Phytoplankton depletion in the benthic boundary layer caused by suspension-feeding Nereis diversicolor (Polychaeta): grazing impact and effect of temperature

Mesocosm and field studies were conducted to examine phytoplankton depletion in benthic boundary layers caused by suspension-feeding Nereis diversicolor populations at different temperatures. It was demonstrated that the ability of the worms to deplete the bottom water of phytoplankton was highly correlated with temperature dependent filtration and water pumping capacity. At the optimum temperature of 17.8 degrees C, the worm population (57 g m(-2) dry wt) was able to deplete the water column of phytoplankton to a maximum height of 26 cm, in unstirred water. The height of the algal depleted layer was controlled by a combination of the clearance capacity of the worms and the strength of vertical water mixing generated by the water jets ejected from the worm tubes. The algal concentration in the boundary layer was reduced from the initial 7000 to 10 000 Rhodomonas cells ml(-1) to a minimum of 1500 to 3000 cells ml(-1) within 2 to 4 h. In the field, the maximum height of the phytoplankton depleted zone was only 4 to 8 cm, but considerable (25 to 40%) phytoplankton depletion occurred in the bottom layer, even at moderate to high wind speeds (6 to 8 m s(-1)). Wind speeds were less than 8 m s(-1) for approximately 70% of the season (March to August 1995) and thus algal depletion above dense populations of suspension-feeding N. diversicolor may occur frequently in the shallow Odense Fjord, Denmark. Due to the presence of an algal depleted boundary layer, N. diversicolor was not able to control phytoplankton biomass at the field site in Odense Fjord, in spite of a considerable increment in filtration capacity from 1.7 to 6.9 m(3) m(-2) d(-1) (3 to 14 times the water column a day) from March to August 1995.