Freshwater food webs amplify microplastic transfer to fish

Summary paragraph Microplastics are pervasive in aquatic and terrestrial environments¹,²,³, yet the processes that govern their accumulation in organisms across habitats remain unclear. Here we show that freshwater fish consistently carry higher microplastic burdens than marine fish. This contrast reflects two drivers: freshwater environments tend to hold higher ambient microplastic levels, and non-selective freshwater zooplankton more efficiently transfer particles to fish. Smaller particles are more readily ingested by zooplankton and passed on to fish. Across datasets, the freshwater–marine difference is strongest in gut and whole-body, detectable in muscle, and weakest in gills, consistent with dietary rather than branchial exposure. Guild-specific patterns align with a zooplankton-mediated pathway, with clear freshwater–marine contrasts in carnivores but not in herbivores. Although fish gastrointestinal contents show trophic dilution, bioaccumulation remains substantial in gut, gills and muscle, consistent with retention in tissues that are consumed by humans. These findings challenge the prevailing view that marine seafood is the dominant dietary source of microplastics⁴,⁵,⁶ and highlight the need to explicitly include freshwater fisheries in exposure assessments.