Microbial sulfate reduction (MSR) in both freshwater and marine ecosystems is a pathway for the decomposition of sedimentary organic matter (OM) after oxygen has been consumed. In experimental freshwater wetland mesocosms, sulfate additions allowed MSR to mineralize OM that would not otherwise have been decomposed. The mineralization of OM by MSR increased surface water concentrations of ecologically important constituents of OM: dissolved inorganic carbon, dissolved organic carbon, phosphorus, nitrogen, total mercury, and methylmercury. Increases in surface water concentrations, except for methylmercury, were in proportion to cumulative sulfate reduction, which was estimated by sulfate loss from the surface water into the sediments. Stoichiometric analysis shows that the increases were less than would be predicted from ratios with carbon in sediment, indicating that there are processes that limit P, N, and Hg mobilization to, or retention in, surface water. The highest sulfate treatment produced high levels of sulfide that retarded the methylation of mercury but simultaneously mobilized sedimentary inorganic mercury into surface water. As a result, the proportion of mercury in the surface water as methylmercury peaked at intermediate pore water sulfide concentrations. The mesocosms have a relatively high ratio of wall and sediment surfaces to the volume of overlying water, perhaps enhancing the removal of nutrients and mercury to periphyton. The presence of wild rice decreased sediment sulfide concentrations by 30%, which was most likely a result of oxygen release from the wild rice roots. An additional consequence of the enhanced MSR was that sulfate additions produced phytotoxic levels of sulfide in sediment pore water.
Bibliographical noteFunding Information:
This work was supported by the Clean Water Fund, created by the Clean Water, Land and Legacy Amendment to Minnesota’s constitution; by the Fond du Lac and Grand Portage Bands of Lake Superior Chippewa with band funds and water quality funds provided by the Environmental Protection Agency; by Minnesota Sea Grant; by NSF 0715808 to Pastor and others; and by NSF 0949962 to Myrbo and others. A partial data set is available in the EarthChem database: https://doi.org/10.1594/IEDA/ 100701. The full data set is available in the Data Repository for U of M (DRUM): https://doi.org/10.13020/D6595Z.
- acid-volatile sulfide
- internal nutrient loading
- radial oxygen loss
- terminal electron acceptor