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					                Bydrological, Chemical and Biological Processes of Transformation and Transporte/ Contaminants in Aquatic
                Environments (Proceedings of the Rostov-on-Don Symposium, May 1993). IAHS Publ. no. 219,1994.




                Aquatic ecosystem stability to acidification:
                experimental modelling and buffering capacity
                calculation



                M. G. TARASOV & A. M. NIKANOROV
                Hydrochemical Institute, 198 Stachki Ave., Rostov-on-Don, Russia 344104

                Abstract A method has been chosen to assess the consequences of
                fresh-water ecosystem acidification. Various regimes of anthropogenic
                acidification and pH level recovery in microecosystems of different
                water bodies (river, lake, reservoir) have been studied. The fresh-water
                ecosystem response was found to be dependent upon the ecosystem
                specific features, types of acids added, regime of acidification and its
                intensity. The buffering capacity in relation to anthropogenic
                acidification has been determined for natural water bodies based on the
                laboratory data.


INTRODUCTION

The recent problem of surface water acidification is of special importance among
global ecological problems. Sulphur dioxide and nitrogen oxides which are the
products of fossil fuel combustion, react with atmospheric moisture to produce acid
rain.
     When introduced to water bodies, the acidic precipitation can change the pH of
water that in turn can change the water chemistry, which can be toxic to biota.
Biological effects are stimulated not only by the action of hydrogen ions but also by
a change in geochemical background, including an increasing concentration of
toxicants in water and heavy metals, aluminium in particular.
     Thus, to approach the problem one initially needs comprehensive information on
all the chemical and biological parameters. Based on field studies carried out in Russia
and abroad, pronounced changes in acidity in water bodies and soils do not occur in
all the areas receiving acid precipitation. The degree of the acid rain impact would
depend upon the geochemical composition of watersheds and bottom sediments,
especially the content of alkaline and alkaline earth elements which can neutralize
airborne acids. It also depends on the buffering capacity of water itself.
     It is quite clear that the ecological system, which is in relatively stable (unchange-
able) condition for quite a long period is capable of resisting some disturbances
(including anthropogenic ones). This ecosystem feature is usually called stability.
Despite the apparent simplicity, it is rather hard to define this term distinctly and
unambiguously. So far this term does not have a common definition. In recent
publications there are many definitions of stability; however, they are generally
controversial and reflect different issues of ecosystem stability (Armand, 1983;
Kupriyanova, 1983; Puzachenko, 1983).
 106                          M. G. Tarasov & A. M. Nikanorov

    It should be recognized that although the interest in the problem of ecosystem
stability is great, its theoretical aspects are poorly developed. In practical terms the
definition of "ecosystem stability" has to answer the question: Will the system be
within the inherent range of natural variations when it is exposed to a certain rate of
pollution regardless whether the pollution is from a permanent source or an accidental
discharge?


THE SELECTION OF THE METHOD TO ASSESS THE EFFECTS OF
ACIDIFICATION
 Studying the problem of ecosystem stability in relation to anthropogenic impact is
 important since the stability assessment is a major factor in the ecological rate setting
 development. Ecological rate setting, which is the background for ecological
 monitoring, requires a comprehensive analysis of all main structures, functions,
 relations, processes and reactions occurring within the ecosystem. The elementary unit
 for study, therefore, should not be a single or a series of reactions but the whole
 microecosystem that simulates chemico-biological processes in the natural system. The
 effort to develop a methodology for such a study was undertaken by the
 Hydrochemical Institute (Trunov & Teplyakov, 1986). A specially designed chemico-
 biological experiment utilizing microecosystems has been conducted under conditions
 as close to a real ecosystem as possible.
     An analysis of the various methods used to address the problems has been
presented in the literature. For instance, Losano (1989) examined the advantages and
limitations of different methods of impact assessment upon aquatic ecosystems. These
methods comprised 1-species biotests, microcosms, mesocosms, experimental ponds,
enclosures and natural bodies of water. Results of laboratory experiments (with
biotopes and microcosms) are not readily extrapolated to field conditions, especially
if the pollutants are influenced by the biogeochemical conditions. Field experiments
provide the best approximation in the study of the processes of transformation or
chemical stability of substances. These studies help to interpret the relation of chemical
effect and ecosystem response and to show system recovery after stress impact.
     The mesocosms studies combine the advantages of laboratory and field
experiments and provide process information not readily available from laboratory
microcosm studies. Besides, these investigations consider the individual peculiarities
of water bodies (physiographical, geochemical, landscape). This fact is very important,
since ecological criteria can not be common for all types of ecosystems or for all
physiographical conditions.
     By definition, mesocosms are microecosystems existing as isolated volumes within
a given natural ecosystem. They are exposed to the natural environment but the factors
influencing them are predetermined and controlled by the scientists.
     While modelling the changes in a natural ecosystem, one should be sure that the
processes occurring in mesocosms are similar to those in natural water bodies. The
principle of ecological similarity has been formulated by Nikanorov (1990) as:
Ecological systems are similar if the values characterizing the state of the system are
proportional to corresponding values (parameters) of another system in similar
ecological conditions, space and time irrespective of the dimensions (scale) of the
systems compared. If the above conditions are met, the regularities determined for
                         Aquatic ecosystem stability to acidification               107

mesocosms can also be applied to natural ecosystems albeit with some corrections.
    Based on experimental data (Nikanorov et al., 1987) and the literature (Brockmann
et al, 1977; Kuiper, 1984, 1985; Kuiper et al, 1983; Takahasi & Witney, 1977) it
can be stated that natural systems and mesocosms of 1.5-2.0 m3 volume are similar
over a one-month period.
    To solve some problems, microcosms can be used with the mesocosms.
Microcosms generally refer to systems created in a laboratory or to an isolated natural
ecosystem in which the regulatory factors are predetermined and controlled by the
researcher.

MESOCOSM METHODS

To set up mesocosms, two types of isolating containers are used by the scientists of
the Hydrochemical Institute (Trunov & Teplyakov, 1986). Containers of the first type
are empty, braced columns of 200-2501 made of organic glass. Braced mesocosms are
used when there is no need for long exposures and where the experimental
methodology can be readily changed.
     The second type is a "flexible" (film) container. It is made of transparent
polyethylene film fixed in the upper portion by a floating neck and at the bottom. The
base is intended to isolate bottom sediments and to fix the mesocosm. Film mesocosms
have different volumes depending on the depth of the water body. Flexible containers
are preferred to braced ones because they have better hydrodynamic properties due to
the wave-like fluctuations of their side walls.
     Water in experimental mesocosms was acidified with H2S04 or by a mixture of
sulphuric and nitric acids. One of the mesocosms (without any additions) was used as
a control. Samples for chemical and biological analysis were taken at the beginning
of the experiment: mainly twice a day in the morning and evening, and later only once
a day. Samples were taken from three horizons at 20 cm below the surface, and at the
middle, at 20 cm from the bottom, or an integral sample was taken. On the first day,
the analysis was carried out in the field, additional samples were taken and preserved
for laboratory analyses.


RESULTS AND DISCUSSION

Mesocosm experiments

Mesocosms experiments on the artificial acidification of fresh-water ecosystems
(Table 1) were conducted during the period 1985-1989.
    At the River Seversky Doniets in July 1986 and 1987 the experiment was carried
out with both mesocosms and microcosms to investigate the artificial acidification of
microecosystems. A pH-shock effect (sharp instantaneous pH decrease) of varying
intensity was identified followed by a pH increase in the water as a result of natural
biogeochemical processes. Ecosystems in the background mesocosms were not
acidified artificially. pH levels decreased to 2.45, 3.65-3.70, 5.1 in experimental
mesocosms due to the addition of diluted sulphuric acid. It can be seen from Figs 1
and 2 that a slight pH increase was registered on the third day in the control
108                               M. G. Tarasov & A. M. Nikanorov

 Table 1 Description of water bodies, mesocosms and acidification regimes.


 Water     Type of  Depth of     Volume of       Water      Description      Acidification    Acid
 body      mesocosm mesocosm mesocosms           pH         of bottom        regime
                    installation (m 3 )                     sediments

Seversky Flexible      1.6-2.0      1.5-1.8      8.0-8.5   Quartz sand       pH-shock,        H2S04
Doniets                                                    containing        gradual pH       sulphuric
River                                                      fragments         decrease,        acid
                                                           of molluscs       sharp pH
                                                           shells in         decrease and
                                                           some sites        pH maintained
                                                           covered by        on low level,
                                                           silt 3-5 cm       pH increase
                                                           thick

Aksay      Braced      1.1-1.5     0.12-0.16     8.0-8.6   Thick organic- pH-shock,           Nitric
River                                                      clay silts of  and increase        and
                                                           black colour                       sulphuric
                                                                                              acids
                                                                                              mixture
                                                                                              (1:2)

Krivoye    Flexible    1.5-1.7     1.4-1.6       8.2-8.9   Thick organic-    pH-shock,       H2S04
Lake                                                       clay silts        permanent
                                                           covered by        acid load,
                                                           algae mats        low pH main-
                                                           algal             tained due to
                                                                             introducing
                                                                             acid microdoses

Isakovo Flexible      1.6-1.7      1.5-1.6       8.7-9.1   Sand contain-     pH-shock,       H2S04
Reservoir                                                  ing fragments     permanent
                                                           of mollusc        acid load,
                                                           shells,           pH increase
                                                           covered by
                                                           silt 5-10 cm
                                                           thick

All the water bodies are located in the Azov Sea basin in the eastern Ukraine and Rostov Oblast areas of
Russia.


mesocosms. It was probably a result of periphytons that appeared on the walls of the
mesocosms and intensified photosynthetic activity in microecosystem. As a result,
dissolved C0 2 concentration decreased, while alkalinity increased.
     In the mesocosm with an initial pH level of 2.45, nearly all the biota died as a
result of the large H 2 S0 4 dosage. This was aimed at defining the influence of chemical
processes alone on pH increase. It can be seen from Fig. 1 that pH increased slightly
from 2.45 to 2.70 during the experiment. It can be easily attributed to biochemical
processes which did not take place in this mesocosm and to the influence of chemical
reactions (mainly acid neutralization due to CaC03 dissolution of mollusc shells) which
were insignificant after 6 days of exposure. The form of the curves of pH recovery
in mesocosms, acidified to pH 3.65 and 3.7, was similar in the experiments of 1986
and 1987. In the 1986 experiment, the pH level remained practically constant on the
first day; for the next two days the pH level increased from 3.9 to 5.6; for the
following three days there was slow pH growth from 5.6 to 6.15. In the 1987
                                    Aquatic ecosystem stability to acidification                                                        109

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                  Fig. 1 pH dynamics in the process of natural modelling of acidification of the
                  Seversky Doniets River, 1986.




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     •ft. Of               •15.07          «.07         (7.07       18. C 7        fS.O','     3 a 07      it.07             22.07 £
               tt.°7
                  Fig. 2 pH dynamics in the process of the field modelling of trie Seversky Doniets
                  River acidification, 1987. Mesocosms: 1 — experimental No. i ; 2 — experimental
                  No. 2; 3 - control.
110                              M. G. Tarasov & A. M. Nikanorov

 experiment, the initial pH level (3.7) remained constant for three days. This first stage
 can be considered as the period of ecosystem adaptation. During the second stage, the
 pH level rose from 3.8 to 6.4 over two days. The third stage was characterized by a
 slow constant pH increase from 6.4 to 7.4 after 4 days. The period of ecosystem
 adaptation to pH-shock in the experiments of 1986 and 1987 was found to be 1 and
 3 days, respectively. In the mesocosm, acidified to pH5.10, the first stage —
 ecosystem adaption period was lacking. pH was growing evenly during the whole
 experiment.
      To estimate the role of phytoplankton, sediments, and suspended matter in the
processes of pH increase, simultaneous experiments have been performed in
 microcosms (aquariums) of 1 litre volume. In the microcosms containers sediments,
water was acidified with H2S04 to pH 5.35. One of the aquariums was exposed to
light, another was isolated from light. Figure 3 shows that pH rose more intensively
in the first aquarium (light) where photosynthesis was taking place. Three microcosms
without bottom sediments were also studied: in the first microcosm biota was
suppressed by formalin addition (5 mg per 1 litre of water), in the second experiment,
suspended matter was filtered; the third microcosm was filled with natural water from
the Seversky Doniets River. All the aquariums were acidified by H2S04 addition: the
first aquarium was acidified to pH 5.45, the second and third to pH 4.95. The
recovery rate of pH in the microcosm with formalin was lower compared to the others.
The curve form is similar to that of the mesocosms with initial pH values 3.65 and
3.7. We also note here the stage of adaption and the second stage of relatively
intensive pH growth. The intensity of pH growth in the second and third microcosms
is similar, suggesting that suspended substances do not actually affect the rate of pH
recovery. It should be noted that the pH growth in microcosms containing bottom
sediment is more intensive.
     In autumn 1988 at the River Seversky Doniets, the mesocosm experiment was
carried out to determine the influence of ecosystem sediments upon the pH recovery
rate in the river. For this purpose flexible mesocosms were set up. The No. 1
mesocosm contained no sediments; No. 2 was acidified by daily additions of H 2 S0 4
microdoses to maintain constant pH level; No. 3 allocated for a pH decrease followed

              pHf




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                         (6.07       •17.07    •18.07   ^.07       20.07    Zi.07     IZ.07           t
              Fig. 3 pH dynamics in microcosm experiments. Microcosms without bottom
              sediments: 1 — with formalin addition; 2 — with filtered water; 3 - with nonfiltered
              water; Microcosms with bottom sediments: 4 - No. 1 (light); 5 — No. 2 (darkness).
                                    Aquatic ecosystem stability to        acidification                          111

 by pH recovery due to chemico-biological processes; No. 4 was a control mesocosm
 without any additions. The mesocosm without sediments was a polyethylene cylinder,
 filled with water and open in the upper part to provide free exchange with atmosphere.
 Mesocosms have been acidified step by step over 2 days. The total amount of
 sulphuric acid introduced into the mesocosms was as follows: No. 1 — 72 ml;
 No. 2 - 140 ml; No. 3 - 143 ml. The pH decreased to 5.40, 5.20, and 4.10,
 respectively. Although various amounts of acid were introduced into the mesocosms
 (much more into mesocosm No. 2 with bottom sediments), the pH resulting remained
 at between 5.2 and 5.4. This attests to the major role of sediments in the process of
 neutralization. Furthermore, the dosage of H 2 S0 4 into mesocosm 3 was only 3 ml
 more than into mesocosm 2, the final pH was 1.1 lower. This suggests that the
addition of 140 ml H 2 S0 4 had resulted in an exhaustion of the ecosystem buffering
capacity and that the additional 3 ml of acid had caused a pH decrease of 1.1 units
(Fig. 4).
      The rate of pH increase in a microecosystem containing sediments is considerably
higher than in a microecosystem with only isolated sediments. In mesocosm No. 3, pH
increased by 2.84 after 6 days; in mesocosm No. 1 by 1.28. To maintain the pH at
the initial level ( — 5.4) 26 ml of H 2 S0 4 was introduced into mesocosm No. 2 during
the 6 days. Although a total of 166 ml of acid was added to the mesocosm, the
buffering capacity of the ecosystem had not been exhausted during the experiment as
evidenced by a lack Of any sharp pH decline.
      In the spring of 1989 at the River Seversky Doniets, an experiment was conducted
to determine the geochemical effect of various regimes of acidification upon the river
ecosystem. The experiment was performed with flexible mesocosms: No. 7 - control,
No. 8 - instantaneous addition of acid followed by a natural pH increase (pH-shock
modelling), No. 9 — gradual pH decrease as the result of daily additions of diluted
H 2 S0 4 , No. 10 — instantaneous acid addition followed by acid microdoses dosing. A


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                      Fig. 4 pH dynamics during the "step" acidification and pH recovery of the Seversky
                      Doniets ecosystem, 1988. Mesocosm: 1 — experiment No. 1; 2 - experiment No. 2;
                      3 — experiment No. 3; 4 — experiment No. 4; 5 - background (the Seversky Doniets
                      River).
112                              M. G. Tarasov & A. M. Nikanorov

 single dose of H2S04 (80 ml) was added into mesocosm No. 8, which caused a pH
 decline from 8.00 to 6.10. The instantaneous addition of 70 ml of concentrated H 2 S0 4
 to mesocosm No. 10 resulted in a pH decrease from 8.01 to 6.04. This was followed
 by acid microdoses totalling 26 ml of concentrated H2S04 over the entire period of the
experiment. Daily doses of H2S04 to mesocosm No. 9 added up to 100 ml.
     During the experiment, pH values in river water and in the control mesocosm
 were found to be very close to each other (Fig. 5), i.e. a similarity of the natural
ecosystem and microecosystem was observed. In water of the mesocosm No. 8, a
stable pH increase from 6.10 to 7.41 was registered over a 6-day period. In mesocosm
No. 9, the initial pH at 6.10 had not been reached, although 100 ml of H 2 S0 4 (20 ml
more than into mesocosm No. 8) had been added during the 6 days. This suggests a
large, long-term buffering capacity of the ecosystem with regard to acidification. In
mesocosm No. 10, daily microdoses of H2S04 have been insufficient to maintain the
initial pH (6.04). This fact also confirms the previous suggestion. We also assume that
the acidification regime in mesocosm No. 8 (pH-shock) could be more dangerous for
the ecosystem of the Seversky Doniets River than the regime of prolonged acid
loading.
     The experiment conducted at the Aksay River was to determine the rate of pH
increase for a fresh-water ecosystem as a function of the intensity of acid loading, the
types of acids and the ecosystem characteristics. In the experiment, braced mesocosms
have been used. On 14 August 1987, the following doses of acids were introduced:
mesocosm No. 1 - 8 ml of H 2 S0 4 , pH decreased to 5.11; mesocosm No. 2 — 12 ml
H 2 S0 4 , pH decreased to 3.20; mesocosm No. 3 - 15 ml of a sulphuric and nitric acid
mixture (2:1), pH decreased to 3.07; mesocosm No. 4 — 9 ml of the same mixture,
pH decreased to 5.15. Mesocosm No. 5 was used as a control with no acid addition.
Diurnal pH variations were observed in the control mesocosm. Rainfall slightly
decreased the pH in the control mesocosm, but pH levels in the experimental




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              Fig. 5 pH dynamics in the process of the natural modelling of acidification of the
              Seversky Doniets River, 1989. 1 - background; Mesocosms: 2 - control; 3 -
              experiment No. 8; 4 — experiment No. 10; 5 — experiment No. 9.
                                  Aquatic ecosystem stability to acidification                  113

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                       Fig. 6 pH dynamics in the process of the field modelling of the Aksay River
                       acidification, 1987. Mesocosms: 1 — experiment No. 1; 2 — experiment No. 2; 3 —
                       experiment No. 3; 4 — experiment No. 4; 5 - control.


 mesocosms were not affected (Fig. 6). In the experimental mesocosms, a sharp
 increase in pH was registered during the first 20 h; from 5.11 to 6.49 in mesocosm
 No. 1, from 3.20 to 5.45 in mesocosm No. 2, and in mesocosm No. 4 - from 5.15
 to 6.36. In mesocosm No. 3, the period of rapid pH increase was 44 h in which time
 the pH changed from 3.07 to 5.30.
      It is interesting thkt in the Aksay River ecosystem the adaptation period for pH-
 shock was lacking even after acidification to low pH values, unlike the ecosystem of
 the Seversky Doniets River. This probably can be related to complex chemical
reactions effecting pH recovery within the Aksay River ecosystem. It is especially
noteworthy that the pH recovery proceeded more intensively in the microecosystems
acidified by H 2 S0 4 alone than in the microecosystems acidified by a nitric and
sulphuric acid mixture. This is apparent from a comparison of the pH curves in
mesocosm No. 2 and No. 3 acidified to low pH levels.
     From 18 August to 4 September 1989, an experiment at Lake Krivoye was
undertaken to study the influence of the lake characteristics and to reveal the role of
bottom sediments in the ecosystem pH recovery. A single H2S04 dose was added to
the test mesocosm No. 6 (pH-shock modelling) and the pH dropped to 4.10. After
single acid dosing of mesocosm No. 7, microdoses of H2S04 were introduced daily
to maintain a low pH. The purpose of the mesocosm No. 8 experiment was to evaluate
the importance of bottom sediments on the microecosystem's ability to recover pH.
This was achieved by isolating bottom sediments from the water column by using a
polyethylene film, i.e. the mesocosm was a polyethylene cylinder filled with water
with the upper part open to free contact with the atmosphere. pH in this mesocosm
dropped to 4.2 as a result of H2S04 additions.
     In both the lake water and control mesocosm, pH exhibited random daily
fluctuations (Fig. 7). In this experiment a pH increase relative to the control was not
 114                                          M. G. Tarasov & A. M. Nikanorov


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                                                                                                                         1988
         18 08 (9 0S     lO.Qi Uol   M.08. Z3.0» 24.08 iS.OS li.OS i70S       «.OS 29.0S 30.08 3108 i.09   2.03   3.09   it.j-09

                             Fig. 7 pH dynamics in the process of the natural modelling of Lake Krivoye, 1988,
                             1 — background. Mesocosms: 2 — control; 3 — experiment No. 6; 4 — experiment
                             No. 7; 5 — experiment No. 8. Figures on the curve 4 are microdoses of H 2 S 0 4 .

 observed, i.e. for 17 days the ecosystem and microecosystem were found to be
 similar. In the experiment mesocosm No.6, a rapid pH increase (1.5 units) was
 registered after several days. During the next 6 days, pH rose slowly, but daily pH
 fluctuations were not recorded during the first 3 days. The pH level stabilized in the
 course of the experiment, although its natural level had not been reached. After the
 seventh day of the experiment, daily pH fluctuations of high amplitude were
 registered; by the end of the experiment, fluctuation amplitude had decreased. It is
 interesting that the amplitude of the daily pH fluctuations in mesocosm No. 6 was
 significantly higher than the one in the control mesocosm. Biological studies had
 shown that the phytoplankton biomass had significantly increased in this period due
 to a growth of green algae. Consequently, pH fluctuations of high amplitude might
have been caused by intensive photosynthesis.
     A slight pH increase of 0.15 was registered for 2 h in mesocosm No. 8 which
contained no bottom sediments. It was probably related to the buffering capacity of the
suspended matter. Throughout the experiment, pH increased by 0.3 in this
microecosystem, while daily fluctuations were not registered. In mesocosm No. 7, the
pH dropped to 4.1 as a result of H 2 S0 4 dosing. Daily microdoses of H 2 S0 4 (1-5 ml)
were added during the rest of the experiment to maintain a pH of 4.5 (the value
similar to that in mesocosm No. 8). The total amount of acid microdoses was 41 ml.
The amount of daily microdoses varied depending on the pH recovery rate. The pH
also remained close to that of the water in the mesocosm without bottom sediments.
During the first period the daily amount of acid dose was 5 ml; later it was about
1-2 ml therefore 41 ml of acid were neutralized by bottom sediments, highlighting the
important role of this process in the natural pH recovery of Lake Krivoye ecosystem.
This phenomenon is illustrated by comparing the data on the pH variation in
mesocosm with bottom sediments (No. 6) and without them (Fig. 7).
     Based on the observations that daily random fluctuations occur in fresh-water
ecosystems, we conducted an experiment at the Isakovo Reservoir in autumn 1989 to
determine the influence of the pollutant input frequency on ecosystem adaption. The
following acidification regimes were studied: a) an instantaneous sharp pH decrease
(pH-shock); b) a constant acid loading at different frequencies of random impact in
order to estimate the frequency at which ecosystem adaption to acidification is
maximum.
                                         Aquatic ecosystem stability to acidification                                                          115

      The parameters have been measured in the water of Isakovo Reservoir
 (background) and four mesocosms. The first mesocosm was considered as a control;
 the second mesocosm simulated the regime of pH-shock (100 ml of H2S04 added, the
pH dropped from 9.08 to 5.20); a H2S04 dosage of 10 ml was added to mesocosm
No. 3 once a day; 5 ml of H 2 S0 4 was introduced into mesocosm No. 4 twice a day.
      Figure 8 shows that the pH of water in the background and control mesocosm was
 fluctuating slightly every day during the experiment. The microecosystem and mother
 system behaved similarly. While simulating the pH-shock after acid dosing, a sharp
pH increase was recorded (from 5.20 to 6.16) during the first day. This is probably
the period when chemical reactions occur. For the following 6 days, relatively
monotonous pH increases from 6.16 to 7.35 were detected. During the next two days,
a rapid pH increase from 7.35 to 8.55 was observed. The disturbance of the relatively
monotonous process of pH recovery in the natural microecosystem was probably the
result of strong winds which lasted for 3 days and waves which overflowed the
mesocosm installation. This observation is suggested by a similar pH increase in the
mesocosms with constant acid loading. For the following seven days, the pH slightly
increased from 8.8 to 8.9 and reached its natural level recorded in the reservoir
(background) and control mesocosm.
     In the mesocosms with constant acid loading strong decreases in pH were observed
during the first period. Then the process became less intensive; during 7 days the pH
decreased by only 0.1 in the mesocosm No. 3, by 0.2 - in the mesocosm No. 4, i.e.
the pH level had stabilized. It should be noted that the pH increased more rapidly in
the daytime as compared to the nighttime. This can be clearly seen from the curve of
pH temporal variations in mesocosm No. 3 (Fig. 8). In the process of the field
experiment, the pH level in mesocosms with constant acid loading decreased by 1.2
to 1.5 compared to the background and control mesocosm; however, much more acid




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4-
                                                                                                                                           -fffâff
     2S.03   U.OS' Z7,cg' Sg.cf29,09'   30.M'   i.lo   ' 2./0 ' 3.10 ' -4./0 ' f / o '   6./0 ' 7.10'   g./c '   ffjo'   io,/e'   f/./o'

                        Fig. 8 pH dynamics in the process of the field modelling at the Isakovo Reservoir,
                        1989. 1 — background. Mesocosms: 2 — control; 3 — experiment No. 2; 4 —
                        experiment No. 3; 5 — experiment No. 4.
 116                          M. G. Tarasov & A. M. Nikanorov

 was introduced into these mesocosms (No. 3 - 170 ml, No. 4 - 155 ml). This
 suggests that the ecosystem of the Isakovo Reservoir has a large great buffering
 capacity for anthropogenic acidification. It also suggests that the regime of pH-shock
 has an even more negative impact upon the ecosystem than that of the prolonged acid
 loading. Based on modelling various random effects upon the ecosystem, the frequency
 at which system adaption to such effects is maximum has not been found.

Water body stability

 The major problem in the quantification of ecosystem stability is related to the lack of
 a direct relationship between the concept of stability and the ecosystem parameters
 which can be estimated quickly. Scientific literature usually recommends controlling
 parameters which require prolonged observation of the ecosystems' functioning, for
 instance, species diversity, trophic and dimensional structures of populations, etc., and
 detailed analysis of the structure of the links within this ecosystem.
     To compare different water body stabilities, Turk (1987) used the titration curves
 of water samples taken from the Lakes Iceland and Marvin and distilled water. These
 curves helped to define which aquatic ecosystem is more resistant to anthropogenic
 acidification.
     The rationale of the approach developed by Batoyan et al. (1989) for quantifying
 stability consists in the following: the external effect upon the natural system might
 cause fluctuation of parameters as a result of the external effect of certain intensity.
In this case, the measure of ecosystem stability is its buffering capacity that
characterizes the ability of the system to attenuate its parameters fluctuations.
     Israel et al. (1985) defined the buffering capacity of fresh-water ecosystem in
relation to heavy metals as the parameters characterizing the amount of metals that do
not disturb the natural ecosystem functioning. A similar definition can be applied to
the buffering capacity of a fresh-water ecosystem in relation to acidification.
Interpretation of buffering capacity as a measure of fresh-water ecosystem stability
with regard to anthropogenic acidification is rather promising.
     Batoyan et al. (1989) have determined several types of stability based on the type
of external effect and information on the response of an aquatic ecosystem:
instantaneous differential stability, short-term integral stability, long-term consumptive
stability, and stationary integral stability.
     Based on titration curves, it is impossible to determine long-term and stationary
stabilities of ecosystems. Only short-term effects can be determined from these curves.
Such is the case when there is no exchange of substance between water and bottom
sediments.
     The buffer capacity of natural water is related mainly to free carbon dioxide and
the bicarbonate-ion content. The buffering capacity can also be influenced by humic
substances or, in some cases, by increased concentrations of readily hydrolysed salts,
carbonates and hydroxides formed as a result of C0 2 uptake in the process of
photosynthesis. Two hydrochemical parameters oxidizability and concentration of
bicarbonate-ion, are therefore most significant in the determination of aquatic
ecosystem stability in relation to acidification.
     We have carried out the H 2 S0 4 titration of filtered and unfiltered water from Don
River (c.Rostov-on-Don), Aksay River (c.Novocherkassk, Rostovskaya Oblast),
                                Aquatic ecosystem stability to acidification                         117

 Isakovo Reservoir (c.Kommunarsk, Ukraine), Belaya River (c.Ufa, Bashkortostan)
 Krivoye Lake (c.Aksay, Rostovskaya Oblast), the River Angara and the Ushakovka
 stream at the City of Irkutsk. For comparison, distilled water also was titrated. The
 buffering capacity of unfiltered water is usually higher, but the difference, however,
is not so significant. It can be explained by the buffering properties of suspended
 matter in fresh-water ecosystems, which contain a mixture of carbonates, organic and
other substances. It is very convenient to titrate filtered water under laboratory
conditions because during the transportation of samples suspended matter can
precipitate. Also, the amount of suspended matter in rivers is subject to temporal
variations. The results of field experiments with mesocosms show that the role of
suspended matter is much less significant than that of bottom sediments.
     The titration curves for the above water bodies are shown in Fig. 9. It can be seen
that distilled water does not have any buffering properties with regard to acidification.
The value of buffering capacity is determined from the ratio of the quantity of added
acid to the value of the pH decrease (x/y). The ecosystem of the Isakovo Reservoir has
the maximum buffering capacity with a numerical value of 2.16. The Angara River
in Irkutsk is characterized by the least buffering capacity (0.52). For the other water
bodies, the values of the buffering capacity are given in Table 2.
     A method of determining the buffering capacity of natural water in relation to
anthropogenic acidification has been developed in the Hydrochemical Institute. The
technique is based on the ability of the natural water matrix to bind free hydrogen ions
in a solution (Nikanorov & Lapin, 1990). This property of natural water is related to
its qualitative and quantitative composition which in turn is regulated by the total
amount of dissolved mineral and organic substances entering the water from the
watershed soil and rock from bottom sediments and from the atmosphere and by
various physical-chemical reactions and biological activity.
     The buffering capacity (SB), which is the measure of fresh-water ecosystem
stability in relation to acidification, can be determined from the equation:

          &**          = -i[ET] + _L
       [Ho+]-tH+]          S
                            B            S K
                                            B


                  Pu
                 10-




                       .       —-f-n            ,           ,    ,      ,          ,       .    p.
                       Q        1       1       5          4     S      6      )   7   8       9
                                                    O.K.
               Fig. 9 Titration curves for distilled and surface water. 1 — Angara River; 2 - Belaya
               River; 3 — Ushakovka Stream; 4 — Lake Krivoye; 5 — Don River; 6 - Aksay River;
               7 — Isakovo Reservoir; 8 — distilled water.
118                              M. G. Tarasov & A. M. Nikanorov

Table 2 Buffering capacity of surface water bodies to anthropogenic acidification.


Water body                          Buffering capacity                SB (mol/1 x 103)
                                    (Batoyanef a/., 1989)             (Nikanorov & Lapin, 1990)

Isakovo Reservoir                  2.16                               6.70
Aksay River                        1.38                               4.40
Don River                          1.20                               3.30
Lake Krivoye                       1.08                               2.90
Ushakovka Stream                   0.77                               1.70
Belaya River                       0.74                               1.60
Angara River                       0.52                               1.13



 [H + ]      -    equilibrium concentration of free hydrogen ions;
 [H 0 + ] — total content of hydrogen ions added;
K           — conventional constant of acid-base balance.
      In the above equation, \ISB is an angle coefficient of the straight line, i.e. tan a.
The buffering capacity of the water matrix with regard to acidification SB can be easily
determined.
      In Fig. 10 the experimental relationships for calculation of the buffering capacity
of water in the Belaya and Don Rivers, Lake Krivoye and the Isakovo Reservoir are
presented. Buffering capacities have been determined for water bodies in various
climatic, geochemical and physiographic zones.
      It is interesting that the ranking of water bodies based on the buffering capacity
of the water matrix determined by two independent methods is similar (Table 2). Thus,
it may be concluded that this method of measuring the ability of a water matrix to bind




                                        -loo                zoo                  wo         ÎH'li°'r
                Fig. 10 Plots of relationships for calculation of buffering capacity of natural water in
                relation to acidification. 1 — Angara River; 2 — Belaya River; 3 - Ushakovka
                Stream; 4 — Lake Krivoye; 5 — Don River; 6 — Aksay River; 7 — Isakovo
                Reservoir.
                                  Aquatic ecosystem stability to acidification                                 119

 hydrogen ions is reasonable. This approach can estimate the buffering capacity of a
 fresh-water ecosystem in relation to anthropogenic acidification. It also allows the
 preliminary ranking of water bodies according to their resistance to anthropogenic
 acidification.
      It should be noted that the advantage of the first method is the simple
 determination of the buffering capacity and the calculation of a "critical acid loading"
 for natural water, i.e. the amount of acid inflow that can cause a sharp pH decrease
 can be determined from the plot. In addition, the first method allows one to use the
buffering capacity of water and maximum acid load to predetermine a pH value, for
 example, to define the boundary conditions of the habitat for aquatic biota sensitive
to pH decrease.
      The advantage of the second method is that it can estimate the constant of the acid-
base balance, K (hydrogen ion interaction with organic and inorganic matter). The
higher the value K, the stronger the H ion binding (Nikanorov & Lapin, 1990).
      In the laboratory experiments on water sample titration, only short-term and
instantaneous buffering capacity of fresh-water ecosystems were considered. Buffering
and ion exchange abilities of bottom sediments and other components were not
considered in these experiments. Because water is in balance with suspended
substances and bottom sediments of a water body and with rocks and soils of the
watershed, buffering capacity determined by titration curves takes into account, to
some extent, all the components of a fresh-water ecosystem. This is proven by the fact
that the field experimental studies with mesocosm acidification indicate that the
ecosystem of the Isakovo Reservoir is characterized by the highest stability (buffering
capacity) to acidification amongst the water bodies studied in the field and in the
laboratory.
     In conclusion, it should be noted that the long-term field experimental studies on
artificial microecosystem acidification in mesocosms along with laboratory experiments
on water titration, are needed to determine the true (complete) buffering capacity and
"critical acid load" of fresh-water ecosystem in relation to anthropogenic acidification.


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