Linking Satellite Observations with Coupled Bio-physical Models of Sargassum

The holopelagic macroalgae Sargassum fluitans and Sargassum natans grow uniquely in the subtropical and tropical North Atlantic. Although their combined distribution and seasonal pattern have been described based on satellite observations, the factors underlying the distribution are not known. Recent high profile wash-up events in the Caribbean highlight the need for improved understanding of seasonal Sargassum distribution. We present a Lagrangian particle approach to modelling Sargassum using output from a 1/12-degree HYCOM model for the years 2011 and 2012. Buoyant Sargassum particles were initialized according to satellite observations and tracked forward and backward in time. Our simulations suggest advection alone can account for up to 60% of the observed Sargassum seasonal distribution at timescales shorter than 2 months. Temperature and light appear to constrain Sargassum's northern extent in the fall and winter months. Modeled Sargassum shows variations in short term dispersal between eddy permitting and eddy resolving resolutions, but for timescales of a year or more, Sargassum aggregates in the Sargasso Sea. This pattern of aggregation requires continuous re-seeding based on observations to maintain populations in the tropics and Gulf of Mexico. Analysis of particles run backwards in time suggests potential source regions for Sargassum in the Atlantic.