Effects of wetland construction on nitrogen transport and species richness in the agricultural landscape-Experiences from Sweden

Constructed wetlands, have been used to decrease nitrogen transport from agricultural catchments to the coast of Sweden, while simultaneously contributing to increased species richness in the landscape. The purpose of this paper is to compile and evaluate data that have been produced in Sweden during some 20 years of using constructed wetlands as an environmental tool. To evaluate effects of different stated objectives for constructed wetlands within different national support systems we focus on nitrogen removal and bird and amphibian species richness. Continuous automatic water sampling for nitrogen removal measurements, during 1.5 to 10 years, has been performed in 7 constructed wetlands located in agricultural catchments in southern Sweden. Nitrogen removal per wetland area varied between wetlands but the results imply that an annual removal of at least 1000 kg N per ha wetland area can be achieved in individual wetlands. Data from the long-term monitoring of the 7 wetlands, together with nitrogen removal data in the literature, were used to model >150 randomly selected constructed wetlands. According to the modeling, the nitrogen removal per created wetland area varied markedly between different wetland creation programs and was generally considerably lower compared to what can be achieved in individual wetlands. Cost-efficiency of nitrogen removal in wetland creation programs can be increased considerably with improved planning and if other desired benefits (e.g. species richness) contribute to share the costs for wetland creation. The effects of constructed wetlands in Sweden on species number and populations of wetland birds and amphibians were large enough to positively affect the occurrence of species in the national red list, i.e. on a national population level. Species richness of wetland breeding birds in the constructed wetlands were relatively stable after 13 years, but different functional groups had very different succession pattern. The mean maximum breeding bird species number in the wetlands occurred after 3.8 years. Wetland birds and amphibians colonized constructed wetlands irrespective of the original objective of the wetland (nitrogen removal or biodiversity). However, some amphibian species showed preference for biodiversity wetlands. There were clear effects of wetland construction on the regional populations of non-wetland bird species. The maximum species number for a given wetland size was found to be higher than previously reported. The analyses show that wetland construction can be a cost effective method for decreasing transport of diffuse pollution from arable land, and that the wetlands are important for the species numbers and population sizes on all spatial scales. (C) 2013 Elsevier B.V. All rights reserved.