Satellite data reveals largest-ever macroalgae bloom

Scientists have used satellite observations to identify the largest bloom of macroalgae in the world, the Great Atlantic Sargassum Belt – a heavy mass of brown algae stretching from West Africa to the Gulf of Mexico. According to the results, the expansion of the Great Atlantic Sargassum Belt (GASB) could signify a seaweedy new norm for the tropical Atlantic and Caribbean Sea, driven by factors including deforestation and fertilizer use. Some species of Sargassum – a genus of seaweed – live on the ocean’s surface and grow into floating, island-like masses that attract many species of fish, birds and turtles. However, the amount and spatial extent of Sargassum has increased substantially over the last decade; entire flotillas of wayward seaweed mats have increasingly begun to wash ashore, inundating Atlantic and Caribbean beaches and resulting in significant environmental and economic problems. Largely due to a lack of large-scale Sargassum data, little is known about the cause of these Sargassum expansions, particularly the role of atmospheric, oceanic and/or climatic conditions in driving them. For the first time, Mengqiu Wang and colleagues address these questions using a 19-year record of measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite to study the GASB, which experienced a major bloom every year from 2011 to 2018, except for 2013. According to Wang et al. – who also analyzed fertilizer consumption patterns in Brazil, Amazon deforestation rates, Amazon River discharge, and more – the GASB annual bloom events since 2011 show connections to two key nutrient inputs: one human-derived, and one natural. In the spring and summer, Amazon River discharge adds nutrients to the ocean, which may have increased in recent years due to increased deforestation and fertilizer use. In the winter, upwelling off the West African coast delivers nutrients from deep waters to the ocean surface. As of June 2018, the GASB, impacted by these and other factors, extended 8,850 kilometers and harbored over 20 million tons of Sargassum biomass. The authors suggest that the results indicate a possibly permanent regime shift in Sargassum blooms. In a related Perspective, James Gower and Stephanie King highlight the way satellite sensors are especially well-suited to monitor Sargassum blooms.

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