Several studies have demonstrated that coastal marine areas with high populations of bivalves/polychaetes/ascidians are characterized by population filtration capacities often exceeding the overlying water column volume several times a day. This may lead to insufficient vertical water mixing in the field and may thereby uncouple the suspension feeders from the overlying water column, ultimately preventing them from grazing on the phytoplankton. In such a case, mesocosm and field investigations were performed in the benthic boundary layer at different temperatures to assess phytoplankton depletion caused by suspension-feeding Nereis diversicolor populations. The capability of the worms to deplete the phytoplanktons in the bottom water was demonstrated to be highly correlated with temperature-dependent filtration as well as water pumping capacity. In unstirred water, the worm population (57 g m-2 dry wt) was capable to deplete the phytoplankton in the water column to a maximum height of 26 cm at an optimum temperature of 17.8 °C. A combination of the clearance activity of the worms and the strength of vertical water mixing generated by the water jets elected from the worm tubes controlled the height of the algal depleted area. In this study, within 2-4 h, the algal concentration in the boundary layer was depleted from an initial of 7000 to l0000 Rhodomonas cells ml-1 to a minimum of 1500 to 3000 cells ml-1. The maximum height of the phytoplankton depleted zone in the field was only 4-8 cm, but considerable phytoplankton depletion (20-40%) occurred in the bottom layer. This happens even at moderate to high wind speeds (6 to 8 ms-1). During the investigation, for approximately 70% of the season (March to August 1995), the wind speeds were less than 8 ms-1 and as such in the shallow Odense Fjord, Denmark, algal depletion above dense population of suspension-feeding N. diversicolor may occur frequently. Therefore, in spite of a considerable increment in filtration capacity from 17 to 69 m3 m-1 d-1 (3 to 14 times the water column a day) from March to August 1995, N. diversicolor was not able to control phytoplankton biomass at the field site in Odense Fiord due to the presence of an algal depleted boundary layer.
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