Elevated methyl mercury (MeHg) levels found in endangered species of fish and birds inhabiting tidal wetlands of the San Francisco Estuary have been attributed to the influx of elemental Hg derived from legacy gold mining in the Sierra Nevada, and sulfidic Hg derived from Hg mines in the Coast Ranges. But, these contaminated sediments contribute to the environmental hazard only if they are solubilized and methylated; processes likely facilitated in the peat-rich tidal wetlands of the Estuary. We sought to quantify the interactions among mercury species and elements of the dissolved and particulate organic matter (DOM, POM) pools in tidal wetlands, and the effects of tidal dynamics on these interactions. We used an in situ instrument package to measure spectral absorbance and fluorescence properties of DOM and POM, which were then correlated to Hg and MeHg concentrations using multivariate regression. Seasonal variations in OM concentration and character were measured in situ at 30-minute intervals over three seasons in month-long deployments. Total Hg and MeHg were measured in hourly discrete samples for a tidal cycle during each deployment. We also collected a suite of ancillary data including NO3-, HS, DO, pH, EC, and SSC. Variation in extrinsic spectral properties measured in situ were strongly related to filtered (r2=0.96) and unfiltered MeHg concentrations (r2=0.95) measured in discrete samples. The observed relationships were robust through both the spring and fall season, permitting the extrapolation of a small number of discrete samples to a full spring-neap tidal cycle. Examination of the full tidal cycle revealed interactions between wetland dynamics and Hg cycling, such as the lowered contribution of particulate MeHg at ebb tide, when the contribution from the MeHg associated with wetland DOM is greatest. Also, carbon quality parameters independent of concentration such as specific UV absorbance - related to the aromatic content of the DOM - are closely coupled to MeHg concentrations, suggesting preferential association with types or sources of DOM. The results of this study provide information about the net MeHg production and transport in Estuary wetlands, and insights into the relation between tidal wetland dynamics and mercury cycling.
Bergamaschi, B., J. Fleck, B. Downing, M. Stephenson, and E. Boss, 2006. Observed Relationships Between Mercury Species and Properties of Dissolved and Particulate Organic Matter Measured in Situ in a Tidal Wetland in the San Francisco Estuary. CALFED Science Conference, October 23-25.