Automatgenererad bild.

21 May 2016

Better knowledge of wetlands’ sink capacity

Wetlands are effective traps for nitrogen and phosphorus. But for their optimal functioning both planning and a smart design are required. The need for a reasonably low water flow is one of the conclusions in a Mistra EviEM report presented in mid-April. On a day trip to Lilla Böslid in the county of Halland, a visit to an experimental facility was also offered.

In much of the world, eutrophication of fresh and coastal waters is a major environmental problem. Heavy inputs of the nutrients nitrogen and phosphorus cause undesired changes in these waters, algal blooms and oxygen depletion being just two examples. Several EU directives have therefore focused attention on the need to reduce leaching of nutrients from farmland to marine ecosystems of all types.

One effective method is to make the water flowing from cultivated areas pass through a wetland before running out into a watercourse. Unfortunately, many of the natural wetland areas in the world have been drained to create new cultivable land. In Sweden the situation is dire: here, only a fraction remains of the bogs and fens, wet forest land and meadows, and transition zones between land and water that existed in the 19th century.

Farmers have therefore, in many cases, received support to restore wetlands or create new ones. However, the genuine effectiveness of the method has been difficult to ascertain.

To answer the question of how and where wetlands should be created, the Mistra Council for Evidence-Based Environmental Management (EviEM) has implemented a systematic review of all available studies. In the course of the work, more than 6,000 articles and research reports have been examined.

Coherent picture from two reports

The EviEM report (How effective are created or restored freshwater wetlands for nitrogen and phosphorus removal? A systematic review) and a corresponding one carried out by the Swedish Board of Agriculture last year were presented in mid-April at the Rural Economy and Agricultural Society’s conference centre in Lilla Böslid, in the county of Halland.

‘One conclusion in our report is that if you want to re-create a natural wetland, after the land has been used for cultivation, you have to dig away the topsoil. If you don’t do that, the phosphorus that has accumulated in the ground during the time it’s been used as agricultural land may be released. The same concern doesn’t seem to exist with newly created wetlands on arable land, and this may be connected with the fact that to create them you usually have to remove a great deal of soil,’ says Magnus Land, a co-author of the EviEM report.

Another key factor is that how effective the phosphorus removal actually is depends on the scale of the water flow through the wetland. The higher the speed, the lower the removal rate.

The two reports have used different methods and starting points. Nonetheless, they have arrived at roughly the same results.

‘It means there’s now a basis for advising people who are going to create a wetland.’

During the day at Lilla Böslid, which is the Rural Economy and Agricultural Society’s conference centre in Halland, the 70 or so delegates were not only treated to reports. They also had the opportunity to don boots and personally inspect nearby wetlands. Among those who did so were administrators at the county administrative boards, farmers’ representatives, researchers and consultants in the area.

What is a systematic review?

In its review, EviEM used a systematic method to summarise available knowledge of created and restored wetlands.

Systematic reviews are based entirely on existing studies. In this respect, they are no different from ordinary literature studies of scientific issues. Instead, the difference lies in the work procedure. A systematic review is characterised by meticulous planning, a methodical approach and full, open disclosure of all assessments carried out in the course of the work. This method was designed to avoid preconceived and partial conclusions and to permit quantitative conclusions to be drawn on the basis of data from a number of different studies.

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