GRAZING BY LARGE RIVER ZOOPLANKTON - A KEY TO SUMMER POTAMOPLANKTON DECLINE - THE CASE OF THE MEUSE AND MOSELLE RIVERS IN 1994 AND 1995

To explain summer declines in phytoplankton biomass in large rivers, w e compared the effect of zooplankton grazing on the planktonic algae o f two large European rivers, the Meuse and the Moselle. In situ grazin g was measured during two years (1994 and 1995), using the Haney metho d. Total zooplankton community filtration rates recorded in the river Meuse ranged between 1 and 32% of the water volume filtered per day. A drastic algal decline was observed early July both years and may be e xplained by high densities of a rotifer-dominated zooplankton communit y (500-700 ind. 1(-1)) with more than 75% of Brachionus calyciflorus. During the summer period in 1994, when grazing was over 20%, edible al gal biomass was controlled by a diversified rotifer community (up to 2 500 ind. 1(-1)), while a non-edible algal assemblage developed. In con trast, phytoplankton biomass remained comparatively low in the Moselle throughout the low-flow period, as did zooplankton numbers during mos t of this time (fewer than 200 ind. 1(-1) during the summer period). T he proportion of crustaceans in this zooplankton was rather higher tha n in the Meuse, and they dominated at times, in biomass as well as in numbers. Nevertheless, measured in situ grazing rates (1-15%) could no t explain the low summer algal biomass, even if low filtration rates m ay at times represent a significant carbon loss for phytoplankton, whe n and where net algal production was low. As a conclusion, the role of phytoplankton - zooplankton interactions in controlling algal biomass in large rivers is discussed.