The origin of high cadmium loads in some bivalve molluscs from Shark Bay, Western Australia: A new mechanism for cadmium uptake by filter feeding organisms

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Abstract
Although Shark Bay is remote from all known industrial and geological sources of heavy metals, the cadmium content of several species of Shark Bay molluscs may exceed 10 mg/kg. The cadmium load in these molluscs varies geographically within the bay, but possible explanations for cadmium distribution involving variation in salinity, saline groundwater influx, the dissolved cadmium concentration, the cadmium concentration in substrate sediments, species, or an anthropogenic source are not supported by analytical data. The cadmium concentration is normal in Shark Bay seawater (0.04 Μg/L to about 0.35 Μg/L), rarely exceeds 0.25 Μg/L in ground waters, bore waters, and salt lake brines, and very seldom exceeds 1 mg/kg in sediments. No direct link between the cadmium loads in molluscs and its concentration in the water or substrate sediment is evident, but the cadmium load in molluscs is usually highest where turbulence is high and the substrate sediment contains fine hematite. Over about 2,000 km2, the water depth in Shark Bay is less than l m and fine sediment is readily suspended by strong winds. The iron-oxides (superfine hematite) are eroded from the Peron Sandstone exposed in some coastal cliffs and constitute up to 2% of substrate sediments near these cliffs. This study reveals that cadmium in the water adsorbs extremely efficiently onto the surface of the hematite, which is negatively charged at the prevailing seawater pH of 8.15, and that suspended hematite particles are ingested by the molluscs. Inside the molluscs, lower pH conditions cause reversal of the hematite charge and the cadmium is released and accumulated by the organism. Effectively, the hematite is pre-concentrating the cadmium and acting like a chemical conveyor-belt in enhancing metal transfer to the molluscs. A link between cadmium uptake by molluscs and hematite in the sediment is supported by the observation that in water with a depth of 3 m, bottom dwelling oysters contain up to 10 times more cadmium than oysters grown in baskets suspended near the surface at the same site. This mechanism for the transfer of cadmium to filter feeding organisms is new, but we have no doubt that it operates in similar environments elsewhere, and that at some sites where cadmium “pollution” has been reported, high metal loads in molluscs may be natural rather than anthropogenic. Conversely, in some environments the discharge of cadmium-free hematite could induce a rise in the cadmium load in the resident mollusc population.