Plague of algae in the Atlantic Ocean – brown tide in the sea

A brown, smelly mass of decaying seaweed rotting on the beaches of Florida, the Caribbean and Mexico. The shimmering, golden-green sea plants originally sparkle to form thick mats, and their decomposition releases toxic hydrogen sulfide, which burns in the eyes and mucous membranes. For a decade, these brown algae swamps have been pouring onto the beaches with increasing frequency.

The origin of brown algae is the Sargasso Sea. An oasis of green gold of biodiversity swims in a high, nutrient-poor sea: a dense network of Sargassum algae that is often two meters long. Unlike other large brown algae, it does not establish itself on land, but floats freely in the water. The algae forest provides shelter and food for many residents and serves as a nursery for fish and sea turtles. Animal secretions and waste, desert dust that is blown into the sea and high water areas off the coast of West Africa supply the algae with nutrients.

But the unique environment is changing: satellite images show that what was once a narrow-band moss forest 8,850 km long has become the Great Atlantic Sargassum Belt. The largest algal blooms in the world extend from the coast of West Africa to the Caribbean Sea and the Gulf of Mexico. But how does that happen?

Serious consequences for the environment

A US research team has now compared a set of 488 Sargassum tissue samples from research trips from the 1980s and 2010s. In the trade journal «Nature Connections» Marine biologists describe drastic changes in important plant nutrients such as nitrogen and phosphorous as the cause of the algal plague. Therefore, the man-made Sargassum belt: the deforestation of the Amazon rainforest and the global increase in the use of synthetic fertilizers have radically changed the marine chemistry of the North Atlantic, with catastrophic consequences for the ecological structure.

An oasis of biodiversity, limited by currents and scarce nutrients, is turning into a dangerous “brown flood”: since 2011, large carpets of dead brown algae have inundated the shores of the Caribbean, Florida and Mexico, burying all life. Even large sea turtles get stuck in it and no longer reach their nesting sites on the sandy beaches due to the difficult barrier.

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The Amazon alone is responsible for 20 percent of the fresh water flow into the Atlantic, and the humus from large-scale deforestation is carried into the river by rain. Satellite images show how its water body, colored with suspended matter, extends up to 620 kilometers into the Atlantic Ocean. But the Orinoco, Mississippi, and Congo rivers also bear their burden from agriculture, livestock, and urban sewage. In Sargassum samples from 2010, the nitrogen content was 35 percent higher than the reference value. According to the authors, this is probably the main reason for the algae to spread so much.

Marine ecologist Hazel Oxenford of the University of the West Indies in Barbados suspects that the Great Sargassum Belt and the brown tide could become the “new normal.” Labpointe and other marine biologists are now calling for solutions to contain nutrient flooding in the sea and help coastal communities fight the brown tide.

Publication date: 06/13/2021, 9:50 pm

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