On average, created and restored wetlands significantly reduce the transport of TN and TP in treated wastewater and urban and agricultural runoff, and may thus be effective in efforts to counteract eutrophication. This suggests that part of the large variation between studies may be explained by less rigorous study designs. Although there are small differences in average values between the two quality categories, the variation is considerably smaller among high quality studies compared to studies with lower quality. Median TP removal efficiency was 46 % with a 95 % confidence interval of 37–55 %. Removal efficiency for TP was significantly correlated with inlet TP concentration, HLR, T, and A. Removal efficiency for TN was significantly correlated with hydrologic loading rate (HLR) and T, and the median was 37 %, with a 95 % confidence interval of 29–44 %. Median removal rates of TN and TP were 93 and 1.2 g m −2 year −1, respectively. Significant relationships were also found for annual average air temperature (T) and wetland area (A). The removal rate of both total nitrogen (TN) and total phosphorus (TP) is highly dependent on the loading rate. Most of the wetlands were situated in Europe and North America. After screening on relevance and critical appraisal, 93 articles including 203 wetlands were used for data extraction. Regressions were performed using generalized additive models that can handle nonlinear relationships and interaction effects. Quantitative synthesis consists of meta-analyses and response surface analyses.
Relevant articles remaining after screening were critically appraised and assigned to three quality categories, from two of which data were extracted. Kappa tests were used to evaluate screening consistency. When screening at full-text level the articles were evenly distributed among the authors. To check that the screening was consistent and complied with the agreed inclusion/exclusion criteria, subsets of 100 articles were screened by the other authors. One author performed the screening of all retrieved articles at the title and abstract level. Searches for primary studies were performed in electronic databases and on the internet. However, the large variation in measured nutrient removal rates in such wetlands calls for a systematic review.
A widely used method to achieve this is to allow water to pass through a created or restored wetland. 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 nutrients to freshwater and marine environments. Eutrophication of aquatic environments is a major environmental problem in large parts of the world.
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