Microplastic-associated metal risks in turbid coastal waters: limited vectoring but sensitivity effects at extreme loadings

Microplastics are often hypothesized to vector contaminants, including metals, to marine organisms, yet in turbid coastal waters they coexist with far more abundant natural suspended particulate matter (SPM). Field-aged microplastics (MPs) and co-located natural SPM, stable-isotope tracers, and a toxicokinetic-toxicodynamic (TK-TD) framework were used to link particle exposure to patterns of metal uptake and toxic effects in the clam Ruditapes philippinarum . Under matched particle volumes (100 mg L −1 aged MPs versus 200 mg L −1 SPM), aged MPs adsorbed Cu and other metals (Pb, Zn, Ni, Cd) far less strongly than SPM and contributed little to particle-mediated bioaccumulation, whereas SPM substantially increased tissue burdens. Among the tested polymers, biodegradable polylactic acid (PLA) exhibited the highest metal adsorption capacity. In 96-h Cu toxicity tests, the extreme MP loading (100 mg L −1 ) slightly increased mortality relative to SPM at a similar total Cu dose, driven by amplified toxicodynamic sensitivity rather than increased Cu uptake. In contrast, at an environmentally realistic MP abundance (≈100 particles L −1 ), no measurable effect on Cu accumulation or mortality was detected. Overall, in turbid coastal waters, natural SPM dominates particle-mediated metal exposure, while MP-associated sensitivity effects emerge only under extreme loadings. Ecological risk assessments of metal-particle mixtures should therefore prioritize natural SPM, with MP vectoring considered secondary under typical environmental conditions. • SPM sorbed Cu, Pb, Zn, Ni and Cd 1–2 log units more strongly than field-aged microplastics. • At matched particle volumes, SPM drove bioaccumulation; field-aged microplastics were minor. • Extreme microplastic loading amplified Cu mortality via toxicodynamic sensitivity. • Realistic microplastic abundance (~100 L −1 ) did not alter Cu uptake or survival. • In turbid coastal waters, natural SPM dominate metal risks over microplastics.