Abstract

Fish farm effluents can significantly impact water quality and aquatic ecosystems, by inducing oxidative stress and sublethal biochemical alterations in organisms. This study assessed the pro-oxidant potential of fish farm effluent by measuring the enzymatic activities of catalase (CAT), glutathione S-transferase (GST), glutathione reductase (GR), glutathione peroxidase (GPx), and the levels of reduced glutathione (GSH) and oxidised glutathione (GSSG) in Lemna valdiviana and Hyalella azteca. Both species were exposed to different concentrations (100 to 3.125% v/v) and to ambient waters collected from sampling stations downstream of a fish farm. L. valdiviana was tested under controlled laboratory conditions, while H. azteca was evaluated both in situ and in laboratory bioassays. Increased conductivity, nutrient concentration, dissolved organic carbon (DOC), and dissolved organic matter (DOM) were recorded downstream of the discharge point, reflecting the influence of fish farming on water quality. Significant changes in antioxidant enzyme activity including CAT, GST, GR, and GPx were observed in L. valdiviana across all sampling sites and different effluent concentrations, alongside variations in GSH and GSSG levels after 120 h of exposure. In H. azteca, significant changes in CAT, GST, GR, and GPx activities were recorded at the sampling sites after 48 h of exposure and at different effluent concentrations after 48 and 72 h. These findings indicate that fish farm effluents induce oxidative stress in aquatic species across different trophic levels. Our results highlight the relevance of antioxidant biomarkers for the early detection of oxidative stress on aquatic ecosystems and support the need for improving environmental management strategies to mitigate these effects.