The increasing anthropogenic release of gadolinium (Gd), a rare earth element, primarily from medical contrast agents, raises concern about its environmental fate and ecotoxicological effects in freshwater ecosystems. Gd can reach remote alpine lakes through long-range atmospheric
transport and deposition, leading to its accumulation in cold, oligotrophic environments. These ecosystems are particularly vulnerable due to their low buffering capacity, reduced biological productivity, and sensitivity to contamination. In this study, we investigated the effects of Gd
 exposure under winter conditions on the copepod Cyclops abyssorum (G.O. Sars, 1863), a cold-adapted zooplankton species commonly found in deep and high-latitude lakes. As a key component of pelagic food webs, C. abyssorum plays a crucial role in energy transfer and nutrient
cycling. Individuals were sampled from an alpine lake in the Western Alps and exposed for 14 days to a gradient of environmentally relevant Gd concentrations (1–150 µg/L) under simulated winter conditions (low temperature and darkness). To evaluate sublethal effects, we measured
four oxidative stress biomarkers: superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione S-transferase (GST), and malondialdehyde (MDA). Results revealed significant biomarker modulation across treatments. SOD activity showed a non-linear response with
marked inhibition at intermediate concentrations, while GPx exhibited a concentration-dependent increase. GST activity decreased at the highest concentrations, and MDA levels rose significantly, indicating cellular membrane damage. Compared to existing literature, our results highlight species-specific responses, with C. abyssorum showing patterns distinct from those observed in standard model organisms. This study provides novel insights into the potential risks posed by rare earth elements in cold freshwater ecosystems and emphasizes the importance of including non-model and ecologically relevant species in environmental risk assessments.