MODELING THE EFFECTS OF WETLANDS ON REGIONAL NITROGEN TRANSPORT

Created wetlands have been suggested as a method to reduce nitrogen tr ansport to the Baltic Sea. This paper presents a dynamic conceptual mo del for simulation of the hypothetical effect of wetlands on nitrogen export to the coastal zone. The study was performed in the Soder-kopin gsan drainage basin (882 km(2)) in southeastern Sweden, discharging in to the Baltic Sea. An empirically based routine for wetland retention was calibrated separately and incorporated in the model. Scenarios wit h different location and size of wetlands were analyzed. It was estima ted that conversion of 1% (8.8 km(2)) of this basin into wetlands woul d reduce the nitrogen transport by 10-16% and that more than 5% (45 km (2)) conversion to wetlands is required to reduce the transport by 50% . It was concluded that creation of wetlands should be considered, pri marily, downstream from major lakes, in coastal areas, and where the s ummer load is a significant portion of the annual load. Some further c onclusions from the study were that: i) the net reduction of nitrogen transport per unit area of wetland decreases with increasing total are a of wetlands in a drainage basin; ii) the wetland retention efficienc y obtained in studies of individual wetlands can not be extrapolated i n a linear fashion to estimate the net reduction of nitrogen transport at the mouth of a whole drainage basin; iii) the seasonal hydrologica l and hydrochemical dynamics are of fundamental importance for wetland retention efficiency, which complicates comparison and extrapolation of results from one region to another.