Museum Preserved Bivalves as Indicators of Long-term Trends in Methylmercury Concentrations

Abstract:

Despite the many efforts to reduce mercury concentrations in the environment, there are relatively few datasets on long-term trends in mercury in biota, especially for the bioavailable form, methylmercury (MeHg). This study used museum preserved bivalves (stored in ethanol) to look at MeHg trends in the Asian date mussel Musculista senhousia and the Asian clam Potamocorbula amurensis, collected from San Francisco Bay, California between 1975 and 2012. For each sampling date, 4 to 15 individuals were obtained from museum collections (N = 156 total specimens), freeze-dried, weighed, homogenized, digested, and individually analyzed for MeHg using trace metal clean techniques. The bivalves were also analyzed for δ¹³C and δ¹⁵N to look for changes in food web structure. P. amurensis specimens were only available from 1988 to 2012, and an increase in MeHg was observed during that time. In contrast, M. senhousia specimens were available for the entire 37 year period and exhibited a significant decline in MeHg in the southern reach of the estuary (South Bay). The median MeHg concentration in M. senhousia was highest at 239 ng/g dw in October 1975. That year was the last year of operations for the New Almaden Mercury Mining District, which drained into South Bay. By the 1990s, MeHg concentrations in M. senhousia dropped significantly to a median of 37 ng/g dw. Isotopic δ¹⁵N values did not support a hypothesis of reduced trophic position causing the MeHg decline. Over the study duration, δ¹⁵N increased in M. senhousia, which we attributed to a baseline shift. We also observed a decline in δ¹³C since 2000, which may represent a shift in bivalve carbon towards greater utilization of planktonic sources. To validate the use of museum specimens, we ran a preservation study, where we collected fresh bivalves, fixed them in ethanol or formalin, and then transferred them to ethanol for long-term storage. Although MeHg concentrations increased after 1 week, they stabilized over time, indicating the suitability of preserved bivalves for collection of historical MeHg data. To our knowledge, this is the first study to report a long-term decrease in MeHg in San Francisco Bay biota. Our results suggest that aquatic biota MeHg exposure has declined since the 1970s, likely due to curtailment of historical mercury mining and industrial use.