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- Mercury cycling in agricultural and managed wetlands: A synthesis of methylmercury production, hydrologic export, and bioaccumulation from an integrated field study
- 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.
- Windham-Myers, Fleck, Ackerman, Marvin-DiPasquale, Stricker, Heim, Bachand, Eagles-Smith, Gill, Stephenson, Alpers
- Spatial and habitat-rased variations in total and methyl mercury concentrations in surficial sediments in the San Francisco Bay-Delta,
- Recent studies indicate significant amounts of mercury (Hg) are annually transported into the San Francisco Bay-Delta (Bay-Delta) as a result of historic gold and Hg mining activities. We examined temporal and spatial variation in concentrations of total Hg (HgT) and monomethylmercury (MMHg) in surficial sediments of various ecosystem types in the Bay-Delta. We sampled surficial sediments across the Bay-Delta system and found HgT sediment concentrations in the central Delta were generally 100-200 ng g -1 and increased westward through Suisun Bay to 250-350 ng g -1. MMHg concentrations in the central Delta were between 1 and 3 ng g-1, while those in sediments in the perimeter waterways and adjacent bays were less than 1 ng g-1. Six sites were monitored monthly for over a year to identify seasonal changes in Hg sediment concentrations. Hg T sediment concentrations ranged from 48 to 382 ng g-1 and varied as a function of location not season. However, MMHg concentrations varied seasonally, increasing from 1 ng g-1 during winter months to 6 ng g-1 during spring and summer. Transects conducted at three marshes in the central Delta revealed MMHg sediment concentrations of 4-8 ng g-1at the interior and 2 ng g-1 at the exterior of the marshes. Habitat type was a major factor controlling MMHg concentration and the MMHg to HgT ratio in sediments of the Bay-Delta. MMHg was significantly correlated to HgT (r2 = 0.49) in marsh sediments. © 2007 American Chemical Society., Cited By (since 1996):24, Oceanography, CODEN: ESTHA, ,
- Heim, Coale, Stephenson, Choe, Gill, Foe