The habitat-modifying suspension-feeding mussel Mytilus edulis may have facilitating or inhibiting effects on seagrass meadows depending on the environmental conditions. We investigated the effects of M. edulis on sediment biogeochemistry in Zostera marina meadows under eutrophic conditions in Flensborg fjord, Denmark. Sediment and plant samples were collected at 5 stations with Z. marina (Eelgrass), 5 with Z. marina and M. edulis (Mixed), and at 2 unvegetated ones, 1 with mussels (Mussel) and 1 with sand (Sand). The Mixed sediment was enriched in fine particles (2 to 3 times), nutrients and sulphides compared to Eelgrass stations. Increased sediment nutrient availability at the Mixed stations was reflected in increased N and P content in eelgrass. However, the plant biomass did not differ significantly between stations, while shoot features (number of leaves and leaf area) were significantly reduced at Mixed stations, suggesting an inhibiting effect of M. edulis on Z. marina. Negative correlations between eelgrass measures and sediment sulphide at Mixed stations indicate that the presence of mussels increases sulphide invasion in the plants. A survey of 318 stations in Danish fjords suggests a threshold of 1.6 kg M. edulis m −2 beyond which no coexistence between Z. marina and M. edulis was found.

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