Sediment-Linked Microplastic Exposure in Edible Freshwater Fish Across Tropical Lentic and Lotic Systems

Abstract

Freshwater sediments may constitute a more persistent microplastic (MP) exposure source for edible fish than the overlying water column, yet this relationship has rarely been evaluated by simultaneously sampling water, sediment, and biota in tropical systems. MPs were quantified in surface water, sediment, and fish gastrointestinal tracts (GITs) at 22 lentic and lotic sites in northern Thailand; 185 fish from 13 edible species were included. Sediment MP concentration had a median of 10.0 items kg⁻¹ dw (IQR 24.0) and showed a robust and consistent positive association with fish GIT burden in negative binomial regression (IRR = 2.42, 95% CI: 1.88–3.12; p < 0.001). Water-column concentration was not a significant predictor of fish GIT burden (IRR = 0.86, 95% CI: 0.69–1.06). Fish GIT burden did not differ between habitat types (W = 3194, p = 0.239) despite higher water-column concentrations in lentic systems (p = 0.032), suggesting that sediment-associated processes may contribute to fish MP exposure across both hydrological settings. Water-based relative compartmental burden ratios (RCBRs) were higher in lotic habitats, a pattern consistent with episodic bed resuspension not captured by routine grab sampling. MP burden differed among the 13 species (H = 28.37, p = 0.005, ε² = 0.095), with Barbonymus schwanenfeldii showing the highest mean GIT burden (1.90 items g⁻¹), whereas feeding guild was not significant. These findings support the importance of sediment as an environmental reservoir associated with MP burden in tropical edible freshwater fish and suggest that water-only monitoring may underestimate organism-level exposure.