Pesticide pollution and its impact on biota: Contrasting shrimp-based biomonitoring results between headwater and middle-course streams.

The Pampean region is characterized by a flat land area with gentle slopes and extensive agricultural land use. Streams and rivers associated with agricultural catchments are usually affected by pesticides that are used on crops and drift into aquatic environments. As streams converge into larger rivers, their water quality may undergo significant changes, particularly in terms of pesticide concentrations. Ten different sites along lowland stream were studied, from the headwaters to the mouth near the Middle Paraná river. Five physicochemical parameters and pesticide concentrations (135 compounds including glyphosate and its metabolite AMPA) in the water were measured. Additionally, Macrobrachium borellii shrimp were exposed for 7 day at each site in a field cage design. After exposure, five enzymatic and non-enzymatic parameters related to oxidative stress were measured in the hepatopancreas and seven in the muscle. At 2 first-order streams and 1 s-order stream the environmental conditions were sufficiently stressful to induce 100 % mortality in exposed individuals. Among surviving shrimp, oxidative stress biomarkers were predominantly elevated in headwaters and 1 s order stream. Significant positive correlations were detected between specific pesticide concentrations and certain biomarkers. Multivariate GLM analysis revealed that atrazine concentrations significantly explained variations in catalase and superoxide dismutase activities in muscle tissue. However, the most pronounced shifts in oxidative stress parameters occurred at sites where physicochemical variables (e.g., dissolved oxygen, pH) were stronger predictors than pesticide levels, with no clear dose-response relationship observed for pesticides alone. These findings underscore that first and second order streams related with headwaters represent the most ecologically stressful aquatic environments in the studied system, warranting targeted conservation efforts and further research to elucidate underlying mechanisms.