Modelling the effects of wetlands on regional nitrogen transport

Created wetlands have been suggested as a method to reduce nitrogen transport to the Baltic Sea. This paper presents a dynamic conceptual model for simulation of the hypothetical effect of wetlands on nitrogen export to the coastal zone. The study was performed in the Söderköpingsån drainage basin (882 km2) in southeastern Sweden discharging into the Baltic Sea. An empirically-based routine for wetland retention was calibrated separately and incorporated in the model. Scenarios with different location and size of wetlands were analyzed. It was estimated that conversion of 1% (8.8 km2) of this basin into wetlands would reduce the nitrogen transport by 10-16% and that more than 5% (45 km2) conversion to wetlands is required to reduce the transport by 50%. It was concluded that construction of wetlands should be considered primarily downstream from major lakes in coastal areas and where the summer load is a significant portion of the annual load. Some further conclusions from the study were that: i) the net reduction of nitrogen transport per unit area of wetland decreases with increasing total area of wetlands in a drainage basin; ii) the wetland retention efficiency obtained in studies of individual wetlands can not be extrapolated in a linear fashion to estimate the net reduction of nitrogen transport at the mouth of a whole drainage basin; iii) the seasonal hydrological and hydrochemical dynamics are of fundamental importance for wetland retention efficiency, which complicates comparison and extrapolation of results from one region to another.