Algal concentration profiles above mussel beds

The distribution of phytoplankton was measured in vertical profiles at three stations in Limfjorden (Denmark) within and above Mytilus edulis beds (-5 to 200 cm above the bed surface) and related to the filtration capacity of the mussels and hydrodynamic conditions. The density of phytoplankton was 2690 to 10 600 cells cm(-3) at 200 cm above the bottom (similar to 1.4 to 3.2 mg Chl-a m(-3)), declining towards the bottom to about 1000 cells cm(-3) at two of the stations. This is a threshold of minimum algal concentration below which mussels are reported to stop feeding. The results of the present study further demonstrate that the near-bed phytoplankton concentrations observed in the field often remained below the concentration level at which mussels reduce their maximal filtration activity. (C) 2000 Elsevier Science B.V. All rights reserved.