Largest macroalgal bloom

Found across the Gulf of Mexico, the Caribbean Sea and the tropical Atlantic, blooms of Sargassum brown seaweed have been rising in recent years, as documented by the satellite-based Sargassum Watch System (SaWS), which is overseen by the Optical Oceanography Laboratory at the University of South Florida, USA.

In June 2022, the Great Atlantic Sargassum Belt (GASB) reached a record peak, with a monthly mean coverage of 6,989 square kilometres (2,699 square miles). This equates to an estimated wet biomass of more than 23.3 million tonnes (25.7 million US tons) of seaweed and the bloom stretched more than 8,000 kilometres (5,000 miles), from the Gulf of Mexico to the west coast of Africa.

This topped the previous record set in June 2018 of 6,317 km2 (2,439 sq mi), equating to an estimated wet biomass of more than 20 million tonnes (22 million US tons). For context, between 2000 and 2010, the monthly mean coverage of Sargassum in this region for all calendar months was 118 km2 (45.5 sq mi), or 0.2-million-tonne (0.22-million-US-ton wet biomass). Possible explanations put forward for the explosion in algae growth including warming ocean waters (as a result of climate change), as well as rising levels of sewage and nitrogen-rich fertilizer being disposed of in the sea.

Prior to 2011, the largest-known macroalgal bloom was of Ulva prolifera in the Yellow Sea, China, which reached a maximum coverage of around 3,489 km2 (1,347 sq mi) in 2008 and 4,994 km2 (1,928 sq mi) in 2009.