Feeding activity of mussels Mytilus edulis related to near-bed currents and phytoplankton biomass

The feeding activity of blue mussels Mytilus edulis was investigated in the field and related to near-bottom current velocities and the phytoplankton biomass in the near-bottom water layers for four days. The body content of Chl-a in mussels and their shell gap size were used as indices of filtration activity. During days 1 and 2 the near-bed current velocities were low (1.2–2.1 cm s−1), and the near-bed phytoplankton biomass was at the same time lower than near the water surface. Between 44 and 69% of the mussels had closed shells and accumulated only small amounts of Chl-a in the body. During day 3 and day 4 the near-bed current velocities increased to 6.5 and 3.9 cm s−1, respectively, and the Chl-a was homogeneously distributed in the water column. Now only 17 and 25% of the mussels had closed valves and they accumulated a larger amount of Chl-a. The actual population filtration rates were estimated from the density of active mussels times the estimated filtration rates (based on the gap size of the mussels and literature values for corresponding filtration rates). Actual population filtration rates ranged from 6.4 to 22.8 m3 m−2 d−1 on days 1 and 3, respectively, corresponding to 27 and 98% of the estimated population filtration capacity.