Influence of a reservoir with surface release on the life history of the mayfly Heterocloeon curiosum (McDunnough) (Ephemeroptera: Baetidae)

The life history of Heterocloeon curiosum was compared in the impounded North Anna River (NAR) and the free-flowing South Anna River (SAR) in Virginia, U.S.A. The study site on the NAR was 32 km below Lake Anna, a surface-release reservoir. Heterocloeon curiosum was bivoltine in both rivers with two summer generations and probable overwintering in the egg stage. It passed through 10 larval instars (range 9–12) in both rivers. The density of larvae was twice as great in the SAR as the NAR. Factors which may have contributed to the lesser success of H. curiosum in the NAR included bottom scouring produced by sudden increases in discharge, absence of the macrophyte Podostemum, quality of available food, and alterations of the temperature regime. Of these four factors, temperature probably had the most significant effect. The emergence of the second generation was 1 month later in the NAR because the reservoir delayed the normal seasonal cooling of the river. The nonoptimal temperature regime appeared to significantly reduce the fecundity of H. curiosum in the NAR. Our data indicate that surface-release reservoirs may have subtle but significant effects on the life histories of benthic macroinvertebrates.