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Saharan Dust: Transport and
Adverse Biological Effects in the
Caribbean
Karen Edwards
Marine Sciences Interdisciplinary Seminar
November 14, 2001
Outline
• Background – Global Dust
• Transport of Saharan dust
– Source
– Transport/ deposition
– Patterns/controls of transport
• Adverse biological effects
– Caribbean sea fan disease
– Red tides in the Gulf of Mexico
• Conclusions
– Climate change
– Human health
Some Relative Sizes
Human Hair = 100 microns
Sand = 63 microns and larger
Dust = 63 microns and smaller
Fungal Spores = 1 – 5 microns
Bacteria = 0.2 – 15 microns
Dust transported over long distances:
- diameter < 10 microns
- Saharan dust ~ 2.5 microns
From: “The Secret Life of Dust”
• ~ 2,350 million US tons of dust put
in atmosphere each year
• 35% (770 million US tons) from
north Africa
Perkins, 2001.
On a Global Scale, Dust
• Affects radiative properties of the atmosphere,
• Serves as a reactive surface for atmospheric gases,
• Adds minerals and nutrients to ocean,
• Affects air quality and visibility; and
• Affects human health.
Atmospheric vs Riverine Inputs to Ocean
Units = 109 moles/yr
Jickells, 1995.
Global Distribution of Deserts
TOMS Satellite Image
Saharan Dust
Major African Vegetation Zones
deMenocal, 1993.
Sources of Fine-grained Dust
• Arid regions
• Ancient sedimentary
basins
• Topographical lows
• Sediments from:
– wadis,
– lake and playa,
– alluvial fans, and
– alluvial floodplains
Expect to see highly weathered particles.
Prospero, 1999.
Size Distribution of Dust Particles
Function of many factors including:
• Physical properties of the soil matrix
• Condition of the surface, and
• Wind field above the surface.
Prospero, 1999.
How Is Dust Mobilized?
16-17 September 1994
Karyumpudi et al., 1999
The Saharan Air Layer
S N
Karyampudi, 1999.
The Saharan Air Layer
Latitude = 20.5N
Karyampudi, 1999.
The Saharan Air Layer
Longitude 22W
Karyampudi, 1999.
The Saharan Air Layer – Dust
Mobilization
• Dust plume within easterly wave troughs
• SAL well-mixed, 5-6km deep
• Rise of the SAL bottom/sinking of top
• Mixing with marine layer
• Inversion layer due to temperature differences
• Middle-level jet near southern edge
• Maximum dust concentration in ridge-region to
north of middle-level jet
Karyampudi, 1999.
Transport Across the Atlantic
Summer vs. Winter Atmospheric
Circulation Patterns
deMenocal, 1993.
Transport Across the Atlantic
Perry et al., 1997.
Saharan Dust over the Caribbean
http://eol.jsc.nasa.gov/debrief/STS065/STS065-75-47.htm
“The wind had been for twenty-
four hours previously E.N.E.,
and hence, from the position of
the ship, the dust probably
came from the coast of Africa.
The atmosphere was so hazy
that the visible horizon was
only one mile distant.”
Charles Darwin, 1845
Average Barbados Dust Concentration
http://coastal.er.usgs.gov/african_dust/barbados.html
Increase in Dust Transport
Prospero & Nees, 1986.
Saharan Dust and the NAO
1986, Low-NAO index (0.28) 1989, High-NAO index (4.73)
NAO Control of Dust Export
Summary – Saharan Dust Transport
• Topographical lows produce heavily
weathered particles
• Easterly waves in the summer pick up dust;
• Saharan Air Layer carries dust;
• ITCZ controls the latitude of transport;
• Correlated to rain fall deficits – controlled
by the NAO
Biological Effects
Biological Effects of Saharan Dust
• A significant source of primary nutrients to
the ocean.
• Experiments have shown ocean basins may
be Fe limited input of iron stimulates
primary production.
• Adverse effects:
– Aspergillosis of sea fans (gorgonian corals)
– Red tides in the Gulf of Mexico
Sea Fan Disease
• In 1983 and again in
the mid-1990s a
Caribbean-wide
epizootic pathogen
affecting the sea fans
Gorgonia ventalina
and G. flabellum was
reported.
http://earthobservatory.nasa.gov/Study/Dust/
Sea Fan Disease
Bahamas Curaçao Saba
(Dec 96) (June 96) (June 96)
Incidence 60% 32% 94%
Virulence 55% 5% 52%
Nagelkerken et al., 1997.
Sea Fan Disease
Nagelkerken et al., 1997.
Sea Fan Disease
• The pathogenic agent:
soil fungus Aspergillus
sydowii.
• Does not reproduce in
seawater.
Shinn et al., 2000., Weir et al., in press.
Microbial data of air samples collected
on St. John, USVI
• July 23 Dust
– 12 isolates: 4 plant pathogens
• July 26 Dust
– 35 isolates: 7 plant pathogens
Griffen et al., 2001.
Microbial data of air samples
collected on St. John, USVI
Griffen et al., 2001.
Microbial Data of Air Samples
• Cultivatable organisms:
– 0.23/liter cultivatable microbes in dust season.
– 0.01/liter cultivatable microbes in non-dust
season.
• Organisms from dust caused disease in
healthy sea fans.
Griffen et al, 2001, Weir et al, in press.
Red Tides: Florida Red Tide Bloom
of Gymnodinium breve
Saharan Dust and Florida Red Tides
Walsh and Steidinger propose a causal chain of events to
permit landfall of large red tides on west Florida
beaches:
1. Summer Saharan dust events
2. Sufficient rainfall
3. Dissolution of aeolian iron
4. Seed stocks of both T. erythraeum and G. breve
5. Release of DON to all dinoflagellate competitors
6. Selective grazing stress on other dinoflagellates as well
as diatoms, and
7. Onshore flows to complete landfall.
1. Dust Concentration at Miami
Lenes et al., 2001.
2. Miami Rainfall
Lenes et al., 2001.
3. Atmospheric vs Riverine Fe Input
Duce and Tindale, 1991.
3. Fe(II) vs. Fe(III)
Duce and Tindale, 1991.
3. Photoreduction of Fe(III)
• At a pH of ~2.5-5 the following reaction may
produce Fe(II):
[Fe(OH)(H2O)5]2+ + H2O hv>[Fe(H2O6)]2++(OH)aq
Duce and Tindale, 1991.
4. ECOHAB Cruise Track
Lenes et al., 2001.
4. Cruise Results
Cruise Date Iron Levels Iron Levels Trichodesmium Phosphorus Nitrogen
(Offshore) (nearshore) populations μmol/kg μmol/kg
nmol/kg nmol/kg Colonies/L
1-3 May 00 <0.1 1-2 0.1-0.2 ~0.5 DOP <0.5 NO3
~0.3 PO4
5-7 Jul 99 16.0 sta 11 20 Undetectable 12
3.1 avg
6-8 Aug 99 1.5 avg 12 Undetectable 8
7-9 Sep 99 1.1avg 5
5-7 Oct 99 <0.1 1-2 0.1-0.2
6-10 Aug 98 0.2 1-2
Large red tide of > 5 x 106 cells/L in October 1999
Lenes et al., 2001.
5. Bloom Dynamics
• July 1999 total dissolved iron = 16 nmol/kg
• Trichodesmium surface stock of 20 colonies/L.
• DON reached 15-20μM
This organic nitrogen could have supported the
red tide of >20μg chl/L of the toxic dinoflagellate,
Gymnodinium breve, found on the West Florida
Coast during October 1999.
Lenes et al., 2001.
Iron Limitation of Trichodesmium
• Molar particulate N/Fe ratios:
– 465 for Trichodesmium
– ~5,000 for nitrate-using diatoms
– ~16,000 for smaller ammonium-using dinoflagellates
• Trichodesmium : severe Fe limitation
– kFe of ~1.0 nmol Fe/kg
– background levels <0.1 nmol Fe/kg
Walsh and Steidinger, 2001.
Time Series of Tricho. and G. breve
Walsh and Steidinger, 2001.
Saharan Dust and Florida Red Tides
Walsh and Steidinger propose a causal chain of events to
permit landfall of large red tides on west Florida
beaches:
1. Summer Saharan dust events
2. Sufficient rainfall
3. Dissolution of aeolian iron
4. Seed stocks of both T. erythraeum and G. breve
5. Release of DON to all dinoflagellate competitors
6. Selective grazing stress on other dinoflagellates as well
as diatoms, and
7. Onshore flows to complete landfall.
Conclusions
Conclusions
• Transport of dust well-established
– Controlled by NAO
– Increased Desertification
• Adverse Biological Effects
– Pathogens in the dust
• Aspergillus in sea fans
• White plague bacteria
– Iron in the dust
• Red Tides in the Gulf of Mexico
• More work needs to be done
Climate Effects
• Saharan dust “cools” climate warming
estimates.
• Dust clouds - suppress rainfall making dry
conditions even drier.
www.gsfc.nasa.gov/gsfc/earth/
Effects on Human Health
• 1997 EPA PM 2.5 standard
• NIH identifies airborne dust as the primary source
of allergic stress worldwide
• 17-fold increase Barbados asthma 1973-1996
Griffen et al., 2001.
Thanks
Eugene Shinn, and Garriett
Smith for sending papers.
Marc Alperin, John Bane,
Conrad Neumann, Hans
Paerl, Cisco Werner
Alfredo Arechavaleta,
Sarah Carr, Melanie Meaux,
Melissa Southwell
References
The Loop Current
http://www.rsmas.miami.edu/~ryan/oc/atlantic/loop-current.html
Processes to Create Fine-grained Dust
• Precipitation in the highlands weathers rocks and
soils.
• Fine particles are carried downstream to the basin
and deposited in river channels and wadis.
• In the dry season, the deposits become exposed,
dry out, crack and flake.
• When wind velocity increases, the disrupted soil
surface is easily deflated, and clouds of fine-
grained dust are carried away.
Expect to see highly weathered particles.
Prospero, 1999.
Diadema 1983 Die-off
• In January 1983 there was a
mass mortality of Diadema on
the Caribbean coast of Panama;
• By September 1983 this
mortality had extended to many
other areas of the Caribbean.
• Population densities of
Diadema were reduced to
– 1.1- 5.8% of their previous
levels in Panama,
– 1% in Jamaica and
– 0.6% in Curaçao.
Lessios, 1984. http://coastal.er.usgs.gov/african_dust/diadema.html
Diadema Mass Mortality
• Initial outbreaks of
Diadema mortality
followed Caribbean
surface water
circulation.
• September 1983, die-
offs occurred at
Barbados, upstream
from any affected
areas.
Caribbean Transport
A dust-born pathogen:
– Impact Panama in
January and
– By summer affect the
entire Caribbean.
– Provide a source of the
pathogen to Barbados
and other isolated
regions.
Shinn et al., 2000 Roberts, 1997.
Theories
• Sedimentation, runoff from land, sewage,
pollution, ship groundings, temperature, etc.
• New hypothesis: Saharan dust is a
contributor to coral reef decline. (Shinn,
2000).
Barbados Mineral Dust Annual Average
and Benchmark Caribbean Events
http://coastal.er.usgs.gov/african_dust/barbados.html
The ITCZ
Australian, Gobi and Saharan Dusts
• Australia
– particle size modes 8 & 12 μm
– highly aggregated soil
– large quantities of clay pellets
• The Gobi
– Stony desert
– Main dust source in China is the
Tarim basin
• Saharan
– mode at 2-3μm
Prospero, 1999.
