Résumés
Résumé
La bioaccumulation des métaux chez les mollusques peut être une conséquence de la désorption des éléments métalliques fixés sur les particules inertes ou vivantes en suspension dans l'eau qui leur servent de nourriture. Les meilleures expériences réalisées jusqu'ici semblent indiquer que les matières en suspension ont un rôle mineur dans la contamination des mollusques. Mais quelles que soient les précautions prises, les expériences ne simulent jamais parfaitement les phénomènes naturels. Aussi, nous avons choisi de reprendre cette question à l'aide d'une expérience en grand volume où le nombre de paramètres contrôlés est restreint mais où les organismes sont dans des conditions aussi proches que possible de la réalité.
Au cours d'un prégrossissement expérimental, les jeunes palourdes reçoivent des quantités variables contrôlées de nourriture phytoplanctonique. La fourniture d'une nourriture plus abondante a pour conséquence d'augmenter les quantités de cuivre et de zinc et au contraire de diminuer celles de plomb contenues dans chaque individu. Elle entraîne également une diminution des concentrations en cadmium et plomb traduisant une « dilution biologique » de ces métaux. L'augmentation de la densité de la population expérimentale a un effet négatif sur les quantités de cuivre et de zinc contenues dans les individus. A âge identique, les individus les plus grands présentent des concentrations plus faibles en cuivre et zinc et plus élevées en cadmium et plomb.
L'utilisation des eaux marines souterraines présente deux avantages: une production algale intense à un coût économique négligeable et la possibilité par échange thermique de réchauffer en hiver les eaux marines naturelles et ainsi de maintenir une croissance des mollusques toute l'année. L'emploi des eaux de forage n'entraîne aucun effet néfaste dans les phénomènes de bioaccumulation des métaux, du moins pour ceux étudiés ici : cadmium, cuivre, plomb et zinc.
Mots-clés:
- Palourde,
- métaux,
- facteurs écologiques,
- bioaccumulation,
- pré-grossissement
Abstract
Metal bioaccumulation in bivalves may occur as a consequence of the ingestion of inert or living particles with fixed trace elements. The best experiments like those carried out by BORCHARDT (1983, 1985) concerning Cd in mussels have shown that the role suspended matter plays in the contamination of molluscs is insignificant. But, however reliable the experimental methodologies, laboratory conditions never reproduce perfectly natural phenomena. We planned therefore to restudy the problem by using a largescale experiment where the procedure was characterized by a restricted number of controlled parameters and the organisms as close as possible to the real conditions.
During an experimental nursing of young carpet shells, we assessed the influence of various controlled quantifies of phytoplankton and of experimental population density on the transfer of metal from their environment to molluscs.
Carpet-shell brood (Ruditapes philippinarum) was distributed in several cylindrical containers the bottom of which consisted in a sieve. Food and seawater were renewed continuously by means of an ascending current (BAUD et BACHER, 1990). Nursing assays were carried out during summer over a period of 74 days. Eight groups of carpet shells were constituted according to food supplies (0, lx, 2x and 4x of Skeletonema costatum grown upon underground seawater plus natural phytoplankton) and population density (25 000 or 50 000 individuals per experimental container). Young molluscs were fed according to a cycle of 3 h-feeding periods and 2 h-periods with no food alternately. The average concentrations of algal cells in mollusc breeding seawater were 17.5, 35 and 70.103 cells/L. This seawater was renewed at a flow rate of 3 m3/h.
At the end of the nursing period, molluscs exposed to different experimental conditions were separated by using sieves of different mesh-size (6, 8 and 10 mm). Young carpet shells were purged for 36 h in order to limit the overvaluation of bioaccumulated metal levels due to ingested matter (AMIARD-TRIQUET et al., 1984; KENNEDY, 1986). In each experimental and size-related categories, 90 individuals were sampled and divided into 3 groups of 30 specimens.
In these groups, soft tissues were separated from the shells and oven-dried at 80 °C for 48 h. The dry samples were powdered and three aliquot parts of about 100 mg each were digested with 1 ml of concentrated nitric acid (HNO3, Suprapur) at 95 °C for 1 h. Then the trace element analyses were performed in this solution diluted with deionized water by dame (Zn) or by flameless (Cd, Cu, Pb) atomic absorption spectrophotometry using the Zeeman effect (AMIARD et al., 1987).
The influence of both food supplies and experimental population density on the dry weight of soil tissues of young carpet shells, their metal concentration and body burdens were examined by means of multi-linear regression analysis.
Increasing body burdens of Cu (4) and Zn (5) and decreasing body burden of Pb (3), corresponded to more abundant food supplies. The increase of phytoplankton supplies induced a decrease of Cd (6) and Pb (7) concentrations as a consequence of a « biological dilution » of these metals. Increasing density induced a depletion of Cu (4) and Zn (5) body burdens. Among individuals of the same age, the biggest ones exhibited the lowest concentrations of Cu and Zn (8 and 9) and the highest concentrations of Cd and Pb (6 and 7).
Increased food supplies induce a biological dilution of Cd and Pb in young carpet-shells. These results are in agreement with previous data concerning Cd, Cu, Pb and Zn in different species (MACKAY et al., 1975; BOYDEN, 1971; PHELPS et al., 1985; BERTHET, 1986). Thus front a sanitary point of view, the use of ground seawater for algal culture is not a risk since metal concentrations in molluscs are not enhanced.
Keywords:
- Carpet shell,
- bioaccumulation,
- heavy metals,
- nursing,
- ecological factors