Size structure of river zooplankton: Seasonal variation, overall pattern and functional aspect

Density-size distributions of a river zooplankton community were examined about biweekly over a year in the upper tidal freshwater portion of a regulated Australian river, specifically the Hawkesbury-Nepean. Results were compared with those of similar studies in lakes (and reservoirs) to characterize the similarities and differences in the structure and grazing function of river and lake zooplankton communities. The density-size distributions of the river zooplankton community were similar to those of lake plankton communities, in terms of a marked temporal variation in shape and overall bimodal shape. They differed in terms of the truncation of the upper body size and the absence of a significant relationship between the slope (regression coefficient) of a log-linear density-size model and environmental variables. The river zooplankton community appeared to have a high average rate of biomass increase when measured against body-mass class and a low mass-specific grazing rate, compared with those of lake communities. Because of methodological differences between studies, intersystem comparisons of size spectra and grazing rates need cautious interpretations and generalizations.