Rapid thermal evolution reduces synergisms with a metal across a thermal gradient

Rapid evolution of tolerance to a natural stressor may impact the tolerance to a toxicant and how toxicity is magnified along a gradient of the natural stressor. While crucial for accurate ecological risk assessment, these two aspects remain understudied. Therefore, we tested the temperature-dependent toxicity of zinc (150 and 300 µg L) across a thermal gradient (12 °C to 30 °C) in the water flea Daphnia magna, and examined how this was shaped by rapid thermal evolution. Two resurrected subpopulations separated by 40 years from a single lake that had experienced a threefold increase in heat wave frequency were compared. In the old subpopulation, high zinc exposure reduced survival (-21 pp) and acute heat tolerance (-0.69 °C CT_MAX) at 20 °C, and more so at higher temperatures (at 30 °C: -31 pp survival, -0.80 °C CT_MAX), indicating synergisms. Rapid evolution of a higher heat tolerance caused a lower tolerance to zinc: the high zinc concentration reduced survival by 41 pp and CT_MAX by 1.0 °C in the recent subpopulation at 30 °C. Rapid evolution caused synergisms between warming and zinc in the recent subpopulation to occur only at 30 °C and no longer at 28 °C, moreover the synergism for survival at 30 °C became 53 % weaker. Our findings highlight the importance of incorporating rapid thermal evolution and realistic thermal gradients including extreme high temperatures when assessing the combined effects of warming and toxicants. Integrating these aspects will advance the ability to forecast temperature-dependent toxicity patterns.