Combined effects of warming and hypoxia on aquatic ectotherms: A multilevel meta-analysis of physiological and fitness responses.

The performance of aquatic ectotherms depends on temperature and dissolved oxygen. Temperature shapes metabolic demand, while oxygen limits aerobic capacity. As warming and hypoxia often occur together, their combined effects across biological levels need clearer synthesis. We analyzed 40 empirical studies using multilevel meta-analysis and phylogenetic sensitivity analyses. To separate physiological deviation from fitness outcomes, we grouped endpoints into two categories: directional responses (fitness traits such as survival and growth) and deviation-based responses (the absolute magnitude of physiological change). We found a clear mismatch between fitness-related responses and underlying physiology. Neither single nor combined stressors produced consistent directional fitness effects (all p > 0.25), and heterogeneity remained high across studies. In contrast, deviation-based responses increased significantly under all stressor conditions (all p < 0.001). This pattern shows substantial physiological change even when pooled fitness responses remain weak or inconsistent. In factorial interaction analyses, deviation-based responses showed a significant positive interaction under combined stress (interaction estimate = 0.494, p = 0.002). This indicates non-additive amplification beyond additive expectations. The pattern is consistent with the idea that warming may increase sensitivity to later hypoxia, although this asymmetry should be interpreted with caution. Overall, deviation-based responses were more sensitive than directional fitness metrics for detecting the impacts of combined stressors. These metrics can complement traditional fitness endpoints in ecological risk assessment.