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, we compared the effect of zooplankton grazing on the planktonic algae of two large European rivers, the Meuse and the Moselle. In situ grazing was measured during two years (1994 and 1995), using the Haney method. 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 explained by high densities of a rotifer-dominated zooplankton community (500-700 ind. l^-1) with more than 75% of Brachionus calyciflorus. During the summer period in 1994, when grazing was over 20%, edible algal biomass was controlled by a diversified rotifer community (up to 2500 ind. l^-1), while a non-edible algal assemblage developed. In contrast, phytoplankton biomass remained comparatively low in the Moselle throughout the low-flow period, as did zooplankton numbers during most of this time (fewer than 200 ind. l^-1 during the summer period). The proportion of crustaceans in this zooplankton was rather higher than in the Meuse, and they dominated at times, in biomass as well as in numbers. Nevertheless, measured in situ grazing rates (1-15%) could not explain the low summer algal biomass, even if low filtration rates may at times represent a significant carbon loss for phytoplankton, when 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.