Abstract
The species composition, diversity, and phenology of a community of water beetles (Dytiscidae) were studied over a 4-year period at George Lake, Alta. There was a pronounced trend of decreasing diversity between years due to the loss of several rarer species. In any year, there were distinct seasonal assemblages of different species, as well as a group of permanent residents. There were few differences in species composition between various microhabitats in the lake. The decline in diversity is attributed to habitat stability and competitive success of one species.