Hydrodynamic and genetic connectivity metrics help inform priorities for conservation of the eelgrass Zostera marina across spatial scales

Connectivity is essential to the persistence and resilience of marine populations, yet it is often overlooked in conservation planning. The eelgrass Zostera marina has suffered important declines in France, and to ensure long-term sustainability of this important habitat-forming species, connectivity among eelgrass beds along the English Channel and Atlantic coasts of France was estimated using two complementary methods: multi-year biophysical dispersal simulations and population genetic analyses. High-resolution hydrodynamic modelling identified three central connectivity hubs (Bay of Lannion, Bay of Douarnenez and Lorient/Île de Groix), while populations south of the Loire River such as Île d’Yeu and Arcachon were more isolated and potentially more vulnerable. Clustering based on connectivity among zones revealed six large hydrodynamically distinct regions that were well-connected internally, but had limited connectivity with other regions. High levels of genetic differentiation were observed among nearly all populations studied, with recent migration estimates being low or null for nearly all population pairs at the national scale, except for three highly-connected populations located on the western point of Brittany. Clonal richness was highly variable among the sampled populations, indicating that both clonal and sexual reproduction contribute to the maintenance of eelgrass populations in contrasting environments. Genetic data also showed unexpected migration events between sites in Brittany and Arcachon, indicating that other dispersal vectors (biotic-mediated dispersal) may be responsible for some long-distance dispersal events. These findings highlight the importance of short-distance and intra-region connectivity for population persistence, the complementarity of hydrodynamic and genetic information, and its relevance for coastal management. ​