Search strategy
The search aims to capture an unbiased and comprehensive sample of the literature relevant to the question, whether published or unpublished. Different sources of information will be searched in order to maximise the coverage of the search. Bibliographies of any review papers, summary reports or books retrieved, will be searched for relevant references.
Electronic databases
The following electronic databases will be searched:
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1.
ISI Web of Knowledge
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2.
Copac
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3.
Agricola
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4.
JSTOR
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5.
EThOS
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6.
DART – Europe E-theses Portal
No restrictions will be applied regarding the year of publication. The search will be refined by language (English) and country (UK, England, Scotland, Wales, Ireland) where this facility is available.
Conservation and statutory websites
The official websites for the following organisations will be searched:
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1.
Department of Environment, Food and Rural Affairs (DEFRA)
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2.
Wildfowl and Wetlands Trust (WWT)
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3.
Ramsar
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4.
Environment Agency (EA)
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5.
English Nature
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6.
Countryside Council for Wales
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7.
Department of Agriculture and Rural Development (DARD)
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8.
Scottish Natural Heritage (SNH)
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9.
Water Framework Directive
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10.
Scottish Environment Protection Agency (SEPA)
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11.
Department of Agriculture, Food and the Marine
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12.
The Irish Agriculture and Food Development Agency (TEAGASC)
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13.
Constructed Wetland Association
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14.
Wetlands International
Websites
The following search engines will be searched:
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1.
http://www.google.com
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2.
http://scholar.google.co.uk
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3.
http://www.dogpile.com
The first 50 hits from each search will be examined for relevance, with any links present being followed only once from the original hit. Bibliographies of articles viewed at full text will be searched.
Authors, recognised experts and practitioners
Authors, recognised experts and practitioners (to include the society of wetland scientists) will also be contacted for further recommendations and for the provision of any unpublished material or missing data that may be relevant.
Search terms
Combinations of the below search terms (Table 2) will be used (where * denotes a wildcard term) to search the databases and websites. Search terms within each group will be combined using the Boolean OR operator, and between groups using the Boolean AND operator where possible. Search terms from group 3 will be used when the facility to refine the search by country is not available. Search terms will be tailored as necessary to the specific data base or search engine used, and as the search progresses.
Study inclusion criteria
Once the search has been conducted, inclusion criteria will be applied in order to identify relevant articles. The articles will be filtered at three levels; by title, then abstract (or introduction section if abstract is not available), and finally by full text. Citations will be stored in Endnote.
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Relevant subjects: Water bodies receiving N, P and SS from agricultural waste in England, lowland Wales and Ireland. All forms of N and P, and all types of agricultural waste will be included.
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Type of intervention: Freshwater constructed or restored wetlands, to include ponds, marshes, fens, floodplains, bogs, mires and reedbeds. Established reedbeds or ponds will also be included.
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Types of comparator: Studies with the following comparators will be included:
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1.
Input concentration/load of N, P, or SS to wetland versus output concentration/load of N, P or SS from wetland;
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2.
Concentration/load of N, P, or SS from agricultural pollution entering receiving water with no wetland versus with a wetland (provided input and geography are comparable);
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3.
Concentration/load of N, P, or SS entering receiving water before installation of a wetland versus after installation of a wetland (provided input and geography are comparable).
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4.
Upstream concentration of N, P or SS in receiving water versus downstream concentration of N, P or SS in receiving water will NOT be included due to the possibility of in-stream processing/additional inputs.
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Types of outcome: The primary outcome is a quantitative change in N, P or SS concentration or load. Quantitative changes in different species of N (e.g. nitrate and ammonium) and P will be included. The secondary outcome is a change in other water quality parameters, biodiversity, or greenhouse gas production. The secondary outcome will not be used as an inclusion criterion.
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Types of study: Studies on both full scale wetlands and pilot scale wetlands will be included. Studies on laboratory mescosms and modelling studies will not be included.
Once the abstracts have been screened, the consistency of the above process will be checked. If in doubt about inclusion at abstract level, the whole text will be viewed. Two reviewers will assess a random subset of 10% of the original list of citations, applying the inclusion criteria at title and abstract level. The level of agreement between the two reviewers will be measured by kappa analysis, with a kappa rating of 0.6 or above (‘substantial’ agreement, compared to chance of agreement alone) considered acceptable. If kappa is less than 0.6, the discrepancies will be discussed and resolved by consensus. The reviewers will then clarify or modify the inclusion criteria as necessary, and one reviewer will then apply the inclusion criteria to the rest of the citations.
Study quality assessment
Articles which meet the inclusion criteria will be viewed at full text, and either be excluded or assigned different categories of study quality (poor, medium, high). At least two reviewers will independently assess a random subset of 25% articles viewed at full text. The level of agreement between the two reviewers will be measured by kappa analysis, with a kappa rating of 0.6 or above considered acceptable. If kappa is less than 0.6, the discrepancies will be discussed and resolved by consensus. A possible ranking system is suggested below:
High: Established wetland (>5 years); Regular monitoring (>=weekly); Long-term monitoring (>=1.5 years); Full scale wetland; Scientifically rigorous (methodological and analytical); Good control.
Medium: Scientifically rigorous (methodological and analytical); Good control; Either full or pilot scale wetland; and one or more (but not all) of either; Established wetland; Regular monitoring (>=weekly); Long-term monitoring (>=1.5 years);
Low: Poor control or lacking scientific rigour.
Data extraction strategy
The following data will be recorded on a specially designed data extraction form to include the following information where available:
General location; geology; soil characteristics; hydro geomorphic landscape setting; type of wetland; type of vegetation; area of wetland; type of management; hydroperiod; hydraulic loading; hydraulic retention time; size of the area generating the pollution; ratio of area generating waste to area of wetland; type of waste; agricultural intensity of upstream area; length of time wetland has been established; frequency of monitoring; length of time monitored; seasons monitored; analytes measured; control type; input and output (without/with; before/after) concentrations/loads/populations of any analytes measured; reduction/increase/no change; % reduction.
Data synthesis
A descriptive statistical overview of the data will be presented. For each analyte this will show: the number of sites, % of sites showing a reduction, % of sites showing an increase and a mean % reduction with standard deviation, calculated from averaged values presented in the source studies.
If possible, a more rigorous statistical analysis of the data will be conducted to examine the effect of the covariates (effect modifiers) on the efficiency of a wetland’s ability to remove (or otherwise) N, P or SS. These covariates include: Geology; soil characteristics; hydro geomorphic landscape setting; type of wetland; type of vegetation; area of wetland; type of management; hydroperiod; hydraulic loading; hydraulic retention time; size of the area generating the pollution; ratio of area generating waste to area of wetland; type of waste; agricultural intensity of upstream area; length of time wetland has been established; frequency of monitoring; length of time monitored; seasons monitored. How these covariates are categorised will depend on the quality and type of data retrieved during the data extraction stage. This will start with constructing a binomial generalised linear model with the possibility of constructing a Bayesian generalised linear model. External expertise will be made use of where necessary in order to formulate these models. In the event of none of the covariates showing a significant control on whether a wetland reduces or increases the analyte concentration using the extracted data, a narrative synthesis will be conducted.