Abstract
The Sargasso Sea, at the centre of the North Atlantic subtropical gyre, draws its name from the endemic floating brown macroalgae, Sargassum, which provides shelter and habitat for life in the pelagic zone. In 2011, the Sargassum footprint expanded to include the Great Atlantic Sargassum Belt in the tropical Atlantic, but little is known about how Sargassum in the Sargasso Sea changed thereafter. Here we use satellite and in situ data to show that Sargassum in the north Sargasso Sea has declined dramatically since 2015. Accompanying this decline is a disruption in local Sargassum seasonal growth cycles, whereby the previously consistent fall-to-winter north Sargasso Sea biomass maxima have shifted to spring-to-summer peaks that mirror those of the Great Atlantic Sargassum Belt-a result of advection from this latter region. We posit that the north Sargasso Sea decline is due to reduced Sargassum supply from a historical Gulf of Mexico source region, possibly attributable to increasing sea surface temperatures and more frequent marine heatwaves in the Gulf of Mexico. Together, proliferation in the Great Atlantic Sargassum Belt and decline in the north Sargasso Sea may represent the beginnings of a regime shift in Sargassum distribution.