Effluents from a Mexican reservoir: A toxicity mosaic inducing teratogenesis and neurobehavioral effects in Xenopuslaevis.

The Madín Dam, a reservoir located in the Mexico Valley Basin, is vital for supplying drinking water to the surrounding population. However, it receives urban and industrial discharges without efficient prior treatment. Given the lack of ecotoxicological data, this study aimed to evaluate the toxicity of Madín waters using the FETAX (Frog Embryo Teratogenesis Assay-Xenopus) protocol with Xenopus laevis as a bioindicator, and to assess behavioral responses of larvae exposed to effluents from different sampling sites during the first 96 hpf. The results showed that the reservoir is a mosaic of heterogeneous toxicity. Sites S1, S3, and S4 were found to be teratogenic, generating a teratogenic profile characterized by cardiac edema and ocular hypopigmentation. S1 was lethal to this species, with nitrite and nitrate concentrations that exceeded Mexican water-quality standards by a factor of 10. The extreme chemical load induces systemic hypoxia, as evidenced in larvae by marked hypoactivity. In contrast, S5 showed a neurotoxic increase in motor activity, indicative of cholinergic system impairment. The toxic effects observed are not attributable to a single compound but rather to the complex synergistic interactions among contaminants, indicating that the reservoir behaves like a mosaic of microenvironments with heterogeneous chemical risk profiles. These findings call for the urgent implementation of continuous ecotoxicological monitoring and remediation measures focused on the most critical discharge sites to preserve public health, regional water security, and safeguard biodiversity.