Rapid chemical changes and N retention during floodplain inundation in the Wisconsin River.
Stanley, Emily*,1, 1 University of Wisconsin, Madison, WI, USA
ABSTRACT- Floodplains are biologically active environments that have the potential to affect forms and amounts of nutrient loads in streams and rivers. The potential influence of floodplains on whole-river nutrient dynamics is influenced both by the extent of hydrologic interactions between the river and the floodplain, and by the initiation and rate of biogeochemical processes that occur as water moves through the floodplain. In particular, for a floodplain to be an important site of nitrogen (N) retention during overbank flooding, conditions favorable to denitrification (low oxygen and high organic matter concentrations) should to be widespread across the floodplain and throughout most of the inundation period. We investigated short-term (hourly to daily) changes in dissolved oxygen (DO), dissolved organic carbon (DOC), and nitrate concentrations during inundation of the Wisconsin River floodplain to determine if conditions favorable to denitrification were present, and how quickly such favorable conditions became established during spring flooding. DO was significantly correlated with discharge and declined by 50-100% within hours of loss of hydrologic connection between the river and the floodplain. Temporal patterns of DOC were erratic, but concentrations were consistently high (>6 mg/L). As with DO, nitrate also responded to changes in discharge, and concentrations declined rapidly during flood recession. These rapid changes in water chemistry indicate prompt initiation of biogeochemical processes conducive to N retention during flooding, and suggest that hydrologic delivery of nitrate to the floodplain (rather than microbial processing) likely constrains denitrification in the Wisconsin River floodplain.
Key words: floodplain, nitrogen, denitrification, flooding
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