sargasso
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A mystery of oceanic proportions: Sargasso algae now form a plant bridge between America and Africa.

It is the largest seaweed bloom ever recorded, stretching 8,850 kilometres across the Atlantic Ocean. Twenty million tonnes of Sargasso, more than the weight of 200 aircraft carriers, are invading the sea surface; and this mass has been growing steadily since 2011. This unprecedented proliferation is believed to be caused by the release of pollutants on both sides of the Atlantic, deforestation and global warming.
 
À Using NASA satellite data and field samples, U.S. researchers have observed the evolution of what has long been known as the "Great Atlantic Sargasso Belt. But what they've just discovered, and which is the subject of an post in the review Science...exceeds all their expectations. This proliferating belt is so prolific that it now forms a junction between the Gulf of Mexico, the West Indies and the mouth of the Amazon River in the west, and the west coast of Africa. According to researchers, the tipping point is in 2011. Since then, the expansion of the Sargasso Belt has been growing steadily and, according to their forecasts, will continue.
 
Source: Wang et al, Science, 2019
 
Researchers are cautious about the exact causes of this unprecedented phenomenon. However, they are establishing a set of presumptions to incriminate the increase in deforestation and fertilizer use in Brazil and throughout the Amazon from the beginning of the decade. « Evidence for nutrient enrichment is preliminary and based on limited field and other environmental data, and more research is needed to confirm this hypothesis. "says Chuanmin Hu, study director and oceanographer at the University of South Florida.
The other likely cause could be a rise in sea level off West Africa that would have lifted nutrients from the deep waters to the surface. 
Finally, scientists believe that rising ocean water temperature and increasing proportions of salinity have played a significant role in the proliferation of these algae.
 
The immediate question is whether this proliferation is a danger to the ocean. The answer is not clear-cut. The sargasso form large schools of brown algae woven into tightly woven mesh, up to one metre thick, and provide habitat for many marine species such as turtles, crabs, some fish and birds. They produce abundant oxygen, which allows marine life to thrive.
On the other hand, too great a proliferation of sargasso can restrict the circulation and respiration of certain species, especially around coastal areas. In addition, when sargassas die, they smother corals and some varieties of marine flora.
 
Sargasso washed up on a beach in Mexico in June 2019
 
The most visible disadvantage, however, is on the beaches of the Atlantic, the Caribbean, the West Indies or Africa affected by this algal bloom. When Sargassum is washed ashore when it decomposes, it produces ammonia and hydrogen sulphide. These gases not only give off an unpleasant smell of rotten eggs, but can be highly toxic, even deadly, to human health or that of land animals. The economic consequences are disastrous for the coastal cities affected by this brown tide: tourists flee and ports are paralysed, depriving some islands and coastal areas of all supplies.
The French West Indies are asking for a state of natural disaster for these invasions which, according to researchers in Florida, will only get worse.
 
 

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