title: Mercury cycling in agricultural and managed wetlands: a synthesis of methylmercury production, hydrologic export, and bioaccumulation from an integrated field study.
abstract: With seasonal wetting and drying, and high biological productivity, agricultural wetlands (rice paddies) may enhance the conversion of inorganic mercury (Hg(II)) to methylmercury (MeHg), the more toxic, organic form that biomagnifies through food webs. Yet, the net balance of MeHg sources and sinks in seasonal wetland environments is poorly understood because it requires an annual, integrated assessment across biota, sediment, and water components. We examined a suite of wetlands managed for rice crops or wildlife during 2007-2008 in California's Central Valley, in an area affected by Hg contamination from historic mining practices. Hydrologic management of agricultural wetlands for rice, wild rice, or fallowed - drying for field preparation and harvest, and flooding for crop growth and post-harvest rice straw decay - led to pronounced seasonality in sediment and aqueous MeHg concentrations that were up to 95-fold higher than those measured concurrently in adjacent, non-agricultural permanently-flooded and seasonally-flooded wetlands. Flooding promoted microbial MeHg production in surface sediment of all wetlands, but extended water residence time appeared to preferentially enhance MeHg degradation and storage. When incoming MeHg loads were elevated, individual fields often served as a MeHg sink, rather than a source. Slow, horizontal flow of shallow water in the agricultural wetlands led to increased importance of vertical hydrologic fluxes, including evapoconcentration of surface water MeHg and transpiration-driven advection into the root zone, promoting temporary soil storage of MeHg. Although this hydrology limited MeHg export from wetlands, it also increased MeHg exposure to resident fish via greater in situ aqueous MeHg concentrations. Our results suggest that the combined traits of agricultural wetlands - slow-moving shallow water, manipulated flooding and drying, abundant labile plant matter, and management for wildlife - may enhance microbial methylation of Hg(II) and MeHg exposure to local biota, as well as export to downstream habitats during uncontrolled winter-flow events.
authors: U.S. Geological Survey, Western Region Branch of Regional Research, 345 Middlefield Road/MS 480, Menlo Park, CA 94025, USA. Electronic address: lwindham@usgs.gov.
authors: Lisamarie
authors: L
authors: Windham-Myers
authors: U.S. Geological Survey, California Water Science Center, 6000 J St, Placer Hall, Sacramento, CA 95819, USA. Electronic address: jafleck@usgs.gov.
authors: Jacob A
authors: JA
authors: Fleck
authors: U.S. Geological Survey, Western Ecological Research Center, Dixon Field Station, 800 Business Park Drive, Suite D, Dixon, CA 95620, USA. Electronic address: jackerman@usgs.gov.
authors: Joshua T
authors: JT
authors: Ackerman
authors: U.S. Geological Survey, Western Region Branch of Regional Research, 345 Middlefield Road/MS 480, Menlo Park, CA 94025, USA. Electronic address: mmarvin@usgs.gov.
authors: Mark
authors: M
authors: Marvin-DiPasquale
authors: U.S. Geological Survey, Fort Collins Science Center, Building 21, MS 963, Denver, CO 80225, USA. Electronic address: cstricker@usgs.gov.
authors: Craig A
authors: CA
authors: Stricker
authors: Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA 95039, USA. Electronic address: wheim@mlml.calstate.edu.
authors: Wesley A
authors: WA
authors: Heim
authors: Tetra Tech, 509 4th St., Suite D, Davis, CA 95616, USA. Electronic address: philip.bachand@tetratech.com.
authors: Philip A M
authors: PA
authors: Bachand
authors: U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, 3200 SW Jefferson Way, Corvallis, OR 97331, USA. Electronic address: ceagles-smith@usgs.gov.
authors: Collin A
authors: CA
authors: Eagles-Smith
authors: Pacific Northwest National Laboratory, Marine Sciences Laboratory, 1529 West Sequim Bay Road, Sequim, WA 98382, USA. Electronic address: gary.gill@pnnl.gov.
authors: Gary
authors: G
authors: Gill
authors: Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA 95039, USA. Electronic address: mstephenson@mlml.calstate.edu.
authors: Mark
authors: M
authors: Stephenson
authors: U.S. Geological Survey, California Water Science Center, 6000 J St, Placer Hall, Sacramento, CA 95819, USA. Electronic address: cnalpers@usgs.gov.
authors: Charles N
authors: CN
authors: Alpers
mesh_terms: D000383:Agriculture; D002140:California; D055669:Ecological and Environmental Phenomena; D004784:Environmental Monitoring; D020387:Food Chain; D062070:Hydrology; D008628:Mercury / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000032:analysis*; D053833:Wetlands
chemical_list: D014874:Water Pollutants, Chemical; D008628:Mercury
keywords: Bioaccumulation; Hydrology; Mercury; Microbes; Rice; Wetlands