Quantifying nitrogen retention in surface flow wetlands for environmental planning at the landscape-scale

Abstract

Surface flow wetlands are valued highly for their high nutrient retention potential and their unique biodiversity. At present, there are an increasing number of activities aimed at restoring these sites as multifunctional landscape entities. The success of wetland restoration is however clearly dependent on the site selection to achieve the specific restoration goals. This study first presents a tool to identify the most suitable areas for the restoration of surface flow wetlands for water quality improvement in a given catchment and secondly compares three different mathematical equations in order to quantify the effect of nitrogen retention when restoring the previously selected, most suitable wetland sites. For site selection, a score system was developed which is linked to a Geographical Information System. The score system combines information from a given catchment in eight data layers including soil substrate, actual land use, relief features, hierarchical drainage basin classification, river proximity and socio-economic factors. The score system was applied to a potential use situation in the Neuwührener Au watershed (40 km 2) in northern Germany belonging to the Baltic Sea drainage basin. Three areas were identified as most suitable for the restoration of surface flow wetlands. Their potential effect on nitrogen retention was evaluated using three different equations: (I) a linear relation between wetland load and a wetland; (II) an exponential equation between wetland load and wetland area; and (III) an exponential equation between wetland load and hydraulic retention time. The linear approach calculates increasing wetland retention with increasing upstream catchment area and appears to overestimate nitrogen retention in wetlands located more downstream. The two exponential equations calculated nitrogen retention in the three wetlands in the same order of magnitude. The results from the siting procedure and the prediction of nitrogen removal rates are useful for decision makers in wetland planning to base their decision on best available data and knowledge. The model comparison allows the incorporation of uncertainty in the decision progress which is seen as a necessary requirement when quantifying biological processes in environmental planning.