Riding the heatwave: the effects of temperature and nutrition on heat-injury accumulation and repair in <i>Daphnia magna</i>

Aquatic ecosystems face increasingly frequent heatwaves and cyanobacterial blooms, which reduce the nutritional quality of resources available to primary consumers. This raises the question of how the nutritional quality of resources modulates aquatic ectotherm heat-induced injury accumulation and repair capacity. We address this question using Daphnia magna raised on diets of contrasting quality (rich versus poor in sterols and polyunsaturated fatty acids) by measuring heat tolerance before and after a sublethal heat-stress and recovery sequence at different temperatures (18-30°C) and durations (2 or 12 h). Overall, the rich diet increased heat tolerance of unstressed individuals, but diet effects on recovery were complex and time-dependent: 2 h post-stress, damage repair in the rich diet exhibited a concave unimodal response to recovery temperature, while in the poor-diet individuals continued to accumulate damage independently of temperature. 12 h post-stress, poor-diet individuals also showed a unimodal damage repair response to recovery temperature. Yet, despite greater repair on average, rich-diet individuals experienced high mortality and interindividual variability in recovery suggesting that the stronger initial repair response could involve costs. Overall, our results highlight the importance of nutritional context for coping with heatwaves and the need to disentangle the interacting effects of temperature and diet.