Ecology of macroinvertebrates in seasonal wetlands

Two seasonal lakes on the Swan Coastal Plain, Western Australia - Forrestdale Lake and Thomsons Lake - were sampled for macroinvertebrates over a period of six months. Three sites were chosen at each lake along a 500m transect from the fringing vegetation to the lake centres. The lakes were sampled when dry to determine which, if any, macroinvertbrates remained in the exposed sediments during the dry phase. The lakes began to fill in May, and sampling was carried out at flooded sites in May, June and July. Between June and August the water was sampled for nutrients (nitrogen and phosphorus), chlorophyll a, pH and conductivity. Organic content was measured in all samples of the sediments. The water chemistry and organic content measurements revealed that both of the lakes were freshwater and meso-eutrophic. Altogether 78 taxa of macroinvertebrates were found, (62 species in Thomsons Lake excluding the zooplankton, and 41 in Forrestdale Lake). Thirteen species were recorded from the dry lake beds, and these and the rest of the aquatic fauna were considered to be well adapted to the seasonal cycle of drying and flooding. After flooding, species richness, density of individuals and diversity increased at most sites on each sampling occasion. Temporal differences between sites were greater than spatial differences. The class lnsecta contained the largest number of species but a crustacean group, the Ostracoda, was the most abundant in terms of numbers of individuals. Sites near the fringing vegetation were found to contain the most species and have the highest density of individuals. The two lakes appear to be very productive, in terms of species richness and density of individuals, compared to other Australian wetlands. This is probably due to the trophic status of the lakes and their seasonal nature, both of which are thought to increase lentic production