March 4, 2021

An immense sea of ​​sargassum blooms in the ocean doped by fertilizers | Science

On his trip to America, Christopher Columbus ran into the worst sea with which a navigator, the Sargasso, could meet. Located east of the coast of the current US and northeast of present-day Cuba, there floated large banks of algae and the play of currents almost stopped the waters. For centuries all sailors feared it. Now, satellites have discovered that less than 10 years ago another huge sea of ​​sargassum is emerging in the Atlantic. Although its origin is still unclear, it is likely to be favored by both natural and artificial fertilizers.

Sargassum is a genus of brown algae (Sargassum) that grow in tropical waters. Most are rooted in the bottom of the sea, but there are some species, such as S. fluitans and the S. natans, which float freely on the surface thanks to the development of bubbles filled with gases. They can grow several meters and their stems are intertwined between them forming a network. They create thus leafy marine jungles where life proliferates. However, in excess they can cause the death not only of the sailors of the old sailing ships, but of many species, especially when one of these vegetable masses reaches the beaches.

For some years, the episodes of tourists who can not reach the water because of a sargassum barrier up to a meter in height and kilometers in front have been repeated from the coasts of south Florida, to the north, to those of Isla Margarita (Venezuela) to the south, passing through the tourist beaches of Mexico. It could be thought that the algae came from the Sargasso Sea, but the different models fed with the currents of the region indicated that they must come from elsewhere.

The anomalous contribution of nutrients from the African coast and the mouth of the Amazon could be the cause

Now the study of 19 years of observations from several satellites indicates that every year millions and millions of sargassum emerge in the central Atlantic, far from the original sea. Rocked by the currents, they move between the African coasts to the south of the Canary Islands and the Gulf of Guinea and the Americans to the south of the Antilles and to the mouth of the Amazon. Its biological cycle reaches its maximum in summer to disappear at the end of the year.

The analysis of these data, published in the journal Science, shows that this new sea of ​​the sargasso began to emerge in the summer of 2011, reaching 8,850 kilometers in July of last year. The estimate of its mass is even more impressive: 20 million tons of vegetable biomass.

"Before 2011, there were already small amounts of sargassum in the tropical Atlantic," says the professor of optical oceanography at the University of South Florida (USA) and co-author of the study. Chuanmin Hu. "But they did not develop massive proliferations until that year," he adds. Indeed, when going back to 2000 (first available data), the satellites Terra Y Aqua from NASA did not detect significant agglomerations of these macroalgae.

Why since 2011? The answer, Hu says, is not simple, although it has to be looked for in a set of conditions "that were not favorable until 2011". Sargasses depend on solar radiation and, in these latitudes, it is guaranteed. The temperature factor, indicated by some studies, does not seem to be the key. "The increase in the temperature of the sea surface is a slow process, in the last 100 years it has risen by about 1.5º," Hu recalls. An average of 0.15 ° per decade should not affect these algae as much. But sargasso, like any plant organism, also need a supply of mineral nutrients, especially nitrogen and phosphorus. This is where the anomaly could be.

At the ends of the area where these macroalgae are proliferating, two large amounts of nutrients come together: to the west, the enormous amounts of sediment that carry the Amazon and other American rivers like the Orinoco. To the east, the outcrop of deep waters in the region that goes from the Canaries to the south of the Cape Verde Islands. They are not the only ones, but they are the main sources of life in the central Atlantic.

"The ocean is a natural recycling machine, water not only moves horizontally, but also vertically, surface water sinks and deep water emerges," explains the director of the Institute of Marine Sciences (CSIC), Josep Luis Pelegrí. In this vertical process, the nutrients accumulated in the depths by the mineralization of the organic matter deposited from the surface emerge and "an explosion of life takes place", adds Pelegrí. In the complicated circuit of currents in this area of ​​the ocean, nutrient-rich waters would move towards the American coasts.

At the opposite extreme, to the west, the Amazon discharges up to 200,000 cubic meters in the Atlantic per second. With the water there are tons of sediments that change the color of the sea and in the last decade the quantity and composition of this discharge are changing. Seeking the key, the authors of the study analyzed the annual average of deforestation since 2000, the patterns of fertilizer consumption in Brazil, which increased by 67% in the period 2010-2018 compared to 2000 and took samples for several years of nitrogen and phosphorus in the western margin of the central Atlantic.

Although their data are preliminary and would require further studies, everything points to the process that goes from deforestation to agriculture, through a greater drag of sediments now enriched would be altering the oceanic chemistry, doping the water with an extra of nutrients that they make the sargasso flourish. Pelegrí, who has not intervened in the study, suggests another possibility, that the warming is stratifying the water column and, once the nutrients take over, they stay there, "favoring a small number of species such as the sargasso". But neither does it rule out a combination of both processes.

The investigator of the governmental Institute of Oceanic Sciences of Canada, Jim Gower, was among the first to see a strange new signal in the data that the satellites captured when passing over this area of ​​the world. Already in 2013 he published a work on the first great emergence of the sargasso, the 2011. As the authors of the study, says that we must confirm the hypothesis, but his suspicion is that "the increase in production and runoff (to the sea) of artificial fertilizers are the cause, but we still have little data concrete ".

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