Dramatic decline of Sargassum in the north Sargasso Sea since 2015

The Sargasso Sea, at the centre of the North Atlantic subtropical gyre, draws its name from the endemic floating brown macroalgae, <i>Sargassum</i>, which provides shelter and habitat for life in the pelagic zone. In 2011, the <i>Sargassum</i> footprint expanded to include the Great Atlantic <i>Sargassum</i> Belt in the tropical Atlantic, but little is known about how <i>Sargassum</i> in the Sargasso Sea changed thereafter. Here we use satellite and in situ data to show that <i>Sargassum</i> in the north Sargasso Sea has declined dramatically since 2015. Accompanying this decline is a disruption in local <i>Sargassum</i> 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 <i>Sargassum</i> Belt-a result of advection from this latter region. We posit that the north Sargasso Sea decline is due to reduced <i>Sargassum</i> 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 <i>Sargassum</i> Belt and decline in the north Sargasso Sea may represent the beginnings of a regime shift in <i>Sargassum</i> distribution.