WETLANDS AND LAKES AS NITROGEN TRAPS

Eutrophication due to excessive nitrogen loading has been a steadily i ncreasing problem in southern Swedish coastal waters during the last f ifteen years. Political decisions now aim at decreasing the anthropoge nic nitrogen emissions by 50% before 1996. Nitrogen export from farmla nd is a major problem in this context. Therefore, the nitrogen retenti on capacity in different types of wetlands in agricultural areas in so uthern Sweden was studied in order to determine whether wetlands could be used to reduce nitrogen export from farmland to the coastal marine ecosystems. Nitrogen retention mechanisms were also investigated in l akes with increased nitrogen loading. The results demonstrate that nit rogen removal in wetlands depends mainly on denitrification. in additi on to denitrification, sedimentation can be quantitatively important i n lakes. The water retention time is the most critical singel factor f or removal of nitrogen. Thus, lakes remove more nitrogen than small we tlands even though the specific nitrogen retention (N retention m(2)) is generally considerably higher in wetlands. Ponds are recommended as the most suitable type of wetland for nitrogen removal. A major probl em for using wetlands as a mean for reducing the nitrogen transport in southern Sweden is that most of the transport occurs during high flow periods in winter when the retention time in the wetlands is too shor t to permit efficient removal by biological processes like denitrifica tion. Large-scale establishment of a great number of wetlands within a catchment may reduce the nitrogen transport by up to about 15%. In mo st cases, the result will probably be less.