Integrated assessment of wastewater pollution in a karstic coastal lagoon: linking nutrients, caffeine, stable isotopes, and fish physiological responses.

Coastal water pollution is a global problem, especially in karstic areas, where pollutants from point and non-point sources reach coastal systems. The Chelem Lagoon in the Yucatan Peninsula exemplifies such environments, where the presence and effects of pollutants have been previously reported. This study presents and evaluates an integrated assessment framework to characterize pollution sources and ecotoxicological impacts in complex coastal systems. This approach combines nutrient concentrations, caffeine (as an urban wastewater tracer), carbon and nitrogen stable isotopes, and a suite of physiological and biochemical biomarkers in the resident fish Sphoeroides testudineus and provides a valuable basis for informing environmental management in karstic environments. The results showed that our integrated approach successfully defined a longitudinal pollution gradient and revealed a complex, tissue-specific physiological response in S. testudineus. In the mid-lagoon (Chuburna area), the presence of caffeine indicated recent or ongoing wastewater inputs. The pollution gradient induced distinct stress profiles: the east (Progreso), characterized by high ammonium concentrations, showed biomarker responses consistent with localized physiological stress, including increased liver protein oxidation. Conversely, the west (accumulation zone), characterized by higher nitrate concentrations and δ15N in suspended and sedimentary particulate organic matter, exhibited different biomarker responses, including muscle acetylcholinesterase inhibition, catalase depletion, and higher liver lipid peroxidation. Overall, this integrated multi-biomarker framework, using S. testudineus as a sentinel species, provides a useful basis for characterising complex pollution patterns and assessing wastewater influence in coastal karstic lagoons. This framework is scalable and can support environmental monitoring and management in similar high-priority ecosystems.