[Coral-List] Sargassum Season and Dust

Pawlik, Joseph pawlikj at uncw.edu
Wed Jul 17 00:54:42 UTC 2019

Thanks for the brief review, Gene,

The Sahara dust hypothesis is an intriguing one, both in terms of iron enrichment resulting in widespread Caribbean nutrification, and as a potential source of pathogens. My colleagues and I have included dust in some recent synthesis papers related to Caribbean reef ecosystem function (see below).

But as I communicated to Gene a few months back, I'm having doubts about dust.

Recent trips to the Red Sea reveal highly oligotrophic reef systems that are dumped on regularly by desert dust. Check out this video of the reefs off Egypt:


In the water column above Red Sea reefs, there was visible trichodesmium, near daily accumulations of dust on the boat deck, but on the reefs, no seaweeds, sponges, or coral disease.

What is different is that there are no river inputs to the Red Sea, and a LOT of freshwater coming into the Caribbean from the Amazon (driven N by surface currents), Orinoco, Magdelena, and Mississippi (see the vicious circle hypothesis paper, below). We've also found that the sponges on the Saudi side of the Red Sea are starving as you move offshore along a gradient of decreasing DOC, something we don't see in the Caribbean, where levels of labile DOC seem to be higher, probably because of all the seaweed.

In short, I think river inputs (and DOC) are much more important than dust in explaining the difference between Caribbean reefs and those in other parts of the tropics.

Any thoughts from the oceanographers at KAUST?



Pawlik, J.R., Burkepile, D.E., Vega Thurber, R. 2016. A vicious circle? Altered carbon and nutrient cycling may explain the low resilience of Caribbean coral reefs. BioScience, 66: 470-476 doi:10.1093/biosci/biw047.

Pawlik, J.R., McMurray, S.E. 2020. The emerging ecological and biogeochemical importance of sponges on coral reefs. Annual Review of Marine Science, 12: 3.1-3.23

Wooster, M.K., McMurray, S.E., Pawlik, J.R., Moran, X.A., Berumen, M.L. 2019. Feeding and respiration by giant barrel sponges across a gradient of food abundance in the Red Sea. Limnology and Oceanography, 64:1790-1801


Joseph R. Pawlik

Frank Hawkins Kenan Distinguished Professor of Marine Biology

Dept. of Biology and Marine Biology

UNCW Center for Marine Science

5600 Marvin K Moss Lane

Wilmington, NC  28409

Office:(910)962-2377; Cell:(910)232-3579

Website: http://people.uncw.edu/pawlikj/index.html

PDFs: http://people.uncw.edu/pawlikj/pubs2.html

Video Channel: https://www.youtube.com/user/skndiver011


From: Coral-List <coral-list-bounces at coral.aoml.noaa.gov> on behalf of Eugene Shinn via Coral-List <coral-list at coral.aoml.noaa.gov>
Sent: Monday, July 15, 2019 5:47 PM
To: coral-list at coral.aoml.noaa.gov
Subject: [Coral-List] Sargassum Season

Melissa is correct. "There has been a lot of Sahara Dust over the same
area where the Sargassum has proliferated.”
Many coral-list readers will remember that I have blamed many events,
including the demise of Caribbean corals, on microbes and toxic minerals
in African dust. As shown by Joe Prospero at the University of Miami,
what is often called Saharan dust is in fact soil dust originating in a
vast area south the Sahara called the Sahel. I spent more than10 years
devoted to the study of African dust and for several years led a USGS
project devoted to its study. Our project included two microbiologists,
a coral biologist who lived in the Virgin Islands and a geochemist. The
study began with funding from NASA to hire our first microbiologist.
Before our congressional funded project ended in 2006 over 200 live
microbes had been identified. Viruses, including one that causes foot
and mouth disease in cattle are many times more abundant. They remain to
be studied. The research was initially stimulated by the demise of
Caribbean corals, including the Florida reef tract. Disease of Acroporid
corals peaked in 1983 the same year the sea urchin /Diadema/ began dying
throughout the Caribbean. What led us to African dust were several peer
reviewed scientific papers demonstrating that the Amazon rainforest
receives most of its essential nutrients form African dust. Tree limbs
high above flood level sprout roots to take advantage of the red
nutrient rich soil that often coats the limbs. Air plants also thrive on
the dust. We soon learned that red soils and hard surface crusts on most
Caribbean islands and the Florida Keys are composed of clay minerals and
fine-grained quartz silt. The red brown color is due to oxidized iron
common to the millions of tons of African dust that reach this side of
the Atlantic Ocean each year. There are no local sources of these
minerals on Caribbean island. Most Caribbean islands surrounded by deep
oceanic waters consist entirely of limestone constructed by corals
including sands and mud precipitated from seawater. The Bahaman banks
and islands cap 15,000 feet of limestone. I will refrain from boring
readers with any more geology.

What is most striking are satellite images indicating the path of this
dust during summer months (July-November). The pattern is identical to
the present distribution of Sargassum mats now extending from West
Africa to the Caribbean and that periodically enters the Gulf of Mexico
and then moves northward to cities in the northern Gulf including
Houston and Dallas. Often these clouds of dust turn eastward and head
back into the Atlantic where they circle the Bermuda High and settle
over the Sargasso Sea. Creation of Red tides in the Gulf of Mexico have
also been attributed to iron fertilization from African dust. (During
winter months the dust clouds take a southerly rout into the Amazon
basin) This year we had our first influx of dust in the Gulf of Mexico
in late June. Meanwhile lake Chad in the Sahel which was about100 miles
in diameter in 1960, has shrunk to less than 10 miles. Its huge drying
lakebed is increasingly blowing across the Atlantic while even more is
arriving from the Bodel depression. Dr. Joe Prospero has been publishing
and monitoring the origin and abundance of these dust clouds since the

Like Melissa and many others who have experienced the dust clouds in St.
Croix, know exactly what she is talking about. Ask any resident or
medical doctor on Caribbean islands and they will tell you that patients
arrive with respiratory problems when the red/hazy clouds of dust
arrive. This is especially true for Barbados and Trinidad. Every
sailboat owner in the Virgin Islands is familiar with the red dust they
wash from sails and decks. Many are familiar with the red mud that
accumulates in their water cisterns. I have collected the red mud from
the bottom of cisterns on several islands in the Caribbean. With this
background I like Melissa, was greatly surprised to see a paper in
Nature attributing the Atlantic Sargassum bloom to run-off from the
Amazon and upwelling of nutrient rich Atlantic waters. There was no
mention of fertilization from atmospheric dust as a possible source.
Possibly it is just another chicken or egg question. Gene


No Rocks, No Water, No Ecosystem (EAS)
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E. A. Shinn, Courtesy Professor
University of South Florida
College of Marine Science Room 221A
140 Seventh Avenue South
St. Petersburg, FL 33701
<eugeneshinn at mail.usf.edu>
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