Dispersal of rafting eelgrass seed and associated population connectivity in the Seto Inland Sea, Japan

Quantifying seed dispersal and connectivity are crucial for effective conservation of the declining eelgrass, Zostera marina. The Seto Inland Sea (SIS), Japan, is a prototypical basin-strait system, a type of marine environment ubiquitous worldwide. The unique geography and hydrodynamic conditions of the SIS exert a profound influence on the connectivity of eelgrass populations via ocean currents. This study assessed connectivity by tracking the rafting of seeds among eelgrass populations in the SIS, using a regional oceanic circulation model coupled with a Lagrangian particle tracking model. The coupled model demonstrated that the interaction between the topographically constrained tidal residual currents and density-driven geostrophic circulation created a physical barrier to seed dispersal in the strait and promoted basin-scale connectivity. The restricted connectivity between the two sides of the SIS is also supported by the genetic structure of the eelgrass populations. Our findings provide a better understanding of the ecological connectivity in marine organisms that are spread by rafting in other basin-strait systems with analogous oceanic conditions, as well as in eelgrass populations.