Potential effect of invasion by an exotic zooplankter, daphnia iumholtzi

We observed an increasing population of Daphnia lumholtzi in Lake Springfield, II, near the northern limit of its known distribution, from 1992 through 1994. The Daphnia lumholtzi population peaked with water surface temperature. Since the establishment of D. lumholtzi, the composition of spring peak crustacean zooplankton has shifted from >75% cladocerans to >75% copepod nauplii. There was also a corresponding decrease in crustacean zooplankton length from 0.41 ± 0.04 in 1992 to 0.18 ± 0.01 in 1994 during peak abundance. Since D. lumholtzi does not appear until summer, direct competition with native zooplankton would not account for this change in spring zooplankton community structure. We propose two indirect mechanisms by which the presence of D. lumholtzi may lead to a decrease in the numbers of native zooplankton overwintering as adults, thus impacting the spring community. Late summer competition among zooplankton species may reduce native zooplankton populations. Also, selective avoidance of D. lumholtzi by fish predators may increase predation pressure on native zooplankton.