Nitrogen Removal in Valley Bottom Wetlands
- Olivier Montreuil and
- Philippe Merot *
Although the reduction of nutrient loading between uplands and streams is sometimes considered evidence of the effect of wetlands acting as buffer zones, the influence of valley bottom wetlands (VBWs) on NO3 − loading has seldom been assessed at the catchment scale. The objective of this study was to quantify the impact of VBWs on NO3 − concentrations in streams in the Brittany region of France. We analyzed the spatial variation in NO3–N concentrations in 18 headwater catchments located in a 400-km2 basin, with varying topographic, climatic, and agricultural intensity conditions. Approximately every 10 d, water was sampled during the high flow season. We investigated the relationships between the mean NO3 − concentration and different characteristics of the catchments: (i) the amount of effective rainfall, i.e., the combined effect of precipitation and actual evapotranspiration on discharge and chemical dilution, (ii) the intensity of farming, i.e., the area used for farming in the catchments and the surplus of the agricultural N budget, and (iii) the relative area of VBWs. Although the first two characteristics were the main factors controlling N concentration variability, a step-by-step regression allowed us to attribute a significant part of the NO3 − concentration decrease to the increase of VBW area in each catchment. For an increase of VBW area from 11 to 16%, the NO3–N concentration decreased from 5.3 to 4.2 mg L−1 Therefore in this basin, VBWs reduced the NO3 − concentrations in streams with sources in agricultural fields by 30%. This work demonstrates the contribution of natural VBWs to NO3 − removal at the catchment scale compared to other sources of variation, which is a current need for integrating water quality criteria into wetland management.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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