TEG Conference Proceedings 104 Cadmium in Oysters and Sediments From Foveaux Strait, New Zealand. Russell D. Frew and Keith A. Hunter, Department of Chemistry, University of Otago, Dunedin, New Zealand. Richard Beyer, Institute of Applied Science, University of the South Pacific, Suva, Fiji. zyxwvutsrqponmlkjihgfedcbaZY Abstract zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Reports that the commercially-fished oysters in Foveaux StraitzyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (Tiostrea chilensis) contain unusually high levels of Cd were confirmed in the present study in which 436 individual oysters firom 29 sites were analysed for their Cd content. The geographical distributions of Cd concentrations in both oysters and sediments of Foveaux Strait were investigated. Average dry weight concentrations exceeded 20 |j.g g''. The age of each individual oyster was determined from the shell mass per unit area, allowing calculation of the net rate of Cd uptake into body tissue over the life of the organism. Highest Cd concentrations and uptake rates were found nearest Stewart Island; this usually corresponding to the region of greatest Cd concentration in the sediments. In general, the highest sediment concentrations were found in the deepest regions of the sea floor, suggesting that Cd is transported into the region largely in a particulate form. It is proposed that phytoplankton uptake of Cd in the Subtropical Convergence region produces this Cd-rich particulate matter. Introduction The direct monitoring of trace metals in the marine environment is often made difficult by short-term variations in the concentrations of trace metals in water. To avoid the analysis of an extensive number of water samples, both marine organisms such as molluscs (Darracott and Watling, 1975; Goldberg et al. 1978; Stephenson et al, 1979; Popham et al. 1980) and sediments (Cauwet, 1987) have been used as indicators of trace metal contamination. Both have the advantages that they are believed to integrate temporal and spatial variations and to concentrate most trace elements to an extent which greatly simplifies subsequent analysis (Martin, 1979). This approach was adopted in the study of Cd in Foveaux Strait by collecting samples of sediments and oysters from this region. The Foveaux Strait dredge oysters Tiostrea chilensis (formerly known as Ostrea lutaria, Buroker et al, 1983) contain exceptionally high levels of Cd (Brooks and Rumsby, 1965; Frew et al., 1989). Nielsen (1975) measured Cd concentrations approaching 9 ^g g ' wet weight in oysters fi-om this region. The source of the elevated Cd concentrations is not obvious as the area near Foveaux Strait is sparsely populated and relatively free of industry likely to cause this contamination. Nielsen (1975) also reported that higher Cd concentrations were found in oysters to the west of Foveaux Strait than to the east. He suggested that the source of Cd was to the west of Foveaux Strait and possibly in Fiordland. However, this study was based on a limited number of samples. In the present paper we report on a large-scale study of Cd in oysters ft-om this region in which a large number of individual organisms of known age were analyzed for Cd. Our study has sought to relate Cd incorporation into the oysters to factors such as age, weight and the geographical distribution of Cd in the fine fraction of bottom sediments. ,., Materials and Methods zyxwvutsrqponmlkjihgfedcbaZYXWVU Sample Collection The geographical distribution of Cd in Foveaux Strait oysters and sediments was investigated using samples collected on two cruises of the RA' Munida. The first sampling cruise carried out in May 1989 involved 29 stations encompassing the region of the commercial oyster beds (Fig. la). The second sampling cruise in May 1990 involved a further 14 stations around Stewart Island (Fig. lb). Station 7 of the first cruise was reoccupied to test the reproducibility of results; this was station 40 of the second cruise. Oysters were collected in a steel cage dredge towed behind the vessel. They were immediately placed into a nylon sack and suspended in clean seawater for about 24 h for depuration (Kerfoot and Jacobs, 1974). The sacks of depurated oysters were then placed into a large plastic bag and frozen for transport back to the laboratory where they were stored at -20°C for analysis. The minimum sample size for each station required to obtain statistically significant results was calculated in two ways. From the results of a previous study on Tiostrea chilensis, presented by Frew et al. (1989), a mean Cd concentration of 30.6 jj,g g' (dry weight) and a standard deviation of 14.6 p-g g"' were calculated for n=40 at a single site. In order that the standard error of the mean be less than 10% of the mean concentration, the sample size must be n=23 or greater at each site. In total, 436 oysters from 29 stations were analyzed for their Cd content. Sampling of sediments from the RA' Munida was achieved with a bucket dredge that was towed beside the boat at the same time as the oyster dredge was deployed. Samples were dumped from the dredge into