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#           Mercury cycling in agricultural and managed wetlands of California: experimental evidence of vegetation-driven changes in sediment biogeochemistry and methylmercury production      

      July 29, 2013    

The role of live vegetation in sediment methylmercury (MeHg) production and associated biogeochemistry was examined in three types of agricultural wetlands (domesticated or white rice, wild rice, and fallow fields) and adjacent managed natural wetlands (cattail- and bulrush or tule-dominated) in the Yolo Bypass region of California's Central Valley, USA. During the active growing season for each wetland, a vegetated:de-vegetated paired plot experiment demonstrated that the presence of live plants enhanced microbial rates of mercury methylation by 20 to 669% (median = 280%) compared to de-vegetated plots. Labile carbon exudation by roots appeared to be the primary mechanism by which microbial methylation was enhanced in the presence of vegetation. Pore-water acetate (pw[Ac]) decreased significantly with de-vegetation (63 to 99%) among all wetland types, and within cropped fields, pw[Ac] was correlated with both root density (r = 0.92) and microbial Hg(II) methylation (kmeth. r = 0.65). Sediment biogeochemical responses to de-vegetation were inconsistent between treatments for “reactive Hg” (Hg(II)R), as were reduced sulfur and sulfate reduction rates. Sediment MeHg concentrations in vegetated plots were double those of de-vegetated plots (median = 205%), due in part to enhanced microbial MeHg production in the rhizosphere, and in part to rhizoconcentration via transpiration-driven pore-water transport. Pore-water concentrations of chloride, a conservative tracer, were elevated (median = 22%) in vegetated plots, suggesting that the higher concentrations of other constituents around roots may also be a function of rhizoconcentration rather than microbial activity alone. Elevated pools of amorphous iron (Fe) in vegetated plots indicate that downward redistribution of oxic surface waters through transpiration acts as a stimulant to Fe(III)-reduction through oxidation of Fe(II)pools. These data suggest that vegetation significantly affected rhizosphere biogeochemistry through organic exudation and transpiration-driven concentration of pore-water constituents and oxidation of reduced compounds. While the relative role of vegetation varied among wetland types, macrophyte activity enhanced MeHg production.

## Citation Information

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#### Lisamarie Windham-Myers

##### Biologist

Water Resources Mission Area

lwindham-myers@usgs.gov

650-329-4447

#### Craig Stricker, PhD

##### Research Biologist

Fort Collins Science Center

cstricker@usgs.gov

303-236-7908

#### Evangelos Kakouros

##### Hydrologist

Water Resources Mission Area

kakouros@usgs.gov

650-329-5784

## Related Content

#### Lisamarie Windham-Myers

##### Biologist

Water Resources Mission Area

lwindham-myers@usgs.gov

650-329-4447

#### Craig Stricker, PhD

##### Research Biologist

Fort Collins Science Center

cstricker@usgs.gov

303-236-7908

#### Evangelos Kakouros

##### Hydrologist

Water Resources Mission Area

kakouros@usgs.gov

650-329-5784

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####             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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