An ecological study of a freshwater zooplankton community was undertaken in the Hawkesbury-Nepean River, a regulated coastal river in NSW, Australia. The study was based on analysis of species composition, density, vertical distribution, biomass, body mass, in situ grazing rate and density-size distribution. Results were compared with those of similar studies in rivers and lakes to characterise the similarities and differences in the structure and grazing function of river and lake zooplankton communities.

The studied freshwater portion of the river was inhabited by a taxonomically rich (116 taxa) and dense (max. 4537 animals/L) microzooplankton community, comparable to that in some of the large regulated temperate rivers in the Northern Hemisphere. The community density was negatively correlated with river flow but positively correlated with temperature, total phosphorus and chlorophyll-a, similar to the findings for other rivers. More than 50% of the dominant taxa (n=18) showed a heterogeneous distribution with depth. There was a negative correlation between mean copepod body mass and river flow. There was a positive correlation between mean zooplankton body mass and community biomass, as reported for lake crustacean zooplankton communities. Because of the dominance of small taxa, mean zooplankton body mass remained low, relative to that in lakes. Despite the marked temporal variation, the time-averaged bimodal shape of the density-size distributions of the river zooplankton community was similar to that of lake communities.

Grazing by the river zooplankton community attained rates (mean 0.2/day, range 0.01-0.59/day, expressed as instantaneous mortality rate of 14C-labelled Chlamydomonas cells), comparable to those reported for lake communities. The multiple regression analysis showed that the measured community grazing rates were predictable largely as a function of zooplankton biomass and surface temperature, with positive regression coefficients in the models. The river zooplankton community appeared to have a lower mass-specific grazing rate than did lake communities.

Overall, the characteristic features of the structure and grazing function of the river zooplankton community seems to be attributable to the absence (or very low abundance) of large planktonic crustaceans, notably Daphnia in the river. In contrast to the Daphnia-based grazing model in lakes, the river zooplankton grazing model is based on microzooplankton. Management strategies for river water quality may need to take account of the possible functional demarcation of grazing (i.e. limited food-particle size range and high degree of selectivity) by river zooplankton.