Seasonal,geographical and size-induced variability in mercury content of Mytilus edulis in an estuarine environment: a re-assessment of mercury pollution level in the Estuary and Gulf of St. Lawrence

Seasonal variability of mercury content of Mytilus edulis (L.) was studied at monthly intervals for 18 mo (June 1978–December 1979) in a natural population from a typical estuarine environment (St. Lawrence Estuary). Body-size-induced variability was observed in the same population in spring 1980. The regression coefficients of log-log relationships between mercury content and body size of the mussels varied with time. However, the regression coefficients were never higher than 1; this indicates, therefore, no net accumulation of mercury with age of the bivalve. A normalization method, taking into account shell length and soft-tissue dry weight, is proposed to minimize the metal variations due to size differences of the mussels. The normalized mercury contents vary seasonally by a factor of two. Minimum values were observed during summer. This factor (2) is taken as the minimal difference which can be accepted as indicating a significant geographical trend in a mussel watch-type study in estuarine environments. In the St. Lawrence Estuary and Gulf, size-normalized concentrations range from 69 to 378 ng Hg g^-1 (dry weight); highest levels were observed in areas where freshwater influence is maximum, while lower contents were encountered in regions of highest Atlantic influence. In the Gulf, mercury in mussel soft tissue is 3x10⁴ times that of the seawater. There is some indication of a decrease in the previous measured levels near the mouth of the Saguenay fjord in the Lower St. Lawrence estuary.