Plankton community cycling and recovery after drought — dynamics in a basin on a flood plain

Abstract

Lake Merrimajeel, a shallow swamp, is part of the Murray-Darling, a flood plain river system with high plankton diversity. It fluctuates drastically in water level and often dries out. The dynamics and seasonality of zooplankton in the face of these fluctuations and the effects of a total drawdown and refilling were monitored between February 1977 and January 1980. During this period thirty species of rotifers, thirty four of microcrustacea and several other species were recorded. Despite erratic fluctuations in water level, temperature and turbidity a community cycle from dominance by rotifers in summer/autumn and crustacea in winter/spring was maintained. The drawdown disturbed this cycle but it was reestablished the following year. Diversity and density reached a maximum just before the drawdown, during hot, turbid conditions. During this drying phase the waters remained relatively fresh and well oxygenated. ‘Phyllopods’ were not recorded. The recovery phase was characterised by a group of rare species. It is postulated that these rare, colonising or successional species are dependent on a brief flush of rich organic material produced on flooding.