Factors controlling the seasonal distribution of pelagic Sargassum

Pelagic Sargassum (S. fluitans and S. natans) is endemic to the tropical and subtropical North Atlantic, where it provides habitat for a diverse and economically important ecosystem. Here, we investigate what controls the Sargassum seasonal distribution using a coupled modelling approach that integrates output from a data-assimilating 1/12 degrees HYCOM simulation, a 1/4 degrees coupled HYCOM-biogeochemical model, and individual-based Lagrangian Sargassum growth models. Passively advected, buoyant particles with no Sargassum physiology aggregate in the central North Atlantic Subtropical Gyre at annual time scales and do not show distributions consistent with satellite observations of Sargassum. However, at shorter time scales, advection alone can explain up to 60% of the following month observed distribution during some periods of the year. Connectivity between the tropical Atlantic and Sargasso Sea is largely one-way, with the Sargasso Sea acting as a 'dead end' for Sargassum. Adding growth, mortality and a simple formulation of reproduction through fragmentation to the passive advection of Sargassum particles generates distributions that match observations with 65 to 75% accuracy across all seasons. Incorporating both ocean circulation and Sargassum physiology appears to be key in successfully reproducing the seasonal distribution of biomass. We propose a conceptual model of the Sargassum seasonal cycle that incorporates new information about a population in the tropical Atlantic. Additionally, we suggest that the Gulf of Mexico and Western Tropical Atlantic are regions whose Sargassum populations may disproportionately influence the basin-wide biomass.