Abstract

Anthropogenic climate change is intensifying multiple environmental stressors in marine ecosystems, including ocean warming (OW) and salinity decrease (freshening), with strong implications for the physiology of coastal ectotherms. Here, we evaluated the combined effects of OW and freshening on metabolic performance and cellular homeostasis in juvenile red sea urchins Loxechinus albus from Northern Patagonia (NP) and Southern Patagonia (SP). Juveniles from both sites were exposed for 30 days to simulated warming and freshening conditions, during which oxygen consumption, survival, protein oxidative damage, total antioxidant capacity, and antioxidant enzyme activity were assessed. Freshening significantly reduced oxygen consumption in both populations, while survival declined only in NP juveniles under combined warming and low salinity. Protein carbonyl levels increased under warming-- freshening conditions in both populations, indicating enhanced oxidative damage. Total antioxidant capacity and enzymatic responses differed between sites: NP juveniles showed stronger antioxidant activation across temperature and salinity treatments, whereas SP juveniles exhibited more constrained antioxidant responses and no mortality. Multivariate analyses revealed temperature as the main driver of enzymatic activity in both sites, with catalase and glutathione reductase contributing most strongly to treatment differentiation. Overall, our results demonstrate site-specific differences in tolerance and redox regulation under combined warming and freshening, suggesting that local environmental history modulates physiological vulnerability to multiple climate-related stressors in L. albus.