Improving transport predictions of pelagic Sargassum

A mechanistic understanding of organismal movement provides context for existing biogeographic and ecological patterns and improves our ability to predict changing patterns in dynamic environments. Here, we examined the movement ecology of pelagic Sargassum in the northern Caribbean Sea, where major inundation events of this brown algae have become increasingly problematic to coastal communities. We used GPS-trackers affixed to Sargassum mats, standard oceanographic drifters, and "pseudo-Sargassum" drifters to acquire empirical data on the movement of Sargassum and ocean currents. We related these observed movements to ocean surface velocity fields from a global ocean circulation model and wind velocities derived from models and satellite observations. Specifically, we assessed whether adding a windage component to ocean surface velocities improved predictions generated by synthetic particles tracked within an ocean circulation model. We found that the inclusion of a windage factor of 1 to 3% reduced separation distances between synthetic particles and the tracks of Sargassum mats, pseudo-Sargassum drifters, and un-drogued oceanographic drifters. When similar analyses were conducted for oceanographic drifters with drogues centered at 15 m depth (which are designed to minimize the influence of winds and waves and thus served as a control for this technique), inclusion of a windage factor did not improve particle-tracking predictions. These findings indicate that adding a windage calculated from satellite-derived winds to the surface layer of an ocean circulation model produces particletracking results that are more relevant to Sargassum, likely by accounting for the buoyant nature of the algae and the influence of the direct forcing of wind (and waves, implicitly) on the part of the object extending above the surface. However, the estimates of an appropriate windage coefficient for Sargassum may differ depending on the wind and surface current products used as well as the particular oceanic region being studied. Future work to identify and resolve systematic biases in the representation of winds and currents will be an important step toward eventually forecasting Sargassum movement and coastal inundation events.