Abstract

In a large (70 ha) shallow (<2m deep) eutrophic wetland investigated between November 1988 and October 1989, there were marked seasonal changes in water quality and the composition of zooplankton, and to a lesser extent phytoplankton communities, with three distinct groupings evident: summer, winter–spring and autumn. Summer conditions were characterized by high levels of total P (689198 ˜g L ^–1 ), N limitation, blooms of Microcystis aeruginosa (mean chlorophyll a = 46630 ˜g L ^–1 ) and zooplankton communities dominated by cyclopoid copepods (Mesocyclops sp. and Microcyclops sp.). Winter–spring conditions were characterized by Secchi disk transparencies to the bed (chlorophyll a <50 ˜g L ^–1 ), low total P (7140 ˜g L^–1 ) and zooplankton communities dominated byDaphnia carinata King. Autumn conditions represented an intermediate state between these two. Significant linear relationships were recorded between chlorophyll a and total P and between chlorophyll a and D. carinata. Abundances of hemipteran predators (Anisops sp.) were correlated with D. carinata, but the relationship did not appear to be causal. The decline in D. carinata in summer was attributed to food limitation. Reductions in nutrient inputs to the lake in 1990–91, resulted in improved water quality and high abundances of D. carinata during summer.