Drifting buoys serve as proxies for invading seaweed

Two species of the brown seaweed Sargassum can form vast, tangled, freely floating mats in the open ocean. The prevalence of such mats gave name to the Sargasso Sea, an extensive elliptical region of the subtropical North Atlantic Ocean. Starting in 2011, large quantities of Sargassum have annually (with the curious exception of 2013) inundated the coastlines of the Gulf of Mexico, the eastern shorelines of Brazil and Florida, and especially the Caribbean Sea during the spring and summer. And the recurring blooms often extend in a “great Atlantic Sargassum belt” (GASB) across the tropical Atlantic to West Africa. The figure below shows the 2015 GASB, plotted as the percentage of Sargassum coverage per pixel, based on images from NASA’s Terra and Aqua satellites.

Carrying large amounts of toxic substances and heavy metals and releasing odious hydrogen sulfide when they decay onshore, the Sargassum blooms are dangerous to coastal ecologies and economies. Understanding the blooms’ origins and spatial and temporal dependencies is an active area of research and debate.

For insights, Francisco Beron-Vera (University of Miami) and colleagues have turned to other floating objects in the Atlantic: drifting buoys from NOAA’s Global Drifter Program . Transported by near-surface currents, the buoys measure local sea-surface temperature, currents, and various other oceanic and atmospheric parameters; since the program started in 1979, 25 587 buoys have been deployed. When the researchers applied a stochastic statistical analysis of 4803 buoy trajectories in the Atlantic, they found that the probability-distribution pattern (below) of certain buoys—ones not towing a holey sock known as a drogue, and thus more affected by wind and waves—closely resembles the observed GASB.

The buoy model yielded information about the Atlantic’s dynamical geography: Just as mountains and other land features determine how water collects in river basins, an ocean’s currents and surface winds determine how buoys collect in various locations. The team’s analysis showed that the Gulf of Mexico is one such collector. It connects only weakly to the Sargasso Sea, but it attracts buoys from the mouths of the Orinoco and Amazon Rivers in South America and from an upwelling area off the northern coast of West Africa. The results lend credence to those nutrient-rich locations as remote sources for the Sargassum blooms. And the researchers identified a new transport route, in addition to the GASB, from West Africa to the Americas: a slower, southern path that runs from the Gulf of Guinea to the mouth of the Amazon.

Sargassum rafts, of course, aren’t isolated rigid particles like the buoys; rather, they are extended, elastic objects that can undergo physiological changes as they’re dragged along by the combined action of ocean currents and winds. The researchers are working to improve their model by incorporating physiological dynamics and seasonal variability. (F. J. Beron-Vera et al., AIP Adv. 12, 105107, 2022 .)