PNNL

Abstract

The seasonal wetting and drying of shallowly-flooded wetlands, such as rice paddies, may enhance the conversion of inorganic mercury (Hg) to the methylmercury (MeHg), the more toxic organic form which biomagnifies in foodwebs. Yet, the net balance of MeHg cycling in such ephemeral wetland environments is poorly understood because it requires an annual, integrated assessment across biota, sediment, and water components. We examined a suite of agricultural and managed wetland habitats from 2007-2008 in California’s Central Valley, which is known to be affected by Hg contamination from historic mining practices. Hydrologic management of rice and fallow fields – specifically the drying of soils for field preparation and harvest, and flooding of soils for crop growth and post-harvest rice straw decay – led to pronounced seasonality and increased sediment and surface water MeHg concentrations above those measured in adjacent permanently flooded wetlands and a winter-flooded seasonal wetland. Still, initial MeHg concentrations in source water largely determined whether a wetland served as a source or sink of MeHg downstream. Despite sediment microbial production of MeHg in all wetlands, MeHg degradation and storage were preferentially enhanced by the high residence times. Vertical hydrologic fluxes led to 2-3fold evapoconcentration for MeHg and temporary root-zone storage due to downward-advection to meet transpiration demand. The apparent tradeoff for limiting hydrologic outflows was increased MeHg exposure to resident fish via greater aqueous MeHg concentrations. Our results suggest that the combined traits of agricultural wetlands - slow-moving shallow water, frequent flooding and drying cycles, high production of labile plant matter, and high use by wildlife - may enhance local and regional MeHg exposure via in situ bioaccumulation in rice seeds, in resident fish and wildlife during the growing season, and via export to downstream habitats during uncontrolled winter flow events.