Methylmercury and Low Dissolved Oxygen Events in Suisun Marsh
Several low dissolved oxygen (DO) events resulting from environmental conditions and management actions in some managed wetlands in northern Suisun Marsh adversely impact the aquatic ecosystem of various sloughs. Co-occurring with these events are elevated methyl mercury (MeHg) conditions. Most managed wetlands are dry during the summer and early fall months when land managers carry out maintenance activities. Fall flood-up typically involves a series of one or more flood-drain-flood cycles after which the wetlands circulate water through the winter and drain at variable times in the spring depending on particular property management objectives. The fall flood-up cycle can create low DO waters that upon release send low DO plumes into receiving waters (tidal sloughs) impacting aquatic organisms, including killing at-risk fish species and impairing valuable fish nursery habitat. Pond releases are also rich in organic matter that stimulates microbial activity increasing biological oxygen demand (BOD) in the receiving waters, which also reduces DO, and results in conditions amenable to MeHg formation. MeHg is a neurotoxin that transfers up the food chain through bioaccumulation and adversely effects fish and wildlife species and poses a health risk to humans.
This study is a large multi-disciplinary multi-investigator study approved by CALFED and funded by the State Water Quality Control Board through the Association of Bay Area Governments. The primary purpose of the USGS portion of this proposed study is to evaluate if spectrophotometric and spectrofluorometric methods are useful for identifying organic sources of oxygen demand by analyzing water-quality samples (DO, BOD, Chl, SSC, Salinity, THg, MeHg) collected by other agencies and project participants. Sources to be evaluated include algal production, vegetation, soils, and wastewater, though the proposal lumps these conceptually as “soil” sources. A secondary purpose of the USGS portion of the overall study is to collaborate with other project participants in some hydrodynamic equipment deployments and, in conjunction with these deployments, to explore whether a combined hydrodynamic and monitoring program may be useful for managing water quality in Suisun Marsh.
Project Map provided by California Department of Fish and Game here:
Project Chief: Brian Bergamaschi