In almost every spring and summer since 2011, a huge algal bloom has formed in the middle of the Atlantic Ocean. Areas of brown floating seaweed, known as sargassum, extend from the west coast of Africa to the Gulf of Mexico in what is known as the “great Atlantic sargassum belt.” In March 2023, scientists found that the sargassum floating in this belt was the largest of any Martian record.
The map above shows the density of sargassum in the mid-Atlantic Ocean (including the Caribbean Sea and the Gulf of Mexico) in March 2023. The red and orange areas show where sargassum densities were highest, by percentage. pixel covered in algae. The data for the map was developed by scientists at the University of South Florida (USF) College of Marine Sciences using data from the Moderated Resolution Imaging Spectroradiometer (MODIS) instrument on board NASA’s Earth and Water satellites.
USF researchers estimate that in March the sorghum belt accumulated a total of about 13 million tons, which is the average for this time of year. “So far this year, the record abundance of sargassum is primarily found in the middle of the Western Ocean,” said Brian Barnes, a marine scientist with USF’s Optical Oceanography Laboratory. “But in other parts of the Atlantic and the Caribbean, their abundance remains high, increasing by 75 percent between 2011 and 2022.”
Dispersed in large quantities in the open ocean, sargassum contributes to the health of the ocean by providing habitat for turtles, invertebrates, fish, birds, and producing oxygen through photosynthesis. But too much of this marine algae near the seashore can make it difficult for some marine species to move and breathe. When sargassum settles on the ocean floor in large numbers, it can suffocate corals and seagrass beds. On the beach, dissolved sargassum emits hydrogen sulfide gas and smells like rotten eggs. This has the potential to create significant issues for both marine ecology and local tourism.
Since its formation in 2011, the Atlantic sardine belt appears to be growing, according to Barnes and his colleagues. A record amount of twenty thousand metric tons was observed in July, the summer of 2018, wreaking havoc on the shores of the tropical Atlantic.
Although the cause of this increase is not immediately clear, researchers have found in previous studies that nutrient inputs from fertilizers and other sources report increased proliferation. Changes in ocean circulation patterns are also an influencing factor, because sargassum grows most rapidly when sea surface temperatures are normal or cooler than average.
Sargassum density peaks in June or July were still a few months away, but there were already signs in March that the 2023 bloom in 2023 was the largest ever recorded. “Major coastal accumulation is inevitable across the Caribbean and along the east coast of Florida as the belt continues to move west,” Barnes said. But the time and place of these arrivals is difficult to predict accurately. This year’s sargassum accumulations have already reached south Florida, on the beaches of Key West (Key West in Spanish), Miami and Lauderdale.
NASA Earth Observatory images by Lauren Dauphin and Joshua Stevens, MODIS data courtesy of Brian Barnes of the University of South Florida (USF), Optical Oceanography Laboratory, and Wang, M., et al. (2019). I’m looking for Emily Cassidy.