Floodplain zooplankton communities reflect combined trophic, hydrological connectivity, and trace metal controls

Floodplain lake assessments are typically centered on nutrient dynamics, although multiple interacting drivers shape these ecosystems. We investigated zooplankton community assembly across three Danube delta lakes representing a gradient from natural hydrological connectivity to controlled inundation. Using redundancy analysis and variance partitioning, we quantified the independent contributions of nutrients, spatial structure, and trace metals to community variation, based on numerical abundance and biomass. Nutrients were the dominant drivers of zooplankton communities, structuring assemblages characterized by small-bodied, fast-reproducing filter feeders (e.g., Moina, Brachionus, Keratella). Spatial structure explained a small but significant fraction of variation, indicating the influence of hydrological connectivity and dispersal processes. Trace metals explained a statistically significant independent fraction of variation (2% for abundance and 4% for biomass). Lead and zinc were significant predictors in abundance-based models, while cadmium emerged as an additional significant predictor in biomass-based analyses. The stronger metal signal detected in biomass suggests effects on growth and energy allocation rather than population size alone. These patterns were detected despite dissolved metal concentrations predominantly remaining below current EU Water Framework Directive regulatory thresholds, indicating that chronic low-level exposure may still influence zooplankton communities. These findings suggest that threshold-based monitoring frameworks may have limited sensitivity to chronic, low-intensity, mixture-driven exposure manifested primarily through biomass redistribution and trait filtering rather than species loss, and support the need to complement nutrient-centered assessments with contaminant-sensitive biological indicators in floodplain lake monitoring.