The effect of food availability on thermal stress in Daphnia magna: trade-offs among oxidative stress, somatic growth, and reproduction

The effects of food availability on thermal stress in Daphnia magna were investigated for 21 days, with a hypothesis that lower food condition would increase the adverse effect of elevated temperature on D. magna. Under higher food conditions (5 × 10⁵ cells mL⁻¹), elevated temperature (25 °C) did not induce higher oxidative stress (levels of antioxidant enzymes and lipid peroxidation) than reference temperature (20 °C). Thus, reproduction (number of offspring per female) was significantly (p < 0.05) higher at 25 °C compared with that at 20 °C. Under lower food condition (2 × 10⁵ cells mL⁻¹), however, elevated temperature significantly (p < 0.05) induced antioxidant enzyme activity (superoxide dismutase and catalase) and reduced adult somatic growth rate (5–21 days), whereas there was no significant change in reproduction between the two temperature groups. These findings suggest that food availability greatly influences the effect of elevated temperature on D. magna, possibly resulting in a trade-off among oxidative stress defense, somatic growth, and reproduction.