Fertilised farmland as a source of cadmium in oysters ☆

Cadmium, added as a contaminant in phosphatic fertilisers to pasture soils of the Mahurangi catchment is mobile through both leaching from the acidic soils and erosion. The high cadmium concentrations on the smallest particles in both soils and freshwater sediments enhances particulate cadmium transport by erosion and fluvial action into the downstream estuary. In the saline conditions of the estuary, cadmium is desorbed from the suspended particles and bed sediments in the lower part of the estuary have low cadmium concentrations. The lowest cadmium concentrations in oysters are found in the riverine section of the estuary where cadmium uptake appears to occur by ingestion of the particulate matter carried into the estuary by the inflowing tributaries. In the lower, permanently saline part of the estuary, where oyster cadmium concentrations are the highest, phytoplankton appear to accumulate dissolved cadmium. The high concentrations of cadmium in oysters result from the selective ingestion of these cadmium-rich phytoplankton.