2023-04-19 15:46:18
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In nearly every spring and summer since 2011, a giant kelp bloom has formed in the middle of the Atlantic Ocean. Areas of floating brown kelp, known as sargassum, have stretched 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 amount of sargassum floating in this belt was the largest of any March on 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 the densities of sargassum were highest, in terms of the percentage of the pixel covered with the 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 Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard the NASA’s Terra and Aqua satellites.
USF researchers estimate that, in March, the sargassum belt accumulated a total of about 13 million tons, which is a record amount for this time of year. “So far this year, the record abundance of sargassum is found primarily in the western mid-Atlantic,” said Brian Barnes, a marine scientist with USF’s Optical Oceanography Laboratory. “But in other parts of the Atlantic and Caribbean, their abundance remains high, at the 75th percentile of measurements made between 2011 and 2022.”
In scattered amounts in the open ocean, sargassum contributes to ocean health by providing habitat for turtles, invertebrates, fish, and birds, and by producing oxygen through photosynthesis. But an excessive amount of this marine algae near the coast can make it difficult for certain marine species to move and breathe. When sargassum sinks to the ocean floor in large numbers, it can suffocate corals and seagrass beds. On the beach, decomposed sargassum releases hydrogen sulfide gas and smells like rotten eggs. This has the potential to cause significant problems for both marine ecology and local tourism.
Since its formation in 2011, the Atlantic sargassum belt appears to be growing, according to Barnes and his colleagues. A record abundance of 20 million metric tons was observed for the month of July in the summer of 2018, wreaking havoc on the tropical Atlantic coasts.
Although the cause of this growth is not immediately clear, in previous research the researchers found that nutrient inputs from fertilizers and other sources correlate with increased proliferation. Changes in ocean circulation patterns are also an influencing factor because sargassum grows fastest when sea surface temperatures are normal or colder than average.
Sargassum density normally peaks in June or July, still within a few months, but there were already signs in March that the bloom in 2023 could be the largest ever recorded. “Major beach accumulation events are inevitable throughout the Caribbean and along the east coast of Florida as the belt continues to move west,” Barnes said. However, the exact timing and location of these arrivals are difficult to predict. Accumulations of part of this year’s sargassum have already reached South Florida, on the beaches of Key West (Key West in Spanish), Miami and Fort Lauderdale.
NASA Earth Observatory images by Lauren Dauphin and Joshua Stevens, using MODIS data courtesy of Brian Barnes of the University of South Florida (USF), Optical Oceanography Laboratory, and Wang, M., et al. (2019). Reporting by Emily Cassidy.
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