Effects of elevated temperatures on threshold food concentrations and possible competitive abilities of differently sized cladoceran species

We tested the hypothesis that at elevated temperatures in temperate lakes small cladoceran species are competitively superior to large species, which would cause a shift to smaller species in pelagic communities. As a measure of competitive ability, threshold food concentrations for growth (C-0) were determined in a flow-through system at 16, 20, 24 and 28 degrees C. Four lake species were tested (in the order of decreasing size): Daphnia pulicaria, D. galeata, D. ambigua and Ceriodaphnia reticulata. C-0 increased at temperatures above 20 degrees C, but the rank order of species was retained, i.e., the larger species had lower C-0 at all temperatures. The two smallest species showed a dramatic increase of C-0 al 28 degrees C, hence their competitive ability was severely reduced. The hypothesis was rejected. In the temperature range tested, small cladoceran species were not competitively superior to large ones. The observed size shift from larger to smaller zooplankton in lakes with increasing temperature may be due to selective factors other than direct competition (e.g., predation, food quality).