down have been discussed in a previous section.
flows along the bottom of the lake until it meets a                      This knowledge about physical processes in
layer of equal density and then spreads into the                    deep and shallow Australian waterbodies has now
water column at that level. Sturman et al. (1999)                   advanced to the point where detailed hydrody-
have studied this process in a shallow lake in                      namic and chemical models have been developed.
Western Australia and have developed a simple                       The Princeton Ocean Model (Blumberg & Mellor,
model of this convective process. They comment                      1987) has been modified to predict the physical
that, even for heavily sheltered waterbodies which                  behaviour of stratified river pools (Bormans &
would normally be assumed to be stratified and                       Webster, 1998) and the growth of a non-buoyant
anoxic, differential cooling effectively prevents                     diatom and a buoyant cyanobacterial species
stratification occurring and can lead to the                         under stratified conditions (Bormans & Condie,
exchange of nutrients between deeper and surface                    1998). The model has been developed specifically
waters. In fact, Sherman et al. (2001) found that                   for the conditions observed in the Maude weir
this process, which has long been recognised but                    pool and so is driven by the stratification process.
little studied, was the most important transport                    It omits both the effects of nutrient limitation on
process during winter in Chaffey Reservoir and                       algal growth and the advection of algal species
was believed to account for the occurrence of                       downstream because neither was observed to be
winter cyanobacterial blooms.                                       important in the Maude study. Also, the DYR-
     The above mechanisms operate regularly during                  ESM hydrodynamic model of water storages has
the annual cycle. Lawrence et al. (2000) and Sher-                  been extended to include water quality (Hamilton
man et al. (2001) describe other mechanisms that                    & Schladow, 1997; Schladow & Hamilton, 1997).
operate spasmodically. These include the entrain-                   It has been applied successfully to a variety of
ment of highly nutrient enriched pore waters into                   water storages (Hamilton, 1999).
                                                                                                          55

    Stratification drives estuarine processes,           of phytoplankton in the dam, with algal species
including phytoplankton production in cold tem-         composition and succession determined by a
perate Tasmania (CSIRO Huon Estuary Study               complicated interplay of flow, temperature regime,
Team, 2000). The Huon estuary is characterised          mixing and nutrients. During times of high
by a shallow, fresher layer flowing seaward over a       inflows, particularly in spring, inputs of silica from
deeper, more saline layer flowing more slowly up-        the catchment and turbulent mixing appear to
river. This is similar to the characteristic physical   favour diatoms. Later, during summer, inflows are
structure of Northern Hemisphere temperate es-          reduced and thermal stratification becomes estab-
tuaries. Nitrogen budgets suggested that inorganic      lished. During that period green algae dominate
N advected in marine bottom waters and driven           the phytoplankton. As nutrients are reduced in the
by the two layer physical circulation dominated         epilimnion during mid to late summer a number of
influx of bioavailable nitrogen to the estuary.          possible scenarios emerge, depending on bioavail-
Based on these physical conditions, the study           able nitrogen. When nitrogen is available as
postulated: strong light limitation of phyto-           nitrate, motile green algae tend to be dominant;
plankton in winter but less in spring and summer;       when ammonia dominates, non-N-fixing cyano-
light limitation of benthic algae all year round;       bacteria are favoured; and when bioavailable N is
depletion of surface nutrients by microalgae; and,      very low, the phytoplankton is dominated by
favourable conditions for dinoflagellates in summer      nitrogen fixing cyanobacteria.
and autumn. These favourable conditions included
a stable stratification regime in which they could       Food chain interactions – biomanipulation
migrate vertically to optimise their position in the    Much of our understanding of predation on algae
water for access to light and nutrients, and the        comes from attempts to manipulate food chains in
potential to maintain a position which prevented        lakes in order to reduce nuisance phytoplankton
them from being flushed out to sea. Dinoflagellates       (Shapiro et al., 1975; Shapiro & Wright, 1984;
may also be able to scavenge nutrients, particularly    Carpenter et al., 1985). ‘‘Top-down’’ control
nitrogen, from the marine bottom waters advected        through biomanipulation (Shapiro et al., 1975)
into the estuary and thus avoid being limited by        was seen as an alternative to the more traditional
nutrients delivered by land runoff.                      ‘‘bottom-up’’ approach through reductions in
                                                        nutrient inputs. In a review, McQueen (1998)
Ecological processes in freshwaters                     suggests that, although biomanipulation can ini-
                                                        tially produce quite spectacular results, longer-
Algal succession in water storages                      term investigations frequently show that these
In their study of Chaffey Dam, Sherman et al.            effects are not sustained and the resulting algal
(2001) recorded 58 taxa of phytoplankton in the         biomass is as high or higher than before the
11 years of sampling between 1987 and 1997. They        manipulation began (see also DeMelo et al., 1992).
found no clear algal successional sequence,             This implies that biomanipulation is not a one-off
although they noted patterns of occurrence. Thus,       activity and that monitoring and further inter-
many of the major taxa showed marked increases          vention will be required.
in biovolume during spring and early summer.                During the early 1990s there was a vigorous
Ceratium biomass was usually low during spring          debate in Australia about whether biomanipula-
and summer, but their biomass increased during          tion would work in local waters (Boon et al., 1994;
autumn together with a number of other major            Matveev et al., 1994b). There are now a number of
taxa (Microcystis, Anabaena and Aulacoseria).           studies from Australia that show the potential for
Cyanobacteria attained their highest biomass            zooplankton grazers to control phytoplankton. In
during mid to late summer and autumn.                   particular, it has been shown that Daphnia cari-
   Lawrence et al. (2000) have reviewed water           nata, a large cladoceran, can be present in high
quality and algal data from the period 1976–1998        densities in Australian systems (Mitchell & Williams,
from Burrinjuck Reservoir, a water storage with a       1982) and that it effectively grazes phytoplankton
capacity of 1026 Gl and a surface area of about         (e.g., Ganf & Shiel, 1985; Merrick & Ganf, 1988;
6000 ha. There was a general successional pattern       King & Shiel, 1993; Kobayashi, 1993; Kobayashi
56

et al., 1996, 1998; Matveev & Matveeva, 1997). In       The triggers of akinete formation and germination
addition, D. carinata is able to feed on cyano-         are very unclear. Paerl (1988) suggested that it was
bacteria (Kobayashi, 1993) while being resistant to     caused by nutrient levels, and Van Dock and Hart
certain cyanobacterial toxins (Matveev et al.,          (1996) showed that P-limitation triggered differ-
1994b). Kobayashi et al. (1998) found that the          entiation in a cultured strain of Anabaena circi-
weight specific clearance rate of D. carinata was        nalis. However, Baker (1999) could find no
between two and 65 times higher (0.27–3.83 l mg-        correlation between environmental variables,
dw)1 d)1) than that of the resident zooplankton         including nutrient concentrations, and akinete
community in a reservoir in Sydney. This suggests       abundance in two species of Anabaena in the lower
that D. carinata is effective at reducing phyto-         Murray River. Blackburn & Thompson (2000)
plankton where planktivorous fish are absent or          concluded that akinetes form and germinate lar-
occur in low numbers and where phytoplankton            gely in the same conditions that are suitable for
production is sufficiently high to sustain large          optimum vegetative growth and that phosphorus
D. carinata populations (see also Burns, 1998). They    limitation is not a primary cause of akinete for-
stress that, although D. carinata can efficiently graze   mation. They found, however, that different
down phytoplankton communities in lakes, it is not      strains of A. circinalis held under different labo-
clear that they are suitable for removing nuisance      ratory conditions react in different ways to P lim-
algae, the targets of such management initiatives.      itation. For example, in one strain they showed a
Burns et al. (1989) and Forsyth et al. (1992) have      correlation between akinete formation and P lim-
shown that the filamentous Anabaena reduce the           itation in batch culture, but not in a chemostat.
grazing rate of D. carinata. Boon et al. (1994)         These anomalies make it difficult to extrapolate
showed that, during a bloom of two Anabaena             laboratory data to field conditions.
species in the Darling River, only a small number of        Blackburn and Thompson (2000) also investi-
D. carinata had ingested these cyanobacteria.           gated A. circinalis dormancy requirements. They
    The first full-scale trial of the effectiveness of    found that not all strains formed akinetes and that
top-down control of phytoplankton in Australia          mixing different strains did not enhance akinete
was commenced in 1998 in two reservoirs in              formation, i.e., there was no evidence of chemical
southeastern Queensland. According to the tro-          cues between different strains. No endogenous
phic cascade model (Carpenter & Kitchell, 1993;         dormancy requirements were found and akinetes
Carpenter et al., 1985) by reducing the biomass of      germinated readily under the right environmental
planktivorous fish, pressure on zooplankton              conditions (Jameson & Blackburn, pers. comm.).
grazers is reduced and, in turn, their grazing on       However, the cell growth rate of germlings was
algae will increase. Australian bass were intro-        greater than that of vegetative cells. This may be
duced into one of the reservoirs to act as predators    important in development of Anabaena blooms.
of the planktivorous fish. Data available to date
indicate that some changes may have occurred in         Ecological processes in estuaries
the zooplankton and phytoplankton communities
of the reservoir after addition of Australian bass      Stratification and anoxia
(Macquaria novaemaculata) compared to the con-          A characteristic of many Northern Hemisphere
trol reservoir which was not stocked. However, no       estuaries is that they are stratified vertically, lead-
clear pattern has emerged and it will be some years     ing to hypoxic or anoxic bottom waters (Diaz &
before it is clear whether the introduction of the      Rosenberg, 1995; National Research Council,
bass has led to the differences in zooplankton and       2000). In that regard, estuaries in Tasmania, such
phytoplankton communities between the treat-            as the Derwent and Huon estuaries near Hobart,
ment and control reservoirs.                            are more similar to those in the Northern
                                                        Hemisphere than to those in mainland Australia.
Algal reproduction                                      Consequently, the drivers of ecological processes
Cyanobacteria can form resting stages (akinetes)        are expected to be similar in these estuaries to those
which can survive for many years in sediments and       in the northern hemisphere. A recent detailed study
germinate when conditions become favourable.            of the Huon estuary has confirmed this (CSIRO
                                                                                                           57

Huon Estuary Study Team, 2000). The study found         marked effects arise from fluctuations in the way
that the estuary was a highly stratified salt wedge      organic matter is processed and in the different
estuary with strong riverine input and, conse-          nutrient cycling pathways between open and
quently, relatively short water residence times of      closed states. A 3-year study in Wilson Inlet in
hours to days in the surface layer and days to          southwest Western Australia (Fig. 1) found that
1 week for the whole water column.                      ecological function could be described in four
    Most estuaries in mainland Australia are very       distinct phases (Fig. 12):
different from those in Tasmania, with longer
residence times and irregular river inflows as the       1.   Dry with closed entrance (mid-summer [Jan] to
major drivers of ecological functions. Although              the start of the winter rains [late May]): River-
there are a number of large estuaries formed by              ine input to the estuary is low, the entrance
drowned river valleys (e.g., the Hawkesbury near             is closed. Wind mixing generates a uniform,
Sydney, NSW), most tend to be small and shallow              vertically mixed water body. Phytoplankton
with intermittent contact with the ocean. These              production is low (<500 mg C m)2 d)1),
types of estuaries are thought to be well mixed              probably due to low nutrient levels of both N
most of the time with few examples of extensive              and P. Chlorophyll a levels do not exceed
hypoxia in bottom waters. Detailed studies in                2 lg l)1.
Wilson Inlet, southwest Western Australia               2.   Wet with closed entrance (winter [Jun–Aug]):
(Tompson & Twomey, 2000), have shown that                    Winter rains increase freshwater flows to the
periodic anoxia does occur even in shallow, see-             estuary, supplying an estimated 20–30% of the
mingly well mixed systems characteristic of most             annual input of both N and P. The water body
of the coast of Australia. This has now been                 is well mixed. Inorganic N levels in the water
recorded in a number of other Australian estuaries           increase to 2.0–3.6 lmol l)1, but chlorophyll
(NSW EPA, unpubl. data). The anoxic layer in                 levels and primary production remain low.
these systems tends to be very thin, patchy and         3.   Wet with open entrance (spring [Aug–Sept]):
ephemeral and is probably driven by saltwater                When the water level reaches 1 m above mean
incursions from the ocean that establish short               sea level the estuary entrance is artificially
periods of stratification. Biochemical oxygen de-             opened, followed by outflow of fresher estu-
mand by sediments can then rapidly draw down                 arine water and a much smaller counter flow
the oxygen in the thin, saline bottom layer, leading         of saline oceanic water into the estuary setting
to hypoxic or anoxic conditions. This has sig-               up a stratified water column. The saline water
nificant implications for sediment nutrient cycling,          forms a thin lens of dense bottom water,
and may decrease the denitrification potential of             within which oxygen is rapidly drawn down.
these systems for periods of time.                           This leads to a hypoxia-driven sediment
                                                             nutrient release of both NH3 and PO4 (Fred-
Effects of estuarine entrance dynamics                        ericks & Heggie, 2000). The annual peak in chl
Many of AustraliaÕs estuaries fall into the category         a occurs some 24 days (23 ± 2.9 – remark-
of ICOLLs, i.e., they have mouths that are closed            ably consistent from year to year) after strat-
for periods of time, occasionally for many years             ified conditions have been established.
(Roy et al., 2000). This is a feature that Australian   4.   Dry with open entrance (spring [Sept] to
estuaries share with estuaries in other Southern             summer [Jan]): Precipitation and runoff
Hemisphere countries (e.g., Koop et al., 1983).              decrease with easing of winter rains and the
Isolation from the ocean imposes significant con-             channel to the ocean shoals and eventually
straints on the estuarine fauna and may, in special          closes by accretion of marine sands. Nutrient
cases, wipe out all of the benthos when the con-             concentrations in the estuary return to low
nection is restored, as in the case of the Bot River         levels and chl a and phytoplankton produc-
estuary in southwestern South Africa (Koop et al.,           tion is low because of nutrient limitation.
1983). It also depopulates fish species that are
dependent on migration to the sea for breeding. In         This study clearly demonstrates that opening
terms of eutrophication, however, the most              the mouth of the estuary after a period of isolation
58


                                                                                                        Bottom
                                                                                                        Surface
                                                                                                        Bar Open

                             1995          1996           1997               1998                1999
                            40                                                                                       40



                            30                                                                                       30
         Salinity (ppt)




                            20                                                                                       20



                            10                                                                                       10
         Ammonium (µgL )
        -1




                           200                                                                                       200



                           100                                                                                       100



                            0                                                                                        0
                             Jan    July   Jan     July   Jan      July       Jan       July      Jan       July
                                    1995          1996            1997                 1998                1999
Figure 12. Salinity and ammonium data during the period 1995–1999 in Wilson Inlet, southwestern Western Australia. Note high
salinities in bottom waters of the estuary immediately after the mouth opened to the sea, leading to hypoxic conditions in bottom
waters (not shown), which resulted in increased fluxes of ammonium from sediments. (Redrawn from Thompson & Twomey
(2000)).


triggers a predictable set of events. Thus, the lag                Western Australia (Fig. 1). Most of the nutrients
between opening of the mouth and peak phyto-                       were taken up by epiphytes, highlighting the
plankton biomass in the estuary was found to be a                  importance of the macrophytes as substratum for
remarkably consistent 50 ± 14.8 days. Further                      these physiologically active plants. The study also
studies of similar systems will need to establish                  showed that Ruppia is able to take up nutrients
whether the pattern detected in Wilson Inlet can be                through its leaves. Thus, the macrophyte beds
generalised across a range of ICOLL systems.                       represent significant competitors for phytoplank-
                                                                   ton and healthy macrophytes decrease the likeli-
Macrophytes                                                        hood of phytoplankton blooms in these systems.
There have been relatively few studies of the effects                  Australia has the greatest diversity of seag-
of nutrients, nitrogen and phosphorus on macro-                    rasses in the world, with 30 species recorded, and
phytes in Australian estuaries, although a study by                the greatest area of temperate seagrass beds
Dudley and Walker (2000) has highlighted the                       (Larkum et al., 1989). The total seagrass area in
importance of Ruppia megacarpa in maintaining                      Australia is around 51,000 km2 (Australian State
low nutrient concentrations in Wilson Inlet in                     of the Environment Committee, 2001), although
                                                                                                       59

there have been significant declines in seagrass        toplankton growth, but a more complex range of
meadows in Australia in recent years. Thus,            responses, including uptake by benthos, vascular
Kirkman (1997) estimated that about 450 km2 of         plants, microbial processes and changes in food
seagrass had been lost in the preceding decade         web structures.
through anthropogenic stress, including nutrient           In Australia, management questions are also
enrichment (see also Abal & Dennison, 1996).           moving estuarine scientists from studying indi-
                                                       vidual phenomena towards an understanding of
Importance of catchment loads                          system function. The most common question to-
Most Australian estuaries are sinks for the            day is how estuarine ecosystems will respond to
majority of the material exported from the catch-      increased development pressures on the coastal
ment and so catchment exports profoundly influ-         zone. Since the development of an ecosystem
ence estuarine processes (Harris, 1996, 2001;          process model for Port Phillip Bay (Murray &
Scanes et al., 1997). There is a range of models       Parslow, 1999), there have been a number of at-
available for estimating catchment exports, but the    tempts to model a range of ecological processes
majority require extensive data for calibration.       involved in estuarine eutrophication in response
The paucity of data exclude the widespread use of      to changes in nutrient loading (Gippsland Coastal
these catchment models. The studies in Port Phillip    Board, 2001; Coade, 2002). Initially, these models
Bay (Harris, 1996) highlighted the importance of       have predicted the effects of nitrogen as a single
estimating catchment loads because of predictions      stressor.
that, above a certain threshold load, denitrification
potential would be drastically reduced and the Bay
would suffer serious eutrophication problems.           Knowledge gaps
Equally, Dennison & Abal (1999) state that muddy
sediments in some parts of Moreton Bay are likely      Although knowledge about eutrophication has
to be sensitive to increases of organic loadings       advanced considerably in both freshwater and
resulting in drastic reductions of denitrification      estuarine systems over the last decade in Australia
efficiency.                                              there are still some important research topics that
    Scanes et al. (1997) have collated estimates of    need to be pursued. The following section is
catchment exports for a range of estuaries in New      divided into two parts: topics where basic research
South Wales and attempted to relate these to           is required to understand processes, and topics
perceived levels of estuarine degradation. Signals     where the findings of the last decade need to be
at either end of the spectrum, i.e., near-pristine     generalised and tested before they can be turned
systems and highly degraded systems were very          into practical tools for managers. Given the recent
well correlated with the estimates of catchment        advances, the majority of the research effort
loads, but the majority of systems with interme-       should be directed towards the latter topics.
diate levels of disturbance had much less clear
relationships. In attempting to relate catchment       New knowledge generation
loads to estuarine nutrient concentrations, the
relationship became even less clear.                   Nitrogen cycle
    Cloern (2001) has pointed out that estuarine       The research reported above makes it clear that
systems appear not to react to nutrient loadings       nitrogen can be as important as phosphorus in
in the same way that freshwater lakes do               driving the development of algal blooms in fresh-
(Vollenweider, 1976). In estuaries, there is only a    waters at particular times of year. Consequently,
poor correlation between loads and phytoplank-         there has been an increase in the research into the
ton biomass. In an earlier review of a large           aquatic N cycle in freshwaters in recent years,
number of estuarine studies, Borum (1996) had          mainly concentrating on 1-D processes in the
demonstrated only a low correlation between            sediments and between the sediments and the water
annual loads of N and phytoplankton production.        column. However, there is only a limited under-
Cloern (2001) argues that coastal eutrophication       standing of N sources and transport processes at
does not exhibit a single response, such as phy-       present, as well as nitrogen loss mechanisms.
60

Ground–surface water interactions                     toxins. Topics that need investigation include:
Groundwaters can be a major source of dissolved       triggers for toxin release from cells; cheap reliable,
contaminants during times of low flow, when algal      robust field-tests for presence of toxins; and novel
blooms are prevalent. Apart from some empirical       methods for degrading toxins.
evidence, there is very little understanding of the
role that groundwaters may play in triggering and     Phytoplankton ecology
sustaining phytoplankton blooms in shallow water      While knowledge relating to physical and chemical
bodies. Thus, there is evidence that the 1991/2       controls over eutrophication has advanced greatly
Darling River bloom was driven by groundwater         in the last decade, there is still only a limited
intrusions rich in both sulphate and salts; moni-     understanding of the ecological interactions
toring data also show that there are high nitrate     involving phytoplankton and cyanobacteria. More
levels (>10 mg/l) in groundwaters adjacent to riv-    sophisticated biomanipulation methods and
ers, particularly in the southern part of the MDB     bloom predictions could eventually emerge from
(Bolger & Stevens; 1999); and the research            improvements to our understanding in this area.
reported above shows that phosphorus can              Topics that could be pursued in this area include:
potentially enter streams via sub-surface flows.       preferential feeding by zooplankton; viral, bacte-
Primary research is needed into these transport       rial and fungal infections of cyanobacteria; and
pathways and the influence of nutrients, sulphate      habitat preferences of algal predators.
and salts on the formation and maintenance of
algal blooms.                                         Akinetes and phytoplankton cysts
                                                      Although recent research has cleared up some of
Biogeochemical processes                              the confusion about the formation and germina-
Ecological processes in waterways are largely dri-    tion of akinetes in Anabaena circinalis, there is still
ven by delivery of organic matter to the water and    very little understanding of the role that these
microbially mediated transformations in situ, par-    seeds play in the life-cycle of Anabaena, or other
ticularly in sediments. In spite of the advances      cyanobacterial species. The finding that cells
described above, our understanding of microbial       grown from akinetes multiply more rapidly than
processes in both fresh and estuarine waters in       vegetatively reproduced cells illustrates how this
Australia is still rudimentary. For example, nutri-   knowledge could be important for the manage-
ent transformation processes in sediments             ment of blooms.
(including nitrification–denitrification couplings)
have been studied in only a handful of systems.       Generalising our current understanding
More information is required on how sediment
nutrient fluxes influence primary production in the     In order to develop sound eutrophication man-
water. In another example, there is good empirical    agement strategies, it is important to be able to
evidence that inorganic phosphorus fluxes from         move from detailed studies at specific locations to
sediments are mediated by microbial action, but       generalisations that are applicable to typical sys-
there is very little understanding of the processes   tems. A framework showing how this generalisa-
involved.                                             tion typically proceeds is shown in Figure 13.

Algal biotoxins                                       Linkages between catchment and in-water processes
Approximately 50% of sampled algal blooms in          Catchment nutrient loads are the principal drivers
the MDB waters are toxic (Baker & Humpage,            of ecological processes in receiving waters. The
1994). However, the major R&D programs funded         loads to the receiving waters generally increase
in the last decade did not address the issue of       with increased levels of disturbance. Although
toxins. A workshop held in 1998 (CRC Water            highly variable across Australia, most of the pro-
Quality and Treatment, 1999), concluded that          cesses that deliver nutrients to waterways are now
research into this topic was urgently needed          well known. Frequently, however, sufficient data
because of the potential human, animal health,        are not available to drive standard catchment
and environmental effects from cyanobacterial          models. In addition, many of the processes that we
                                                                                                                       61




          Figure 13. Process for generalising research results from specific studies and developing management tools.


now know are important in delivering nutrients to                 ventions. While there is a small number of site
waterways – gully erosion, macropore pathways,                    specific ecological-physical models (Harris et al.,
streambank collapse, floodplain deposition and                     1996; Bormans & Webster 1999; Hamilton &
entrainment – are not included in the models                      Turner, 2001) we need to gain more experience in
developed in the northern hemisphere. Recently,                   applying these models to diverse waterbodies
Young & Prosser (2001) have developed models of                   before sufficient confidence can be gained to apply
nutrient loading in Australian rivers (NLWRA,                     them as routine management tools.
2001) that are suited for data-poor situations and
that incorporate some of these nutrient transport                 Biomanipulation
processes. These models need to be expanded to                    Preliminary results from the biomanipulation
include dynamic processes and with finer spatial                   project in deepwater storages in SE Queensland
resolution to make them more relevant for                         suggest that the technique may work in Australian
regional managers.                                                conditions. Assuming that the technique continues
                                                                  to show promise, it should be trialled in other deep
Linkages between in-stream ecology and                            storages where experience can be built up under
hydrodynamics                                                     different operating conditions, different climate
Hydrodynamics is clearly the principal driving                    regimes and in using other native fish species.
force in many waterbodies and models of these                     Other food chain manipulations, apart from top-
physical processes are well advanced, while models                down predation, should also be trialled.
of freshwater sediment-water column interactions
are at an earlier stage of development. Both these                Modelling tools for managers
types of models need to be linked with in-stream                  There is a demand for user-friendly modelling
ecological models in order to assess the ecological               tools that can be used directly by natural resource
effects arising from various management inter-                     managers without recourse to the primary scientists.
62

A recent promising way of achieving this has been      salinity, eutrophication and pesticides, and the
to provide managers with a simple interface to a       recent National Action Plan for Salinity and Water
look-up table that has been developed from the         Quality also requires quantifiable targets. The
detailed scientific modelling of a large number of      above research shows that the setting of targets for
‘cannedÕ scenarios (Gippsland Coastal Board,           eutrophication must involve the main factors
2001; NLWRA, 2001; Coade, 2002). In this way           causing nuisance algal growths, not just phospho-
the scientific models and their outputs are vetted      rus and nitrogen targets as in the past. Thus, in the
by scientific experts, while managers do not need       MDB, targets for nuisance algae would have to be
to acquire the specialist expertise required to run    set taking light (turbidity), stratification (flow) and
these models when wanting to explore options for       nutrients into account. There is sufficient infor-
managing particular aquatic ecosystems.                mation available for this to be done in the MDB
                                                       and possibly southwest Western Australia and
Implications for management                            some coastal Queensland rivers.
                                                           Monitoring is a critical part of ensuring that
The outputs from the eutrophication research           these targets are met. The design of the monitoring
conducted over the last decade in Australia have       program, like the establishment of targets them-
provided a range of management actions that are        selves, must be based on a thorough knowledge of
likely to be effective in the Southern Hemisphere.      the underlying processes to ensure that the critical
Many of the options are not new, but the new           parameters are monitored at the appropriate
understanding will help managers make better           intervals and locations, and that the data collected
choices among the range of options.                    can be interpreted correctly.

Nutrient management
                                                       Appropriate modelling – solid foundation
Diffuse sources dominate nutrient input to most         for management
Australian rivers. Techniques to control these
sources are relatively mature in Australia, given      The site-specific insights that have been gained in
the advances in knowledge about nutrient sources       the last 10 years need to be generalised to a range
and pathways and the formation of community            of different waterbodies. The development and
groups to tackle this form of pollution.               application of computer models, based on these
    For waterbodies where the volume of annual         insights, is a step in that direction. However, these
inflows is much less than the volume of the water-      models often require input data that simply are not
body, management of catchment nutrient sources         available without prohibitive investment. Con-
may not immediately lead to a reduction in algal       struction of simple, conceptual models and deci-
production and biomass because of the dominance        sion trees (albeit based on a modern understanding
of internal nutrient sources. In these larger water-   of the processes involved) provide an alternative.
bodies, internal nutrient sources can be managed       While they do not have the predictive capability of
once the particular mechanisms that govern nutri-      quantitative computer models, they can be applied
ent release and transport from the sediments in that   relatively easily to broad classes of waterbodies
waterbody are identified. The research reported         using readily available data. The study of Moreton
above provides the basis for this identification.       Bay in southeastern Queensland illustrates
                                                       the power of this approach. Simple diagrams of
Target setting and monitoring                          the processes that control the eutrophication of the
                                                       Bay, based on an understanding of the processes
There is increasing emphasis these days on estab-      occurring (Dennison & Abal, 1999) have generated
lishing region-specific targets that reflect desirable   sustained investment in a wastewater strategy
and achievable standards for water quality within      integrated with an ongoing research and
specific regions. Thus, the MDBC has endorsed an        monitoring effort. Furthermore, these diagram-
Integrated Catchment Management Strategy that          matic models can be used widely in managing si-
includes quantifiable targets for issues such as        milar coastal ecosystems.
                                                                                                           63

   In another example, Oliver et al. (2000) have         these ecosystems, phytoplankton biomass may be
developed binary decision trees for a weirpool on        able to be controlled by controlling the light cli-
the Darling River and for a lake (Fig. 14)               mate, either through maintaining turbidity or
integrating the main factors leading to the risk of      deepening the surface layer in which buoyant cells
blooms or the likely composition of blooms.              are circulating. While it is feasible to deliberately
Neither of these trees, by themselves, allows            induce turbidity (through strategies such as
management interventions to be assessed but they         upstream releases of turbid water) precisely when
do provide a structured approach to considering          cells are starting to grow rapidly, the practicality
interventions.                                           of this technique has yet to be assessed. Increases
                                                         in turbidity will affect all primary producers and so
                                                         the techniques may have undesirable effects on
Controlling catchment sources of N and P                 macrophytes and benthic microalgae.
                                                             Deepening the surface layer can also control
Subsurface sediments have been identified as the          the light climate. This technique has been used in
predominant sources of particulate phosphorus in         non-Australian waterbodies and an initial trial is
the MDB (except in areas of intensive agriculture)       being undertaken in Adelaide of this technique
and presumably in other heavily eroded, poorly           (Burch et al., 2000). Thus far the trial has dem-
fertilised parts of the country. Gully erosion and       onstrated that the surface layer can be deepened
slumping of streambanks need priority attention in       but algal cell numbers have been too low to show
these areas. Healthy riparian vegetation may             whether this will control phytoplankton numbers.
reduce the influx of P to rivers more by stabilising      Sherman et al. (2000) have shown that this tech-
streambanks than by reducing overland transport          nique may not work in all situations. The cost
of nutrients.                                            effectiveness of the technique also needs to be
    Surface erosion processes dominate in crop           demonstrated.
areas of tropical Queensland and southwest Wes-
tern Australia. The sources of phosphorus differ in
these two areas; excessive fertiliser applications are   Flow management
most likely responsible in Queensland, whereas
low adsorptive sandy soils transmit even moderate        We now know that stratification in deep storages,
fertiliser applications to steams through surface        shallow weir pools, and in many shallow estuaries
and shallow sub-surface pathways in Western              is an important driver for eutrophication because
Australia. Management programs, focussing on             of its profound effect on mobilising internal sour-
reductions in fertiliser application, are already        ces of nutrients from the sediments. River flows
underway in these regions to limit nutrient losses       can be used to break the stratification in weir pools
to waterways.                                            (Webster et al., 2000) and so remove the advantage
    Knowledge about catchment sources, trans-            that buoyant nuisance species of algae possess
formations and transport of nitrogen is less             (Oliver & Ganf, 2000) as well as preventing anoxic
advanced than that of phosphorus. Nevertheless,          conditions becoming established in bottom waters.
it appears that the riparian zone plays an               This technique has been trialled with success in
important role in promoting nitrogen transfor-           some inland rivers of the MDB.
mation processes, including denitrification, and so           In coastal rivers, algal blooms sometimes
establishment and management of vegetated                occur upstream of the salt wedge, particularly
riparian areas is a critical part of diffuse nitrogen     during low flows (Kerr et al., 1996). Water
management.                                              releases from upstream water storages can break
                                                         the salt wedge and effectively ‘‘flush’’ the system.
Light limitation reduces algal growth                    Trials are presently being conducted in the
                                                         Hawkesbury-Nepean River near Sydney to gauge
It is now clear that primary productivity is limited     the effectiveness of this technique and to assess
by light availability for significant periods of the      whether it has adverse effects on other parts of
year in many Australian aquatic ecosystems. In           the ecosystem.
64




Figure 14. A decision support tree identifying environmental conditions leading to particular types of cyanobacterial blooms along
with indicator threshold values for the interactions. Key: Bclad biomass of cladocerans, Bphyto biomass of phytoplankton, ZEU euphotic
depth, ZMIX depth of mixing, u* shear velocity, V floating or sinking velocity of phytoplankton, tw time that the wind blows, L lake
fetch, cs current speed. (Redrawn from Oliver & Ganf (2000)).
                                                                                                          65

Managing internal loads                                     The vertical position of nutrients that are
                                                        injected into a storage can be simply estimated
Algal biomass is usually limited by nutrient            from knowledge of the temperature and salinity of
availability in the euphotic zone once there is         the inflows compared with the temperature and
sufficient light. There are many pathways for             salinity profile of the storage. While not in a
nutrients to reach the euphotic zone. In many deep      position to control these inflows, the manager can
storages, the annual autumnal mixing of bottom          anticipate their consequences. Thus, injections into
waters into the euphotic zone carries nutrients         the euphotic zone (especially surface injections) are
released from sediments during extended periods         likely to lead to rapid algal growth if algae are
of anoxia the preceding spring and summer. In           nutrient limited. If the nutrients are injected below
these cases, management can focus on preventing         the euphotic zone, then they will be sequestered
hypolimnetic anoxia or suppressing sediment             until the surface layer deepens enough to bring
release of nutrients. Aeration of bottom waters         them into the euphotic zone. Thus, they can fuel
may work in some cases, although energy costs can       blooms some months after the inflows. If the
be high. Breaking stratification through devices         inflows are dense enough, they can flow along the
such as bubble plumes can also be effective if the       bottom, re-oxygenating the bottom waters and
energy costs are not excessive. However, if turned      shutting down sediment nutrient release for a
on too late after anoxia has set in or after deep       period. With well-targeted monitoring and good
intrusions of nutrient rich fresh inflows have           scientific advice a water storage manager can be
occurred, these devices can be counterproductive        prepared for these events.
and promote algal blooms by mixing large loads of
nutrients from the hypolimnion into the euphotic        Nitrogen vs. phosphorus
zone (Sherman et al., 2001). They need to be
operated with a good understanding of the               The current consensus in Australia is that both
waterbody. Alternatively, anoxia can be left to         nitrogen and phosphorus, rather than just one
occur but the sediments can be capped with both         supposedly limiting nutrient, need to be considered
passive and chemically active barriers (Azcue           when developing management strategies to reduce
et al., 1998), including modified clays that bind        nutrient inputs to waters. This is important when
phosphorus even under strong anoxia (Douglas            considering options for managing point sources,
et al., 1999).                                          such as STP. The Moreton Bay Catchment Water
    Nutrients can be transferred throughout the         Management Strategy for southeastern Queens-
year by seiches that can set-up internal waves that     land committed to drastic reductions of nitrogen
break on shores and mix bottom waters to the            in sewage effluent after the importance of N in
surface. In some circumstances, planting wind-          eutrophication had been demonstrated (Dennison
breaks on strategic shores may reduce the extent of     & Abal, 1999).
seiching, but this is likely to be successful only if       The origins, transformations and movement of
detailed hydrodynamic modelling supports the            nitrogen through the landscape is less well known
strategy.                                               than are the sources and movement of phos-
    For water storages that are periodically heavily    phorus. There is a growing consensus (Hart &
drawn down, Sherman et al. (2001) suggest that          Grace, 2001) that nitrogen loads need to be con-
sediment nutrient releases can be inhibited by          trolled either at source or by creating favourable
using these periods to kill the bacteria in the sed-    conditions for denitrification in receiving water
iments that drive the nutrient release mechanisms.      bodies. Differences in hydrology mean that river
However, they note that this technique is not           sediments are able to denitrify a relatively small
suitable if downstream users are dependent on           proportion of the N load, whereas estuaries may
adequate water supplies. Lawrence et al. (2000)         remove more than 50% of the N load and fre-
also point out that rapid drawdown from epilim-         quently up to 70–80%. Thus maintaining or cre-
netic waters will quickly reduce the epilimnion         ating favourable conditions for denitrification in
thereby promoting blooms by bringing nutrient           estuaries will have a relatively greater effect on N
enriched waters into the euphotic zone.                 removal than it will in rivers.
66

   Although there remain concerns about the            relationship between flow and cyanobacteria
potential for promoting nitrogen fixing cyano-          abundance observed in rivers of the MDB the
bacterial species if algae become nitrogen limited,    early 1990s.
there is evidence that nitrogen fixing species, be-         It is also clear that the characteristics of the
cause of their special physiological requirements,     Australian environment do influence the occur-
do not always take advantage of nitrogen deficient      rence of algal blooms. Relatively low settlement
conditions.                                            densities mean that diffuse nutrient sources pre-
                                                       dominate over much of the country; episodic
                                                       rainfall patterns, particularly in tropical regions,
Integration with other natural resource                mean that the water in even large storages can be
management initiatives                                 replaced within days with major effects on nutrient
                                                       dynamics and possibly ecology; the impoundment
Eutrophication is only one of a range of environ-      of most inland rivers of South-East Australia for
mental issues facing natural resource managers in      irrigation means that significant sections of these
Australia. Salinisation of Australian dryland          rivers stratify during the summer creating ideal
areas, for example, is currently a high priority.      physical conditions for cyanobacteria; and the
Revegetating catchments and installing engineer-       high turbidity of many rivers and some estuaries
ing works to control dryland salinity will inevita-    from sediments makes the phytoplankton light
bly modify runoff and the delivery of sediments         limited for significant periods of the year.
and major and minor ions to rivers and estuaries.          In spite of these advances in our understand-
In another example, flow management for river           ing, there are still important knowledge gaps.
health reasons should be integrated with flow           Prime amongst these are: a detailed understand-
regulation to control algal growth in weir pools.      ing of the nitrogen cycle from sources through
Thus, management strategies developed for deal-        transformations to sinks; the factors governing
ing with other environmental issues are likely to      the succession of one algal species by another as
have significant effects on eutrophication. There        in-water conditions change; the factors governing
will need to be a high degree of coordination          nutrient release from sediments, including bacte-
between different initiatives to ensure efficient use     rial mediation of phosphorus release; the role of
of resources and to obtain the best environmental      akinetes in seeding blooms; and the reasons why
outcomes. This will pose a significant challenge to     certain strains of cyanobacteria release more
natural resource managers, the community and the       toxins than others.
political process.                                         The most important task now facing research-
                                                       ers is to generalise from the studies that have led to
                                                       our current understanding of eutrophication pro-
Conclusions                                            cesses, so that the results can be extended to most
                                                       fresh and estuarine waterbodies across Australia.
It is clear that eutrophication in Australian con-     Models are a powerful means of generalising. The
ditions is now considerably better understood than     models that have been developed so far of physical
it was in the early 1990s. Much of the speculation     and chemical processes within waterbodies need to
about the dominant processes operating in fresh-       be tested and refined in diverse environments
water bodies at that time has now been replaced by     before they can be applied across Australia. On the
a process-based understanding founded on solid         other hand, catchment nutrient sourcing and
experimental evidence. These advances have been        transport models have yet to include the current
greatest in the identification of the different sour-    knowledge about different species of nutrients, the
ces of phosphorus in different catchments across        role played by streambank erosion and floodplain
Australia and in the understanding of the physical     processes in some parts of Australia, and the role
processes operating in freshwater systems. It is       of subsurface pathways for nutrient transport.
now clear, for example, that stratification of turbid       Waterbody managers will need to have a wide
weirpools, and the consequent advantage that           array of tools at their disposal. We now possess
confers on buoyant cyanobacteria, lies behind the      sufficient understanding to make more efficient use
                                                                                                                            67

of long-standing techniques for managing water-                    enhanced by grazing on components of the microbial web.
bodies, such as destratification and biomanipula-                   Mitteilungen. Internationale Vereinigung fur Theoretische
                                                                                                                ¨
tion, and to develop and trial novel management                    und Angewandte Limnologie 24: 879–883.
                                                                Atech, 2000. Cost of algal blooms. LWRRDC Occasional Pa-
solutions. Control of stratification through flow                    per 26/99. Land and Water Resources Research and Devel-
management, control of light accession (or, more                   opment Corporation, Canberra, ACT, Australia, 42 pp.
specifically, the ratio between euphotic and mixed               Australian State of the Environment Committee, 2001. Coasts
depth), the use of special nutrient binding materi-                and Oceans, Australia State of the Environment Report 2001
als, and the deliberate drying of reservoir sedi-                  (Theme Report). CSIRO Publishing, Canberra, Australia,
                                                                   106 pp.
ments to inhibit nutrient release are all examples of           Avon River System Management Committee, 1993. Avon River
such novel techniques.                                             System Management Strategy. Waterways Commission,
    There is an increasing demand from both                        Perth, Western Australia.
communities and governments for improvements                    Azcue, J. M., A. Zeman & U. Forstner, 1998. International
in the quality of AustraliaÕs fresh and estuarine                  review of application of subaqueous capping techniques for
                                                                   remediation of contaminated sediments. Proceedings of the
water resources. The research of the last decade                   3rd International Congress for Environmental Geotechnics,
has put Australian managers in a much stronger                     Lisbon, Sept, 7–11, 1998.
position to meet this demand for eutrophication                 Baker, P. D., 1999. Role of akinetes in the development of
management. However, the existence of knowl-                       cyanobacterial populations in the lower Murray River,
edge, even when it is widely accepted within the                   Australia. Marine and Freshwater Research 50: 265–279.
                                                                Baker, P. & A. R. Humpage, 1994. Toxicity associated with
scientific community, is no guarantee that it will be               commonly occurring cyanobacteria in surface waters of the
utilised. Considerable effort is required to turn new               Murray-Darling Basin, Australia. Australian Journal of
knowledge into a new paradigm, from which                          Marine and Freshwater Research 45: 773–786.
practical changes, such as changes in management                Baldwin, D. S, 1996. The effects of exposure to air and sub-
                                                                   sequent drying on the phosphate sorption characteristics of
structures and new field programs, can emerge.
                                                                   sediments from a eutrophic reservoir. Limnology and
While there has already been good progress in                      Oceanography 41: 1725–1732.
transmitting this understanding to managers                     Baldwin, D. S., A. M. Mitchell & J. M. Olley, 2002. Pollutant-
through workshops, presentations and publica-                      sediment interactions: Sorption, reactivity and transport of
tions, there is still much more to be done to turn it              phosphorus. In Haygarth, P. & S. Jarvis (eds), Agriculture,
into practical management outcomes.                                Hydrology and Water Quality. CAB International Press,
                                                                   New York, 265–279.
                                                                Bauld, J. & N. F. Millis, 1977. Role of bacteria in nitrogen
                                                                   cycling in the Werribee zone. Environmental Studies Series
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