Grazing by a river zooplankton community: Importance of microzooplankton

Grazing rates by a zooplankton community were measured in situ by a radiotracer cell method at depths of 1 m and 4 m at the upper tidal freshwater portion of a regulated river over a year. The objectives were to evaluate the likely grazing impact on the river phytoplankton community and to produce predictive models by regressing the measured grazing rates against zooplankton biomass, temperature and food concentrations (represented by chlorophyll a). Grazing attained rates (overall average 0.2 day^-1, range 0.01-0.59 day^-1, expressed as instantaneous mortality rates of algal cells) comparable to those reported for lentic zooplankton communities. The measured community grazing rates were predictable largely as a function of total biomass or rotifer biomass and surface temperature for 1 m depth, and as a function of total biomass or juvenile copepod biomass and surface temperature for 4 m depth, with all-positive regression coefficients in the models. Owing to the predominance of microzooplankton in the river, the impact of zooplankton community grazing appears likely to be linked to a small-size fraction of the phytoplankton community all year. Management strategies for river water quality may need to take account of possible functional demarcation of grazing by river zooplankton.