Abundance and dispersal trajectories of floating Fucus vesiculosus in the Northern Baltic Sea

Floating macroalgae play a fundamental role for the connectivity of littoral marine habitats. They can be found throughout the world's oceans but information on their abundance and dispersal abilities is scarce. This information, however, is essential to evaluate their ecological role and the potential connectivity between populations of both the algae and associated biota. Here, we survey the abundance and spatiotemporal dispersal patterns of the floating brown alga Fucus vesiculosus in the Northern Baltic Sea. By incorporating a biophysical modeling approach, based on ocean circulation and surface winds, we demonstrate potential dispersal ranges, such as floating distances and directional trajectories of F. vesiculosus at different times of their reproductive season. Abundance of floating algae varied both spatially and temporally, with the highest densities being found during reproductive seasons. Evidenced by the fact that many algae were held back and accumulated in archipelagoes, the model suggests that a combination of local oceanographic features and seascapes strongly influences the dispersal of floating algae. However, a relatively high fraction of rafts spilled to more open sea areas, causing them to travel in multiple directions (e.g., Baltic Proper, Bothnian Bay, and Gulf of Finland) that are hundreds of kilometers away from their original sources, implying long-distance dispersal. We propose that the Northern Baltic Sea rocky littoral habitats are potentially well connected through abundant and long-reaching floating algae, with a chance to contribute to gene flow among F. vesiculosus populations.