Neundorf_2026_Sci.Total.Environ_1029_181740

Urban expansion in areas of the Atlantic Forest, one of the planet's biodiversity hotspots, has intensified the anthropogenic pressure on its rivers, promoting the introduction of distinct stressors, including organic contaminants, and microplastics (MPs). This study evaluated the health of fish assemblages along a contamination gradient in the Guaraguau River (southern Brazil), located within the Atlantic Forest Biosphere Reserve, that encompassing upstream preserved areas and downstream sites impacted by sewage and landfill inputs. Sediment organic contaminants, water physico-chemical parameters, and biochemical biomarkers were assessed in Deuterodon langei and Geophagus iporangensis, while MPs in the gastrointestinal tract were quantified in these and additional species representing different trophic guilds. Overall, midstream sectors showed increased biomarker activities compared to the less impacted upstream area. Gills responses were similar between D. langei and G. iporangensis, whereas muscle acetylcholinesterase activity was lower in upstream G. iporangensis. MP abundance was highest in carnivores at midstream areas than less impacted sector, and MP concentrations were overall greatest in omnivores than carnivores and invertivores. These findings reveal species-specific biochemical plasticity and trophic-dependent MP accumulation along the Guaraguau River gradient. The study provides evidence that even sectors considered least impacted may experience biochemical alterations, emphasizing the need for continuous biomonitoring in the Atlantic Forest Biosphere Reserve.