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#           Methylmercury production in sediment from agricultural and non-agricultural wetlands in the Yolo Bypass, California, USA      

      June 18, 2014    

As part of a larger study of mercury (Hg) biogeochemistry and bioaccumulation in agricultural (rice growing) and non-agricultural wetlands in California's Central Valley, USA, seasonal and spatial controls on methylmercury (MeHg) production were examined in surface sediment. Three types of shallowly-flooded agricultural wetlands (white rice, wild rice, and fallow fields) and two types of managed (non-agricultural) wetlands (permanently and seasonally flooded) were sampled monthly-to-seasonally. Dynamic seasonal changes in readily reducible ‘reactive’ mercury (Hg(II)R), Hg(II)-methylation rate constants (kmeth), and concentrations of electron acceptors (sulfate and ferric iron) and donors (acetate), were all observed in response to field management hydrology, whereas seasonal changes in these parameters were more muted in non-agricultural managed wetlands. Agricultural wetlands exhibited higher sediment MeHg concentrations than did non-agricultural wetlands, particularly during the fall through late-winter (post-harvest) period. Both sulfate- and iron-reducing bacteria have been implicated in MeHg production, and both were demonstrably active in all wetlands studied. Stoichiometric calculations suggest that iron-reducing bacteria dominated carbon flow in agricultural wetlands during the growing season. Sulfate-reducing bacteria were not stimulated by the addition of sulfate-based fertilizer to agricultural wetlands during the growing season, suggesting that labile organic matter, rather than sulfate, limited their activity in these wetlands. Along the continuum of sediment geochemical conditions observed, values of kmeth increased approximately 10,000-fold, whereas Hg(II)R decreased 100-fold. This suggests that, with respect to the often opposing trends of Hg(II)-methylating microbial activity and Hg(II) availability for methylation, microbial activity dominated the Hg(II)-methylation process, and that along this biogeochemical continuum, conditions that favored microbial sulfate reduction resulted in the highest calculated MeHg production potential rates. Rice straw management options aimed at limiting labile carbon supplies to surface sediment during the post-harvest fall–winter period may be effective in limiting MeHg production within agricultural wetlands.

## Citation Information

## Related Content

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#### Lisamarie Windham-Myers

##### Biologist

Water Resources Mission Area

lwindham-myers@usgs.gov

650-329-4447

#### Evangelos Kakouros

##### Hydrologist

Water Resources Mission Area

kakouros@usgs.gov

650-329-5784

#### Jacob A Fleck

##### Research Hydrologist

California Water Science Center

jafleck@usgs.gov

916-278-3063

#### Charles N Alpers

##### Research Chemist

California Water Science Center

cnalpers@usgs.gov

916-278-3134

#### Craig Stricker, PhD

##### Research Biologist

Fort Collins Science Center

cstricker@usgs.gov

303-236-7908

## Related Content

#### Lisamarie Windham-Myers

##### Biologist

Water Resources Mission Area

lwindham-myers@usgs.gov

650-329-4447

#### Evangelos Kakouros

##### Hydrologist

Water Resources Mission Area

kakouros@usgs.gov

650-329-5784

#### Jacob A Fleck

##### Research Hydrologist

California Water Science Center

jafleck@usgs.gov

916-278-3063

#### Charles N Alpers

##### Research Chemist

California Water Science Center

cnalpers@usgs.gov

916-278-3134

#### Craig Stricker, PhD

##### Research Biologist

Fort Collins Science Center

cstricker@usgs.gov

303-236-7908

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####               Sacramento Projects Office (USGS California Water Science Center)          

6000 J Street, Placer HallSacramento, CA95819United States

916-278-3000

916-278-3070

####             Water Resources Mission Area - Headquarters        

USGS Headquarters12201 Sunrise Valley DriveReston, VA20192United States

####               Ecosystems Mission Area Headquarters          

12201 Sunrise Valley DrReston, VA20192United States

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