Geochemical controls on production and transport of methylmercury in the St. Louis River Estuary

Abstract

The production of methylmercury (MeHg) in freshwater systems is a concern due to its bioaccumulative properties in aquatic food webs and potential neurotoxicity of consumers of fish, including predatory fish, wildlife, and humans. Transformation of inorganic mercury to MeHg is primarily driven by naturally occurring sulfate and iron reducing bacteria (SRB and FeRB) in anoxic sediment and water columns. The first objective of this research was to examine factors known to be important in the production of MeHg (including the quantity and quality of organic carbon, sulfate concentrations, and bioavailability of inorganic mercury to SRB and FeRB) in a historically sulfate-impacted freshwater setting. The second objective was to determine the significance of MeHg flux from sediment to the St. Louis River estuary relative to upstream sources. A laboratory sulfate addition experiment was performed using 20 cm wide intact cores obtained from three sites in the St. Louis River estuary. The intact cores from each site were exposed to a high (50 mg/L sulfate), control (15 mg/L sulfate), and low (5 mg/L sulfate) overlying water treatment for an incubation period of six months. Results from this six-month sulfate incubation indicated that when overlying water sulfate concentrations increased to 50 mg/L, MeHg production did not increase in surficial sediment (0 to 4 cm) in any sites. In sediment cores with sulfate concentrations lowered to 5 mg/L in the overlying water, production of sediment methyl mercury did not decrease in surficial sediment (0 to 4 cm); however, the MeHg concentration in sediment from the 4 to 10 cm interval was lower at the end of the six month incubation in sediment from one site. Collectively, results indicate that, in sediment from the St. Louis River estuary, the quantity and quality of carbon in sediments were more important factors for MeHg production than overlying water sulfate. In addition to understanding geochemical controls on MeHg production, flux measurements provided a means to compare MeHg loading from sediment relative to MeHg loading from upstream sources. The estimated MeHg loading from habitat zones represented by the three sites included in this study (45% of total estuary area) contributed a small amount MeHg to overlying water relative to upstream sources during low, median, and high flow conditions.