The Odonata of a new reservoir in the southeastern United States

Lake Anna is a recent impoundment of the North Anna River in Virginia, USA. As part of a comprehensive ecological assessment the odonates were studied for 2 years prior to impoundment and 3 years following impoundment. Lake Anna was colonized immediately by odonates during the first summer after the river was impounded. After impoundment there was an initial reduction in the number of species present, but the diversity of odonates increased in subsequent years. The species composition of odonates in the reservoir was also quite different from that of the previous riverine ecosystem. Several obligate rheophilic species disappeared the first year after impoundment, but the reservoir was colonized by an early facultative or ’’pioneer” species. In succeeding years several additional or late facultative species became abundant, and several limnophilic species colonized the reservoir in significant numbers. From a review of the autecology of the species involved it appears that several physical factors determine the success of odonates during transition from a lotic to a lentic ecosystem: rate of flow, vegetation, and bottom composition.