Complex and lasting impacts of heatwaves on life-history traits and fitness in Daphnia magna.

Rapid climatic fluctuations, such as heatwaves, are key drivers of ecological disruption and pose significant physiological challenges to ectothermic organisms, yet their capacity for short- or long-term adaptation and transgenerational effects remain poorly understood. Using the model freshwater zooplankton Daphnia magna, we experimentally tested the physiological resilience, acclimation and evolutionary responses in D. magna across multiple generations under simulated heatwave conditions. Heatwaves significantly compromised the development and reproduction function of D. magna, with detrimental effects amplified under low food availability. Moreover, general temperature responses (stable warming) failed to predict heatwave responses, even within the same temperature range. This appears to be largely due to the complex interplay of warming and cooling effects during heatwaves, where the cooling phase within heatwaves unexpectedly elicited physiological disruption, highlighting overlooked complexity in heatwave dynamics. We found no evidence of short-term acclimation to heatwaves, whereas the impact of a second heatwave was additive or even multiplicative. Molecular biomarker profiles, coupled with physiological responses, revealed transgenerational impacts of heatwave exposure in D. magna and pointed to a potential trade-off between life-history investment and heat tolerance. Despite repeated exposure to multigenerational and periodic heatwaves, D. magna populations may still struggle to develop lasting heatwave tolerance. These findings underscore the physiological complexity of heatwave responses and raise concerns about the adaptive potential of aquatic ectotherms facing increasingly variable thermal regimes.