Daytime vertical distribution of microzooplankton in the Hawkesbury-Nepean River

Daytime vertical distribution of 18 dominant taxa of freshwater microzooplankton was examined in mid-channel open water at North Richmond in the Hawkesbury-Nepean River, New South Wales, by measuring their densities at two depths about biweekly throughout the year. The objective was to test whether or not there was any significant depth-related distributional pattern for the dominant microzooplankton. Ten taxa were heterogeneously distributed with depth over the sampling period. Among the taxa that exhibited vertical heterogeneity, rotifers were distributed more abundantly either near the surface or in the deeper water, whereas microcrustaceans were distributed more abundantly in the deeper water. The observed vertical distributional patterns appeared to be largely independent of river flow rate. For the estimate of density of zooplankton in the water column, depth-integrated collection of quantitative samples may generally be recommended even in rivers to reduce sampling bias deriving from the likely heterogeneous distribution of river zooplankton with depth.