FERTILIZED FARMLAND AS A SOURCE OF CADMIUM IN OYSTERS

Cadmium, added as a contaminant in phosphatic fertilisers to pasture s oils of the Mahurangi catchment is mobile through both leaching from t he acidic soils and erosion. The high cadmium concentrations on the sm allest particles in both soils and freshwater sediments enhances parti culate cadmium transport by erosion and fluvial action into the downst ream estuary. In the saline conditions of the estuary, cadmium is deso rbed from the suspended particles and bed sediments in the lower part of the estuary have low cadmium concentrations. The lowest cadmium con centrations in oysters are found in the riverine section of the estuar y where cadmium uptake appears to occur by ingestion of the particulat e matter carried into the estuary by the inflowing tributaries. In the lower, permanently saline part of the estuary, where oyster cadmium c oncentrations are the highest, phytoplankton appear to accumulate diss olved cadmium. The high concentrations of cadmium in oysters result fr om the selective ingestion of these cadmium-rich phytoplankton.