PHYTOPLANKTON REDUCTION IN NEAR-BOTTOM WATER CAUSED BY FILTER-FEEDINGNEREIS-DIVERSICOLOR - IMPLICATIONS FOR WORM GROWTH AND POPULATION GRAZING IMPACT

Studies of vertical profiles of phytoplankton in the field combined wi th laboratory experiments demonstrated that reduction in phytoplankton concentrations in the near-bottom water layer, 5 to 10 cm in thicknes s on calm days, may play a significant role for the filter-feeding pol ychaete Nereis diversicolor in realizing its grazing capacity (estimat ed at 13.8 m(3) m(-2) d(-1) in the shallow bay of Kertinge Nor, Denmar k, in July 1994). Field-growth experiments were performed with worms t ransferred to U-shaped glass tubes placed at different heights (0, 4, 10 and 20 cm) above the seafloor. A considerably reduced specific grow th rate of worms at the sea floor (0.2 +/- 1.1% d(-1)), compared to wo rms elevated just 10 cm above the sediment surface (2.5 +/- 0.8% d(-1) ), indicates that extremely meagre food conditions may be prevailing a t the bottom. Experimental laboratory data on the development of verti cal algal cell profiles were compared with predicted values obtained b y means of a simple diffusion model.