Temperature as a Co-stressor modulating the ecotoxicity of 3,4-dichloroaniline in Daphnia magna

Rising temperatures associated with climate change may alter the toxicity of persistent contaminants in freshwater ecosystems. 3,4-dichloroaniline (3,4-DCA) is an aromatic amine widely used as an industrial intermediate and produced as a degradation product of herbicides such as diuron and propanil, contributing to its frequent detection and persistence in aquatic environments. However, its chronic effects on aquatic biota under climate-relevant temperature scenarios remain poorly understood. This study evaluated the effects of environmentally relevant concentrations of 3,4-DCA (0.00-6.00 µg/L) on Daphnia magna under 20 °C (standard) and 26 °C (global warming scenario). A 21-day reproduction assay was performed to assess life-history traits alongside biomarkers of antioxidant defence, lipid peroxidation, neurotoxicity, genotoxicity, and cellular energy allocation. Significant interactions between temperature and 3,4-DCA were observed. Warming alone altered reproductive strategy by inducing earlier reproduction and increasing the number of broods. In contrast, fecundity per brood declined, and individuals reached maturity at smaller body size, indicating altered growth patterns. At the sub-individual level, elevated temperature intensified oxidative stress responses. Combined exposure further enhanced antioxidant responses, neurotoxicity, and genetic damage, particularly at 26 °C, indicating increased sensitivity under thermal stress. Energy allocation analyses revealed compensatory responses at low-to-intermediate 3,4-DCA concentrations (0.07-0.67 µg/L), whereas energetic imbalance emerged at higher exposures. Overall, temperature acted as a key co-stressor, reshaping dose-response patterns and amplifying adverse effects on physiology and reproduction. These findings highlight the need to incorporate climate-warming scenarios into ecological risk assessment frameworks to improve environmental realism and ensure adequate protection of freshwater ecosystems.