NASA Satellites Show Sargassum Thinning in Sargasso Sea as Tropical Atlantic Belt Hits Near-Record Levels

Sargassum, a brown floating algae long associated with the Sargasso Sea in the North Atlantic, has undergone a major range shift in recent decades, thinning dramatically there while proliferating across the tropical Atlantic, according to NASA Earth Observatory imagery and analysis released as the Image of the Day for July 29, 2026 (science.nasa.gov).

The trend, underway since about 2011 with the emergence of the Great Atlantic Sargassum Belt, continued in 2026. Satellite data showed the belt’s abundance peaking in June at levels that made 2026 the second-highest year in the continuous satellite record dating to 2000, slightly behind 2025, reported scientists at the University of South Florida (USF) College of Marine Science Optical Oceanography Laboratory (science.nasa.gov).

Regionally, the Caribbean Sea and the Gulf of Mexico (referred to in some USF materials as the Gulf of America) both reached all-time highs in June 2026. The western Caribbean held about 3.6 million metric tons and the eastern Caribbean about 9 million metric tons, while the Gulf reached 5 million metric tons—nearly double its previous record set in 2025—according to USF’s June 2026 Sargassum outlook bulletin (optics.marine.usf.edu). Severe beaching events were reported along southeast Florida and continued around Caribbean islands.

Maps derived from the Ocean Color Instrument (OCI) on NASA’s PACE satellite, launched in 2024, along with longer-term records from MODIS instruments on Terra and Aqua, illustrate high concentrations stretching nearly continuously from West Africa toward the Gulf, though the “belt” consists of scattered discrete mats rather than a solid mass (science.nasa.gov). Ocean currents and winds shape its distribution and width. USF’s Brian Barnes noted that satellite observations uniquely capture both the basin-scale phenomenon and its local impacts, aiding community preparation.

A December 2025 study in Nature Geoscience led by Yingjun Zhang (then at USF) documented the complementary decline: Sargassum biomass in the north Sargasso Sea fell drastically after 2015, from a mean of roughly 0.175 million tons (2000–2014) to 0.014 million tons (2015–2023), with supporting in-situ data showing reduced densities (nature.com). Seasonal peaks there shifted from the traditional fall-winter maximum to spring-summer, mirroring the tropical belt due to advection. The authors attributed the north Sargasso Sea drop largely to reduced supply from a historical northwestern Gulf of Mexico source region, possibly linked to rising sea surface temperatures, more frequent marine heatwaves, and temperatures exceeding the preferred range for local morphotypes (usf.edu; nature.com).

Chuanmin Hu of USF, senior author on the Nature paper and long involved in the belt’s discovery and monitoring, stated that total Sargassum in the Atlantic has more than doubled every five years since 2011, with mechanisms still under investigation but potentially involving ocean warming, nutrient inputs, and self-sustaining processes within large mats (science.nasa.gov). The study concluded that the simultaneous proliferation in the Great Atlantic Sargassum Belt and decline in the north Sargasso Sea may mark the beginnings of a regime shift in holopelagic Sargassum distribution (nature.com).

In moderate open-ocean amounts, Sargassum provides critical habitat for turtles, fish, invertebrates, and birds while producing oxygen. Excessive nearshore accumulations, however, can tangle and smother marine life, sink to smother corals and seagrasses, and release hydrogen sulfide gas upon decomposition on beaches, creating odor and health concerns that affect tourism and coastal economies (science.nasa.gov). USF’s Sargassum Watch System continues to supply near-real-time maps and monthly outlooks using PACE OCI data, which offers improved coverage and sensitivity over earlier sensors.

Biomass declined through July 2026 after the June peak, but beaching risks were expected to persist in the Caribbean and parts of Florida, per the USF bulletin.