Sargassum, the brown floating algae that gives the Sargasso Sea its name, has apparently decided it prefers a warmer, more tropical lifestyle. Since 2011, it's been thinning out in its traditional North Atlantic home while throwing an absolute party in the tropical Atlantic, and 2026 was no exception. According to scientists at the University of South Florida (USF) College of Marine Science, the Great Atlantic Sargassum Belt hit its annual peak in June 2026, making it the second-highest Sargassum year on satellite record - just a hair behind 2025.

Regionally, the Caribbean Sea and the Gulf of America (Gulf of Mexico) both set record highs. The western and eastern Caribbean hosted 3.6 and 9 million metric tons of the stuff, respectively, while the Gulf saw 5 million metric tons - nearly double its previous record, also from 2025. "The belt is a basin-scale phenomenon that can have devastating local-scale impacts throughout the Caribbean and Gulf," said Brian Barnes, a marine scientist at USF's Optical Oceanography Laboratory. "Our tracking helps communities know the current extent of Sargassum and prepare for what's to come." Translation: It's not just a floating mat; it's a floating problem.

In moderate amounts, Sargassum is a swell ecosystem - habitat for turtles, fish, and birds, plus it photosynthesizes oxygen like a champ. But too much of it near shore? That's when it turns into a marine menace, tangling and suffocating life, smothering corals, and decomposing on beaches to release hydrogen sulfide - a gas that smells like rotten eggs and is bad for both ecosystems and tourism. So, great if you're a sea turtle in the open ocean; less great if you're a tourist trying to enjoy a piña colada.

The map above, based on data from NASA's PACE satellite, shows Sargassum density in the tropical Atlantic in June 2026, with red and orange indicating hotspots. Despite appearing as a continuous belt, it's actually a patchy collection of mats scattered across the ocean surface. Ocean currents and winds shape this belt, which stretches from West Africa to the Gulf. Chuanmin Hu, another USF optical oceanographer, noted that currents spared Florida's west coast this summer but dumped large amounts on the Florida Keys and east coast. The bulk, however, remains in the Caribbean, where inundation problems have been more severe.

The data comes from Lin Qi at NOAA, who uses PACE's Ocean Color Instrument (OCI) to generate Sargassum maps. This extends earlier work using MODIS on NASA's Terra and Aqua satellites and VIIRS on NOAA-20, all part of USF's Sargassum Watch System. Satellites detect Sargassum by its near-infrared reflectance spike compared to plain seawater, then convert that to density and biomass. The continuous MODIS record since 2000 shows an uptick starting around 2011, with seasonal dips in winter and peaks in spring/summer. The record high was July 2025, followed by a quick rise in early 2026 peaking in June, then a decline in July.

"Since the initial appearance of the Great Atlantic Sargassum Belt in 2011, the total Sargassum amount in the Atlantic Ocean has increased substantially, more than doubling every five years," Hu said. The exact cause is still under investigation, but possible culprits include ocean warming, multiple nutrient sources, and the fact that large mats attract nitrogen-fixing bacteria that fuel further growth. So, it's a self-perpetuating algae party, and everyone's invited.

OCI data now feeds into the Sargassum Watch System's near-real-time maps. A recent study led by Qi found that OCI offers advantages over predecessors, observing more ocean with greater sensitivity. Hu noted that added pixels improve monitoring of short-term fluctuations, and higher sensitivity yields better maps in winter. Plus, OCI's hyperspectral capability can discriminate Sargassum from other floating algae like Trichodesmium "without ambiguity." "I think I can speak for all project members in sharing how rewarding it is to see the promise of PACE’s advancements come to life," said Jeremy Werdell, NASA's PACE project scientist. "OCI has started a true renaissance in aquatic ecosystem monitoring from space."