Coastal ecosystems are characterized by strong diel fluctuations in dissolved oxygen driven by primary producers, generating dynamic ‘oxyscapes’ that can modulate organism physiology. While oxygen supersaturation can buffer climate-related stressors, its role in mediating contaminant effects remains poorly understood. We investigated whether oxygen supersaturation influences physiological impact of mercury (HgCl₂) in the clam Ruditapes philippinarum. Clams were exposed for 7 days to a fully factorial combination of two oxygen regimes (normoxia: 90% saturation; diel hyperoxia: >150% for 5 h day⁻¹) and two Hg concentrations (0 and 1 μg L⁻¹). Haemocyte traits (count, diameter, volume) and a battery of biomarkers were measured in gills and digestive gland. Bioaccumulation of mercury was also investigated. Hyperoxia did not buffer mercury toxicity but modulated organism responses across multiple biological levels. Mercury and hyperoxia reduced haemocyte size, but not cell abundance. Mercury increased lipid peroxidation, while antioxidant responses were tissue-specific and inconsistently regulated. Hyperoxia independently increased oxidative stress, whereas combined mercury and hyperoxia exposure affected gill CAT activity. Oxygen regime influenced baseline mercury levels but did not reduce bioaccumulation under exposure conditions. These findings demonstrated that oxygen supersaturation reshapes rather than mitigates contaminant effects, highlighting a decoupling between metabolic support and physiological stress. Within dynamic oxyscapes, oxygen availability emerges as a key driver of both exposure and response pathways, with implications for organism vulnerability under multi-stressor scenarios. Our results challenge the assumption that biogenic oxygen production can buffer chemical pollution and emphasise the need to integrate oxygen dynamics into ecophysiological and risk assessment frameworks. © 2026 The Authors.

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