Designing with dispersal: Connectivity-informed site selection for bivalve recruitment

Bivalve beds play an important role in coastal ecosystems and understanding factors that influence their persistence and recruitment is critical, especially given the strong impact of human activities on these habitats. Consequently, marine mussel bed restoration projects have gained momentum in recent years. However, bivalve bed restoration success critically depends on well-planned site selection that optimises larval connectivity to ensure recruitment and long-term population viability. Here, we present a comprehensive connectivity analysis to determine recruitment and potential restoration of blue mussel (Mytilus edulis) reefs, exemplified for Roskilde Fjord, Denmark. A biophysical model was used to predict larval dispersal. Hotspots for potential new blue mussel reefs were first identified based on habitat suitability and environmental conditions and then overlapped with site selection guidelines. Graph theory metrics were used to characterise the connectivity patterns in the basin. This analysis showed that carefully planned reef placement can improve survival of the mussel population by achieving more homogeneous connectivity and increasing inter-site connections. Modelling of the external larval supply demonstrated minimal recolonisation potential from outside the system, highlighting the importance of internal connectivity for recruitment. While nutrient loads remain a key pressure affecting mussel health in this system and reducing them is a major concern, our results provide quantitative evidence-based guidance for optimising bivalve recruitment. Moreover, integrating biophysical modelling with network analysis provides an improved framework for identifying sites with a higher chance of establishing new bivalve reefs through enhanced larval connectivity, with broader applications for potential marine restoration efforts in semi-enclosed coastal systems worldwide.