INFLUENCE OF SOME ECOLOGICAL AND BIOLOGIC AL FACTORS ON METAL BIOACCUMULATION IN YOUNG OYSTERS (CRASSOSTREA-GIGAS THUNBERG) DURING THEIR SPAT REARING

Introduction Metal bioaccumulation in bivalves may occur as a conseque nce of the ingestion of inert or living particles which have bound tra ce elements. The best experiments, such as those carried out by Borcha rdt (1983, 1985) about Cd in mussels, have shown that suspended matter is only a small part of the source of contamination in molluscs. But whatever the reliability of the experimental methodologies, laboratory conditions never perfectly mimic natural phenomena. Thus, we have pla nned to restart these works using a large-scale experiment, the proced ure of which was characterized by a small number of controlled paramet ers with a high degree of representativeness of reality. During experi mental nursing of young oysters, we have assessed the influence of dif ferent controlled quantities of phytoplankton and of various experimen tal population densities on metal transfer from their environment to m olluscs. Materials and methods Oysters (Crassostrea gigas) were distri buted in several cylindrical containers the bottoms of which consisted of a sieve. Food and seawater were renewed continuously by means of a n ascending current (Baud and Bacher, 1990). Nursing assays were carri ed out during summer over a 74 day period or during winter over a 91 d ay period. Twelve groups of oysters were set up according to food supp ly (0, 1x, 2x and 4x of Skeletonema costatum grown on underground salt -water in addition to natural phytoplankton) and population density (2 5,000 or 50,000 individuals per experimental container) (Table 1). You ng molluscs were fed according to an alternating cycle of 3 h-feeding periods and 2 h-periods without food. The mean concentrations of pigme nt cells in the rearing seawater of the molluscs were 0, 10, 20 and 40 mug/l. This seawater was renewed at a flow rate of 3 m3/h during the summer experiment and 1 or 3 m3/h during winter. At the end of the nur sing period, molluscs exposed to different experimental conditions wer e separated using sieves with different mesh-sizes (6, 8, 10, 14 and 1 8 mm; Table 2). Young oysters were purged for 36 h with a view to limi ting the overvaluation of bioaccumulated metal levels due to ingested material (Amiard-Triquet et al., 1984; Kennedy, 1986). In each experim ental and age-related category, 90 individuals were sampled and divide d into 3 groups of 30 specimens. In these groups, soft tissues were se parated from the shells and oven-dried at 80-degrees-C for 48 h. The d ry samples were powdered and three aliquots of about 1 00 mg each were digested with 1 ml of concentrated nitric acid (HNO3, Suprapur) at 95 -degrees-C for 1 h. Then trace element analyses were performed in this solution diluted with deionized water by flame (Zn) or flameless (Cd, Cu, Pb) atomic absorption spectrophotometry using the Zeeman effect ( Amiard et al., 1987). The influence of food supplies, flow rate and ex perimental population density on the dry weight of the soft tissues of young oysters, their metal concentrations and body burdens were exami ned using multi-linear regression analysis (Tomassone et al., 1983; Da gnelie, 1973). Results and discussion In Table 3 the metal concentrati ons in the seawater [Table 3(a)], phytoplankton and seston [Table 3(b) ] are shown. In summer, the average weight and metal (Cd, Cu, Pb, Zn) burden in the soft tissues of the oysters increased with the quantity of food available and the size of the individuals, whereas it decrease d concomitantly with increasing density of the experimental population of oysters (Table 4). The individual weights increased with increasin g food supply (Fig. 2). The increase of food quantity and of the size of the oysters was concomitant with enhancement of the weight of their soft tissues and a decrease of metal concentrations (Table 5). In win ter, the population density was correlated negatively with the weight of soft tissues but not with metal concentrations or burdens (Table 6) . In summer, the experimental population density was correlated with C u concentration [Table 7 and Fig. 5(B)] but not with Cd, Pb or Zn conc entrations [Fig. 5(A) and (B)]. The oyster size was correlated negativ ely with Cd and Pb concentrations during summer [Figs 3 and 5(B)] and with Cu and Zn concentrations during winter (Fig. 2 and Table 8). The influence of season was clearly established (Fig. 4): in summer, dry w eights were two times higher than in winter and Cd, Cu and Zn concentr ations and body burdens were more elevated in winter whereas the oppos ite pattern was shown for Pb. The influences of food supply, populatio n density and individual size on the animal's weight, metal concentrat ions and corresponding body burdens are summed up in Table 9 for both the studied seasons. Conclusion Increased food supplies induce a biolo gical dilution of Cd and Pb in young oysters. These results are in agr eement with previous data about Cd, Cu, Pb and Zn in different species (Mackay et al., 1975; Boyden, 1974; Phelps and Hetzel, 1987; Berthet, 1986). Thus from the sanitary point of view, the use of underground s eawater for algal culture is not at risk since metal concentrations in molluscs are not enhanced.