Factors which affect the abundance of an invasive cladoceran, Daphnia lumholtzi, in US reservoirs

1. Daphnia lumholtzi is a subtropical, Old World species which is rapidly spreading throughout reservoirs in the southern U.S.A. It was first recorded in Lake Texoma (Oklahoma-Texas) in September 1991. 2. Southern U.S. reservoirs typically have strong spatial and temporal gradients in temperature, conductivity, turbidity and in the distribution of organisms. Therefore, the present experiments examined the reproductive and moulting rates, and survival of D. lumholtzi in relation to extremes of food concentration, temperature, conductivity and turbidity. 3. Increases in temperature (range 15-29 degrees C) and decreases in turbidity (range = 0.6-470 NTU) increased reproductive and moulting rates, whereas food concentration only affected the rate of reproduction, while conductivity had no effects. Survival was affected only by temperature. 4. Daphnia lumholtzi tolerates high temperatures (27-30 degrees C) at which other cladocerans in Lake Texoma disappear. Therefore, D. lumholtzi may exploit resources in midsummer, when there are few potential competitors.