Phytoplankton reduction in near-bottom water caused by filter-feeding Nereis diversicolor - Implications for worm growth and population grazing impact

Studies of vertical profiles of phytoplankton in the field combined with laboratory experiments demonstrated that reduction in phytoplankton concentrations in the near-bottom water layer, 5 to 10 cm in thickness on calm days, may play a significant role for the filter-feeding polychaete Nereis diversicolor in realizing its grazing capacity (estimated at 13.8 m3 m-2 d-1 in the shallow bay of Kertinge Nor, Denmark, in July 1994). Field-growth experiments were performed with worms transferred to U-shaped glass tubes placed at different heights (0, 4, 10 and 20 cm) above the seafloor. A considerably reduced specific growth rate of worms at the sea floor (0.2 ± 1.1% d-1), compared to worms elevated just 10 cm above the sediment surface (2.5 ± 0.8% d-1), indicates that extremely meagre food conditions may be prevailing at the bottom. Experimental laboratory data on the development of vertical algal cell profiles were compared with predicted values obtained by means of a simple diffusion model.