[Coral-List] Sargassum Season

[Eugene Shinn]

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 
1960s.

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)
------------------------------------ -----------------------------------
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>
Tel 727 553-1158
---------------------------------- -----------------------------------