Lisa M. Gardner, Oceanography and Coastal Sciences, Louisiana State University, Wetland Biogeochemistry Institute, Energy, Coast & Environment Bldg, Rm. 3221, Baton Rouge, LA 70803 and John R. White, Wetland Biogeochemistry Institute, Louisiana State University, 3239 Energy, Coast & Environment Bld, Dept. of Oceanography & Coastal Sciences, Baton Rouge, LA 70803.
A percent of the Mississippi River is diverted into Louisiana’s coastal wetlands to reduce land loss. The Davis Pond Freshwater Diversion discharges nutrient-rich river water to a 3,760 ha receiving wetland in upper Barataria Basin. Excess nitrate in the Mississippi River has been linked to algal blooms and hypoxia in the Gulf of Mexico and could negatively impact Barataria Basin. Denitrification enzyme activity (DEA) was measured in the soils of the Davis Pond wetland to determine the percent of wetland area receiving river nitrate at a given discharge rate. At a discharge rate of 1,100cfs (10% of the diversion capacity), elevated rates of DEA (0.46 to 2.1 mg N2O-N kg-1 h-1) were found in an 840 ha area proximal to the inflow, while background rates (0 to 0.45 mg N2O-N kg-1 h-1) were measured in areas not receiving direct river loading. Similarly, nitrate concentrations in the surface water decreased from 2.0 to 0.5 mg NO3-N L-1 from the inflow to outflow of Davis Pond wetland. The correlation between DEA and nitrate loading was studied in the laboratory using intact cores flooded with specific concentration of nitrate (0ppm, 0.5ppm, 1.0ppm, and 2.0ppm). After 20 days of flooding, DEA rates in the 1.0 and 2.0ppm nitrate treatments were significantly higher than the background rate (P < 0.05). On average, DEA increased with increasing nitrate loading. The greatest increase occurred in the uppermost soil horizon (0-5cm). DEA rates in the 5-10cm horizon remained low, but also showed a positive relationship with nitrate loading. This study indicates a positive correlation between DEA and nitrate concentration, demonstrating DEA is an effective tool for mapping areas of elevated nitrate. The Davis Pond wetland is effectively removing anthropogenic nitrate at low discharge rates, utilizing only 22% of the wetland area.
Audio File