CAN A COMMUNITY OF SMALL-BODIED GRAZERS CONTROL PHYTOPLANKTON IN RIVERS

1. Phytoplankton, zooplankton and grazing were monitored throughout th e growing season for three years (1994-96) in the Belgian section of t he River Meuse. 2. A size structure analysis of the algal community sh ows that there was a summer shift toward larger algal units, following a decline in phytoplankton biomass. These changes occurred after an i ncrease in zooplankton biomass and diversity. 3. Daily filtration rate s of grazers ranged from 1 to 113% day(-1) and maxima were observed du ring the summer period. Higher rates tended to correspond with peaks o f rotifer biomass. A decline in total phytoplankton biomass within two weeks followed the increase in zooplankton biomass and filtration rat e. A rapid biomass recovery was then observed, along with a shift of t he algal community toward larger units. When grazing activity was not sustained, due to zooplankton fluctuations, the change in phytoplankto n size structure was less marked. 4. We suggest that the composition o f the phytoplankton community of large rivers may at times be controll ed by grazers. However, such biotic interactions can take place only w hen physical constraints are reduced, i.e. when discharge is low, and when increased transfer time, high temperature and availability of gra zeable algae allow high zooplankton biomass.