A Lagrangian framework to simulate Sargassum transport and growth

Abstract. Blooms of the seaweed Sargassum have been reported in the Tropical Atlantic Ocean and Caribbean Sea since 2011. Large-scale inundation events of this seaweed in coastal regions have a negative impact on both the economy and the environment. To predict the timing, location and quantity of Sargassum strandings, model frameworks that link open ocean distributions to coastal regions are required. Here, we develop an open-source customizable Lagrangian simulation framework and growth model for Sargassum that combines transport by currents, wind and waves with a biological modelling framework that includes dynamic growth limitation depending on temperature, nitrate availability and salinity. The framework can use satellite detections of the Sargassum Watch System (SaWS) to initialise virtual Sargassum particles in the Tropical Atlantic Ocean. We demonstrate that the combination of physical transport and biological growth strongly affects Sargassum distribution, with substantial variability on spatio-temporal scales. We show that temperature is the strongest growth-limiting parameter, and in particular that elevated surface temperatures, together with low salinity from the Amazon River plume, play a crucial role in Sargassum decline.