Influence of seasonal growth, age, and environmental exposure on Cu and Ag in a bivalve indicator, Macoma balthica, in San Francisco Bay

Temporal and spatial variations in Cu and Ag in the deposit-feeding clam Macoma balthica and in surficial sediments were analysed at 8 stations in San Francisco Bay at near-monthly intervals for periods ranging from 3 to 10 yr during 1977 to 1986. Strong seasonal variations in metal concentrations of M. balthica were associated with seasonal variations in soft tissue weight. Aperiodic fluctuations in metal concentration appeared to be driven by changes in metal content of the soft tissues. Metal content of clams of standard shell length was less variable than tissue metal concentration, and generally followed changes in the concentrations of Cu and Ag in the sediments. Correlations between metal content and sediment concentrations were improved when content was standardized to age rather than shell length. Metal content of M. balthica displayed few consistent temporal trends among stations, evidently reflecting different sources of input and complex hydrologic and geochemical processes affecting metal availability in San Francisco Bay. Increases in Cu and Ag were noted at several stations in South Bay during 1977 to 1980. A continuous 10 yr record at one of these stations showed that the 1977 to 1980 increase and the subsequent decline beginning in 1981 coincided with fluctuations in metal inputs from a nearby source.