MUSSEL BEDS - LIMITING OR PROMOTING PHYTOPLANKTON

Seasonal variation of phytoplankton over an intertidal mussel bed was measured in the Wadden Sea near the island of Sylt between February 1984 and April 1985. To quantify the uptake of phytoplankton by a mussel bed, an open flow-through system, the Sylt flume (20 x 2 x 2 m), canalized the tidal water over a bed of Mytilus edulis L. Changes in the content of phytoplankton in the water passing through the flume were used to calculate phytoplankton uptake over three tidal cycles in the summer of 1986. Phytoplankton biomass was reduced by 37 +/- 20% between the inflow and outflow of the flume. This figure includes active filtration of mussels and sedimentation. Phytoplankton biomass was reduced by the mussel bed over the whole size range from the smallest cells of 4-mu-m (or a few pg C.cell-1) up to the largest diatoms of several hundred-mu-m (or a few thousand pg C.cell-1). The higher the phytoplankton concentration, the higher the uptake by the mussel bed. There was a significant positive correlation between both concentration and uptake of phytoplankton. Parallel with the uptake of phytoplankton by the mussel bed, a high nutrient release by the mussel bed was measured. Potential primary production based on the N:C ratio of 16:106 was estimated, assuming that the released ammonium would be taken up entirely by phytoplankton. Following this assumption, the potential primary production induced by the nutrient release of the mussel bed is higher than the uptake of phytoplankton by the mussel bed. It is also probable that mussels extract N from particulate organic material other than phytoplankton. While mussels strongly reduce phytoplankton biomass, mussel beds also have the potential to significantly promote primary production.