How effective are reedbeds, ponds, restored and constructed wetlands at retaining nitrogen, phosphorus and suspended sediment from agricultural pollution in England?

A high priority topic within the Department for Environment, Food and Rural Affairs (DEFRA) water quality programme is the mitigation of diffuse rural pollution from agriculture. This programme was set up to meet the requirements of the European Water Framework Directive (WFD ) [1]. Nutrient loss from agricultural land has been suggested as a major cause of elevated nutrient concentrations in surface waters in the UK [2]. Nitrogen (N) and phosphorus (P) are of particular concern as an excess of either nutrient can lead to eutrophication of freshwater systems and coastal waters. Agriculture has also been identified as a significant source of suspended sediment concentrations in UK rivers [3]. Suspended sediment (SS) can lead to loss of ecological integrity [4] and agriculturally derived sediment has been identified as a source of increased bed-sediment P concentrations in rivers [5].Wetlands are often cited as being effective at reducing nutrient and sediment loadings to receiving waters. However, the research in this area is inconsistent, and whilst most studies have shown that both natural and constructed wetlands retain nutrients and sediments, others have shown that they have little effect, or even increase nutrient and sediment loads to receiving water bodies [6-8]. Many factors may have contributed to these disparate results, including the length of time the wetland has been established for, seasonality, the hydrogeomorphic landscape setting, type, size, level and type of management, and the input concentrations/loads and historic loading of the wetland. For example, a constructed wetland system in South-west England switched from a net annual sink to a source of phosphorus over a 10 year period, and from being a sink to a source of soluble reactive phosphorus (SRP) and ammonium between spring and summer [9,10]. Phosphorus removal has been shown to correlate positively with an increase in wetland area, and a minimum wetland to farmyard area ratio of 1.3 has bee