Hidden ecological risks in reservoir sediments: grain-size dependent enrichment of toxic metals in the black sea basin.

Toxic metal accumulation in freshwater sediments poses a significant ecological threat, yet bulk sediment approaches often fail to account for particle-size-dependent variability in contamination and risk. This study investigates grain-size-controlled metal enrichment and seasonal dynamics in reservoir sediments from the northeastern Black Sea region (Türkiye). Surface sediments collected from the Borçka, Deriner, and Torul reservoirs were separated into four grain-size fractions (0.063-0.5 mm) and analyzed for Cu, Pb, Zn, Ni, and As. The finest fraction (< 0.063 mm) showed markedly elevated concentrations, reaching up to 409 µg/g for Cu, 156 µg/g for Pb, and 346 µg/g for Zn, substantially exceeding sediment quality guideline thresholds (PEL and ERM in several cases). Metal concentrations consistently decreased with increasing grain size (F1 > F2 > F3 > F4), confirming strong granulometric control. Seasonal patterns revealed higher concentrations in autumn, associated with enhanced fine particle deposition and reduced hydrodynamic conditions. Spatially, the Borçka reservoir exhibited the highest contamination levels, reflecting the influence of mining-derived inputs and anthropogenic pressures. Importantly, bulk sediment assessments may not fully capture localized metal enrichment associated with fine-grained fractions. These finding demonstrate that integrating grain-size-resolved analysis with ecological risk indices provides a more accurate and environmentally relevant framework for sediment assessment, with important implications for monitoring and management of reservoir ecosystems.