How effective are created or restored freshwater wetlands for nitrogen and phosphorus removal? A systematic review protocol

Background Eutrophication of aquatic environments is a major environmental problem in large parts of the world. In Europe, EU legislation (the Water Framework Directive and the Marine Strategy Framework Directive), international conventions (OSPAR, HELCOM) and national environmental objectives emphasize the need to reduce the input of plant nutrients to freshwater and marine environments. A widely used method to achieve this is to let water pass through a constructed or restored wetland (CW). However, the large variation in measured nutrient removal rates in such wetlands calls for a systematic review. The objective of this review is to quantify nitrogen and phosphorus removal rates in constructed or restored wetlands and relate them to wetland characteristics, loading characteristics, and climate factors. Wetlands are created to treat water from a number of different sources. Sources that will be considered in this review include agricultural runoff and urban storm water run-off, as well as aquaculture wastewater and outlets from domestic wastewater treatment plants, with particular attention to the situation in Sweden. Although the performance of wetlands in temperate and boreal regions is most relevant to the Swedish stakeholders a wider range of climatic conditions will be considered in order to make a thorough evaluation of climatic factors. Methods Searches for primary studies will be performed in electronic databases as well as on the internet. One author will perform the screening of all retrieved articles at the title and abstract level. To check that the screening is consistent and complies with the agreed inclusion/exclusion criteria, subsets of 100 articles will be screened by the other authors. When screening at full-text level the articles will be evenly distributed among the authors. Kappa tests will be used to evaluate screening consistency. Data synthesis will be based on meta-regression. The nutrient removal rates will be taken as response variables and the effect modifiers will be used as explanatory variables. More specifically, the meta-regression will be performed using generalized additive models that can handle nonlinear relationships and major interaction effects. Furthermore, subgroup analyses will be undertaken to elucidate statistical relationships that are specific to particular types of wetlands.