Plankton of the lower River Murray, South Australia

Abstract

Seasonal changes in the composition of plankton in the lower River Murray reflect disproportionate contributions from the Murray and its major tributary, the Darling River. Flows in the Murray are closely regulated, whereas Darling flows are relatively unimpeded. Typically, Murray flows carry blooms of Cyanobacteria (Anabaena and Anacystis spp.) in summer, and diatoms (Melosira and Cyclotella spp.) in winter and spring. The zooplankton, dominated by endemic microcrustaceans, is derived principally from upstream reservoirs. The most common taxa are eurytopic. although there are seasonal occurrences of cold stenothermal species. The Darling flows typically have a high abiogenic turbidity. low algal biomass, and zooplankton dominated by warm stenothermal pantropical rotifers. Of 133 zooplankton taxa recorded from the lower Murray in 1976-80, more than half were pseudoplanktonic or littoral incursions. Of the dominant potamoplankters, more than half were endemics. Plankton density and diversity were distributed contagiously across- and down-river. Mean zooplankton density at Mannum, S.A., was 119 plankters I^-1, and average community species composition was 14.4 rotifers, 6.0 cladocerans and 2.4 copepods. Most common zooplankters were eurytopic and indicative of eutrophic conditions; some were influenced by changes in temperature, turbidity and salinity.