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, <i>Zostera marina</i>. 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.