Trace metals in Gulf of Mexico oysters

Oysters (Crassostrea virginica) from 50 to 69 locations (sites) along the Gulf of Mexico coastline, collected annually in 1986, 1987 and 1988, have been analyzed for 13 trace metals, including most of the metals of concern from an environmental quality perspective. Essentially the entire U.S. Gulf coastline was sampled, from far south Texas to far south Florida. Pooled samples of 20 oysters from three different stations at each site were analyzed by atomic absorption spectrophotometry. The concentrations found were generally less than or equal to literature values from other parts of the world thought to be uncontaminated by anthropogenic trace metal inputs. A few sites did, however, show apparent trace metal pollution and other sites gave anomalous values that cannot readily be explained by either known anthropogenic or natural causes. The range of values for the overall data set (maximum/minimum) varied from 15-fold for Mn to 624-fold for Pb, whereas the coefficient of variation (standard deviation/mean) was generally in the 50-60% range for most metals. Variations were much greater between stations than between years at a given station. Enrichments usually occurred in suites of three to four elements with Ag, Cd, Cu and Zn being the most common suite, thus several strong inter-element correlations were found. There was, however, little correlation between metal levels in oysters and in sediments from the collection sites even when sediment data were rationed to Al (sediment data are not given here). There was likewise little correlation between oyster metal levels and size, sex or reproductive stage of the oysters (data given elsewhere). Geographically, appreciably elevated (greater than 3 times average) metal levels were generally restricted to single sites within bays or estuaries, implying local control. On the other hand, regionally, Ag, Cd and Se levels were somewhat higher in Texas oysters than in those from Florida, whereas the reverse was true for As and Hg. Concentrations were lower than average for several metals in oysters from central Louisiana, especially Ag, Cd and Cu. Thus, the Mississippi River outflow and extensive offshore oil development do not seem to enrich oysters in trace metals.