25-Year Record of In Situ Pelagic Sargassum Observations in the Western North Atlantic and Caribbean Reveals a Shift in Distribution and Range Expansion of Three Common Morphotypes

Holopelagic Sargassum drifts with surface currents and winds and functions as a keystone habitat supporting high biomass and biodiversity compared to surrounding waters. This unparalleled 25-year record of 7500+ Sargassum species-level field observations reveals that the broader North Atlantic has long sustained three distinct source regions, each with a predominant genetically distinct Sargassum morphotype: the Sargasso Sea for S. natans I; the Gulf of Mexico for S. fluitans III; and the Tropical North Atlantic for S. natans VIII. Sargassum Source Regions are linked by ocean circulation and non-static in environmental conditions and boundaries, recent changes in which have driven range expansion and unprecedented overlap of common Sargassum morphotypes. Historic observations noted S. natans I dominant across the western North Atlantic, followed by S. fluitans III; both were most abundant in the Sargasso Sea, abruptly decreased in the north/west Caribbean, and were entirely absent from the south/east Caribbean. S. natans VIII was historically rare. These patterns also held for the early decades of our dataset; however since 2011, Sargassum has appeared in unprecedented quantities in unexpected locations, with severe economic and ecological impacts. A 200-fold increase in pelagic Sargassum in the tropical Atlantic and eastern Caribbean has been linked to region-wide blooms in S. natans VIII. Furthermore, while S. natans I is now frequently observed in the Caribbean and Gulf Stream, the overall balance has shifted in favor of S. fluitans III, which is presently most abundant and widely distributed. Such dramatic changes in the total abundance and geographic distribution of Sargassum are harbingers of significant oceanographic and ecological shifts throughout the central and north Atlantic regions.