Grazing effects of blue mussel Mytilus edulis on the pelagic food web under different turbulence conditions

Benthic filter feeders have the potential to control phytoplankton biomass and size composition in shallow estuaries and fjords, while the in situ impact on the zooplankton community is so far unknown. In a companion study, the blue mussel Mytilus edulis was shown to graze on both micro- and mesozooplankton causing depletion above the mussel bed (Nielsen & Maar, Mar Ecol Prog Ser 339:185-198). However, the vertical zooplankton distribution varied between sampling days, and we thus examined the influence of turbulence on the escape capability of different zooplankton organisms. Two sampling days were selected for further analysis representing relatively low (epsilon = 10(-7) m(2) s(-3)) and high (epsilon approximate to 10(-6) m(2) s(-3)) near-bed turbulence (estimated by a physical water column model, General Ocean Turbulence Model, GOTM). The grazing impact on the zooplankton was estimated by a random walk model taking zooplankton motility and escape behaviour into account as well as turbulence conditions. Protozooplankton and bivalve larvae were always captured efficiently by mussels irrespective of turbulence conditions, while the grazing impact on nauplii and copepodites was greatly reduced due to increased escape success in low turbulence conditions. Thus, the zooplankton community will be more diverse during low turbulence conditions and will compete with mussels for phytoplankton food. Seasonal importance of the different components of the pelagic food web for mussel consumption was evaluated by comparing the model results with plankton data from the studied fjord system. The analysis stresses the need, when evaluating the ecological role of mussels, to also consider the heterotrophic components of the food web.