Wetlands as Biogeochemical Hotspots Affecting Water Quality in Catchments

Water quality in freshwater lakes and coastal areas has deteriorated in many densely populated areas with increasingly intensive agriculture. For example, eutrophicationof aquatic environments has become a major environmental problem in large parts of the world. Biogeochemical transformations occurring in wetlandsgenerally result in reduced nutrient content of water, but quite commonly these ecosystems have been drained or filled to create new forest and arable land. In recent decades efforts have been made to compensate for the losses of natural wetlandsby creating new wetlandsor restore drained wetlands. However, the large variation in measured nutrient removal rates in such wetlandshas made it difficult to assess the effectiveness of such interventions. In this chapter we discuss the role of created wetlandsin regulating water quality at the catchment scale. First, we pay attention to a recent systematic review of nitrogen and phosphorus removal in single wetlandsin boreal, temperate and sub-tropical regions. Second, in a more focused case study, we evaluate the efficacy of large numbers of constructed wetlandsin southern Sweden to remove nutrients from runoff in the context of the eutrophicationof the Baltic Sea. The removal efficiency of total nitrogen and total phosphorus in single wetlandsis in general relatively high (median values are 37% and 46%, respectively). However, to make a significant difference on a catchment scale, more and larger wetlandareas need to be created, and they need to be wisely placed where nutrient loading rates are high.