Polluants métalliques et chaînes alimentaires marines

Heavy metal pollution in marine food chains

¹ INSERM U.303 Mer et Santé - Laboratoire de Toxicologie Marine Faculté de Médecine - Université de Nice Sophia-Antipolis - 06107 Nice Cedex 2 (France)
² Laboratoire de Toxicologie Marine - Faculté de Médecine Université de Nice Sophia-Antipolis - 06107 Nice Cedex 2 (France)

Résumé

Les concentrations en métaux traces (Cd, Cu, Hg, Pb et Zn) ont été déterminées à différents niveaux d'une chaîne alimentaire côtière de Méditerranée (phytoplancton, zooplancton, et crustacés nectoniques). La synthèse de nos résultats montre qu'en général les concentrations en métaux sont plus faibles dans les prédateurs que dans les proies. Cependant des poissons tels que le thon, la bonite et le congre présentent, selon différents auteurs, des concentrations en mercure plus fortes que celles des maillons inférieurs. Ce problème de contamination en mercure de certains poissons de Méditerranée (phénomène qui est peut-être naturel) reste posé.

D'autre part, des déterminations de métaux traces, associées à des mesures de calcium total, ont été effectuées dans des zones de pollution reconnue sur des organes-cibles, les branchies de poissons et de moules. Ces organismes marins sont situés au sommet de chaînes alimentaires. Ces déterminations tendent à montrer que des analyses de calcium (faciles, rapides et peu onéreuses) pourraient servir d'indicateur de stress toxique général pour évaluer la pollution.

Abstract

Trace metal (Cd, Cu, Hg, Pb and Zn) concentrations were determined at different levels of a Mediterranean food chain (phytoplankton, zooplankton, nektonic crustaceans). The synthesis of our results showed that generally metal concentrations are lower in predators than in preys. Nevertheless, fish species such as tuna, bonito, conger present higher mercury concentrations than lower trophic levels, as reported in the literature. These concentrations may exceed the recommendations done for human consumption. The problem of mercury contamination in fish (which may be a natural phenomenon) still remains in the Mediterranean.

Moreover, determinations of trace metals associated with total calcium measurements were carried out in target-organs, the gills of fish (comber Serranus cabrilla and red mullet Mullus barbatus) and of mussel (Mytilus galloprovincialis). These marine organisms, situated at the highest level of marine food webs, were collected in several locations of the North-Western Mediterranean Sea, some of which were known to be polluted. Cadmium concentrations in Mytilus gallo-provincialis gills were higher in some samples collected in November 1989 compared to those of July 1990. The results may indicate a temporary local pollution. In this case, high values of cadmium were associated with high values of calcium. The same trend for cadmium concentrations was found in the gills of Serranus cabrilla collected together with the mussels, nevertheless high values of cadmium were not associated with high values of calcium. High concentrations of cadmium, copper, iron and zinc were observed in the gills of Mullus barbatus collected near the outlet of waste waters from the town of Marseille. These values were associated with high concentrations of calcium. Iron concentrations were even significantly correlated with calcium concentrations. The gills of Serranus cabrilla collected together with Mullus barbatus in the plume of waste waters showed high concentrations of copper and iron, but not correlated with calcium contents. Although these two fish species belong to the meadow of the sea grass Posidonia oceanica, they are different, since Mullus barbatus lives nearer to the sediments (which are a sink for pollutants) than Serranus cabrilla, this could explain the differences found in metal concentrations between both animals. The determinations done aimed at evaluating whether total calcium analyses (easy to carry out, rapid and cheap) could be used as general toxic stress indices to monitor pollution. This could be true in the case of the gills of Mytilus galloprovincialis and of Mullus barbatus but not in the case of Serranus cabrilla. Gills generally presented higher metal concentrations than those found in the mussels in toto or in the muscles of fish. Nevertheless the weight of these organs is low compared to the total weight of animals. This means that cadmium, copper, iron and zinc concentrations in fish or mussels belonging to benthonic food chains of the NW Mediterranean are below the norms established for consumption.