Agricultural wetlands, such as rice fields, may be important contributors to methylmercury contamination, due to their global prevalence, periodic flooding schedules and high use by wildlife.
In a 2010 article published in Environmental Science and Technology
, USGS scientists Dr. Josh Ackerman and Dr. Collin Eagles-Smith assessed methylmercury bioaccumulation within agricultural and permenant wetlands common to California’s Central Valley, during the summer when the majority of wetland habitats are shallowly-flooded rice fields.
The authors introduced western mosquitofish (Gambusia affinis) into cages in white rice fields, wild rice fields and permanent wetlands, with cages placed at water intake inlets, field centers, and water discharge outlets. Wild mosquitofish and wild Mississippi silversides (Menidia audens) were also sampled in the three wetland types.
After only 60 days, mercury levels in caged mosquito-fish increased rapidly in all locations. Total mercury concentrations exceeded initial concentrations by 135% to 1,197%, while mercury body burdens increased by 29% to 1,566%.
Mercury bioaccumulation in caged mosquitofish was greater in rice fields than in permanent wetlands. For fish caged at water outlets, mercury levels were 12 times greater than initial concentrations for white rice fields after 60 days. The levels were 6 times greater for wild rice fields and only 3 times greater for permanent wetlands, respectively. In fact, mosquitofish caged at white rice outlets accumulated 721 ng of mercury per fish in just 60 days.
Wild fish concurrently sampled at water outlets in white rice fields and wild rice fields also had more mercury than those in permanent wetland fields. Within a white rice field, mercury concentrations and body burdens of both caged and wild fish exhibited an increasing gradient from water inlets to outlets. This gradient was not found in permanent wetlands.
Overall, fish mercury concentrations in agricultural wetlands were considered high. In 82% of caged fish and in 59% of wild fish, mercury concentrations exceeded 0.2 μg/g wet weight, a commonly used fish toxicity threshold.
Wetlands provide numerous ecosystem services, but they also can be sources of methylmercury production and export. These results suggest that rice agricultural wetlands in particular may be important hotspots for methylmercury bioaccumulation, and warrant additional research.
White-faced Ibis foraging in a white rice field in California’s Central Valley, which holds over 555,000 acres of white and wild rice fields. Image: Josh Ackerman/USGS.