Sargassum seen where the plume of the Amazon River (in the foreground) and the Atlantic Ocean (in the background) meet / Credit: Marizilda Cruppe / Greenpeace
Scientists are still debating the real dimension of the sediment load that reaches the ocean through the Amazon River and the contribution of human activities to the nutrient load, especially nitrogen. But the LME study estimates that the discharge of nitrogen by large rivers will continue to feed the proliferation of algae along the Venezuelan to the Brazilian coasts over the coming three decades.
In 2010, researcher Emilio Mayorga, from Washington State University, published the NEWS (Nutrient Exports from Watersheds) model, a tool for measuring dissolved nitrogen in watersheds. The methodology is now being adopted in large-scale studies — such as the one that determined the high concentration of nitrogen in 2000 off the northern coast of South America.
According to Mayorga’s study published in 2016 in partnership with researchers at the University of California, most of the nitrogen found in South American waters today comes from manure and sewage, unlike areas in Asia where the origin is predominantly fertilizers. However, this same study predicts that the nitrogen load will grow by 45% by 2050, mainly due to increased fertilizer use. The Amazonia region in Brazil has the worst national indicators for basic sanitation, and it does the least to collect and properly dispose of urban and rural sewage (see other adjoining story). Streams flowing through regional cities carry garbage and sewage into the Amazon and many other rivers.
In addition to taking advantage of the “food” brought by large riverine sources, the growth of sargassum is stimulated by waters warmed by climate change. Changes in global temperature also alter marine currents, spreading algae and other plants to new regions. On the other side of the Atlantic, discharges from the Congo River and sand from the Sahara carried by winds can increase algae growth. But these seem to be less of a factor when it comes to feeding the sargassum that affects the Caribbean.
“Among the datasets on sargassum that need further investigation are the contribution to the phenomenon arising from the real deposition of dust on the seas and oceans, the availability of nutrients in surface waters, and how the plant responds to these various associated factors,” explains Chuanmin Hu, one of the researchers at the University of South Florida.
"Among the datasets on sargassum that need further investigation are the contribution to the phenomenon arising from the real deposition of dust on the seas and oceans, the availability of nutrients in surface waters, and how the plant responds to these various associated factors." Chuanmin Hu, researcher from the University of South Florida.
He is the co-author of a study looking into the marine macro-algae blooms published in Science in July, 2019. Evaluating 19 years of NASA satellite images, this research confirms that the proliferation of sargassum has been gaining strength since 2011 and that its principle nutrient sources are, in the west, large rivers such as the Amazon and the Orinoco, and, in the east, the uprising of deep, saltier waters off the African coast.