Biotic interactions regulate the fate and trophic transfer of particulate contaminants in sediment-water systems: an overlooked role of water mite-chironomid interactions.

Engineered nanoparticles and microplastics are particulate emerging contaminants that can accumulate in sediments and interact with benthic organisms, but their uptake and transfer through biotic interactions remain difficult to predict. To address this issue we exposed a simplified predator-prey system consisting of Chironomus riparius larvae and predatory adult water mites (Mideopsis roztoczensis) to environmentally relevant concentrations of nanoparticles FeO (0.0105 g kg⁻¹), TiO (0.0525 g kg⁻¹), CeO (0.2625 g kg⁻¹) and a microplastic mixture (2.5 g kg⁻¹ sediment; PE: PVC: PA = 50:25:25 by weight). Predator presence reduced chironomid emergence from approximately 80% to 22.5-32.5% and largely eliminated contaminant-specific effects on chironomid life-history traits. Nano-sized contaminants reached water mites through different exposure pathways with contaminant-specific accumulation patterns. The presence of prey increased the body burden of Fe in watermites by twofold, whereas Ce and Ti were primarily associated with direct environmental uptake. In contrast, micrometer-sized plastic contaminants did not reach water mites, and were exclusively present in Chironomids, indicating that their transfer was constrained by the fluid feeding mode of predators. Moreover, microplastic loads were approximately eight-fold higher in exuviae than in adults, suggesting molting as an important pathway of contaminant redistribution. Our findings show that particulate contaminant transfer in this two-species system depended on particle properties and species-specific traits, leading to different pathways of uptake, transfer, and redistribution. These findings underscore the importance of integrating species interactions into ecotoxicological testing and environmental risk assessment.