Linking land use, pesticide pollution, and bacterioplankton responses to enhance small stream resilience

Pesticides remain a major threat to freshwater ecosystems, yet monitoring data, particularly linking catchment land use with rivers pollution and ecological impacts, are still limited. This study assessed pesticide contamination and the mitigating role of forested riparian zones in four Central Poland catchments dominated by mixed agriculture and apple orchard production. Over a 14-month period, 20 pesticide compounds were detected in 85% of samples with an average total concentration of 367.2 ng/L, and a maximum of 3088.5 ng/L. The herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) was the most frequently detected compound (67%). A significant positive correlation was observed between pesticide concentrations and orchard cover, however, this effect was mitigated by forested riparian zones. Rainfall-driven pesticide export was observed only in the orchard-dominated catchment lacking riparian buffering, indicating the importance of hydrological connectivity in contaminant transfer. Microbial analyses using flow cytometry and the Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization (CARD-FISH) revealed elevated total cell counts (3.87 ×10⁶-3.55 ×10⁶ cells/mL) and a dominance of Proteobacteria (70.0-73.6%) in orchard-influenced catchments, positively associated with total phosphorus and pesticides levels. In contrast, the catchment with substantial forest cover (25.4%) exhibited lower Proteobacteria dominance (32.2%) and higher community heterogeneity, suggesting enhanced ecological stability. These findings demonstrate that orchard intensity and riparian configuration jointly determine pesticide export and microbial community responses. Maintaining or restoring forest cover beyond a critical threshold and re-establishing continuous riparian buffers may substantially reduce pesticide transport, strengthen river self-purification processes, and enhance freshwater ecosystem resilience in agricultural landscapes.