Environment and food web structure interact to alter the trophic magnification of persistent chemicals across river ecosystems.

Legacy organic pollutants persist in freshwater environments, but there is limited understanding of how their trophic transfer and effects vary across riverine ecosystems with different land use, biological communities and food webs. Here, we investigated the trophic magnification of polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs) and a suite of organochlorines (OCs) across nine riverine food webs in contrasting hydrological catchments across South Wales (United Kingdom). Pollutants biomagnified through the food webs in all catchments studied, in some cases reaching levels sufficient for biological effects on invertebrates, fish and river birds such as the Dipper (Cinclus cinclus). Trophic magnification differed across food webs depending on pollutant characteristics (e.g. octanol-water partitioning coefficient) and site-specific environmental conditions (e.g. land use, water chemistry and basal resource composition). The trophic magnification of PBDEs, PCBs and OCs also reflected food-web structure, with greater accumulation in more connected food webs with more generalist taxa. These data highlight interactions between pollutant properties, environmental conditions and biological network structure in the transfer and biomagnification of POPs in river ecosystems. We advocate the need for further investigations of system-specific transfers of contaminants through aquatic food webs as these factors appear to have important implications for risk assessment.