Context-dependency of eelgrass-clam interactions: Implications for coastal restoration

Facilitative interactions between co-occurring species sustain diverse communities and constitute a vital functional component of coastal marine ecosystems. In seagrass ecosystems, facilitation ensures the survival and resilience of this important habitat. As seagrass meadows are in decline, innovative restoration strategies incorporating facilitative interactions could open new avenues in marine restoration. Here, we investigated the interactions between eelgrass Zostera marina and the Baltic clam Macoma balthica, and tested whether clams could enhance early survival and biomass increase of transplanted eelgrass shoots in the northern Baltic Sea. We measured eelgrass responses to differing densities of clams, as well as porewater ammonium (NH₄⁺) and phosphate (PO₄^3-) concentrations in field and aquarium experiments. Overall, survival of transplanted plots was high, independent of clam density. Specifically, we found that clams facilitated eelgrass above- and below-ground biomass in low porewater nutrient conditions, potentially through nutrient release, but inhibited growth in high-nutrient conditions, particularly where clams were added at high densities. Our results show the important role of infaunal bivalves for nutrient fluxes within seagrass meadows. Most notably, we highlight the importance of considering and testing context- and density-dependency when studying interspecific interactions, as clams could both benefit and hamper Zostera biomass increase. This becomes particularly crucial when incorporating such interactions in a restoration context.