Rare earth elements as emerging contaminants from Norwegian salmon farms: Contrasting bio-enrichment in mussels and kelp

Data supporting the risk assessment of the emerging, technology-critical rare earth elements and yttrium (REY) are still scarce. Because phosphate rock used as aquafeed mineral supplement can be enriched in REY, we hypothesised that sea-based aquaculture promotes their environmental dispersal in association with feed/faecal wastes. To test this, we conducted a four-month biomonitoring at three salmon farms in Norway using mussels (Mytilus edulis) and kelp (Saccharina latissima). Their tissues, seawater, salmon feeds, and sediments were analysed for REY and other elements as proxies of exposure. In both species, the REY patterns were shaped by the local hydrogeology, with maximum levels at a riverine influenced site and lowest furthest offshore. Feed REY concentrations were in the μg/kg range (four orders of magnitude above dissolved seawater levels), implying REY emission with suspended wastes. The elemental patterns in seaweeds exposed at two of three farms further suggest dispersal of the REY within the upper epipelagial. Contrastingly, the bivalves showed no increased REY-burden near the net-pens, likely owed to physiological factors. Our study is the first to indicate a link between salmon aquaculture and REY dispersal to the marine ecosystem, while underscoring the complex interplay of environmental and species-specific factors in driving biotic enrichment.