Temperature reaction norms of Daphnia carinata fitness: The effects of food concentration, population density, and photoperiod

Increasing concerns over climate change have renewed interest in temperature effects on aquatic organisms. The effects of temperature, food concentration, population density, and photoperiod on Daphnia carinata fitness were examined using life-table experiments in a full factorial design. The high temperature enhanced fitness at the high food concentration, but depressed it at the low food concentration. The temperature-food interactions were significant in the four combinations of two population densities and two photoperiods. The temperature-population density interactions were significant in the two combinations of two photoperiods and the high food concentration, in which the high population density masked the temperature effects. The temperature- photoperiod interaction was only significant in one combination with the low population density and the low food concentration, in which the long day length weakened the temperature effects. Such complicated interactions suggest that multiple ecological variables need to be taken into account simultaneously when assessing temperature effects on zooplankton in nature.