The role of wetlands and other landscape features for large-scale nutrient retention

Andrew Quin, Fernando Jaramillo & Georgia Destouni, Stockholm University

Various ecosystems and other, physical features of a landscape contribute to a combined large-scale ecosystem service, attenuating the waterborne loading of nutrients and pollutants from various sources to downstream waters and ecosystems. Without this ecosystem service, nutrient and pollutant loading from the landscape would be greater and require more, and more costly, abatement measures to achieve the same level of water, environmental and ecosystem protection. This regulating ecosystem service is further closely linked to and may critically affect also other types of ecosystem services, such as the provisioning of clean water, the supporting of nutrient cycling and the cultural service of sustaining recreational water environments. The current values of the latter services depend all on the large-scale service of nutrient-pollutant regulation in the landscape.

In particular wetlands have been put forward as essential landscape features for the retention of nutrients and pollutants from both point (e.g. wastewater treatment plants and industry) and diffuse (e.g. agriculture, forestry, unconnected wastewater pipes, storm water and atmospheric deposition) sources in the landscape. A number of studies have assessed the local effectiveness of wetlands in retaining nutrients and thus lowering the nutrient load carried by the water flow through them. Generally, there is evidence that wetlands can achieve such local load reduction under favourable conditions. However, the question of what the corresponding large-scale effect of multiple wetlands distributed throughout the landscape remains open and needs to be further investigated and quantified.

This study tackles this question by compiling and analysing relevant available data for assessing the large-scale nutrient retention by wetlands and other landscape features. Nutrient retention data is obtained from the Fifth Baltic Sea Pollutant Load Compilation (PLC5), carried out under the auspices of Sweden’s Environmental Emissions Data (SMED). Data on wetlands is obtained from the Swedish Land Cover Data (SMD) and the Swedish Wetlands Inventory (VMI). The analysis is carried out for two main Swedish Water Management Districts of the North and South Baltic Proper, with the former including the EkoKlim reference area of the Norrström drainage basin.

Results reveal undetectable wetland effects on the landscape-scale nutrient retention, accounting for a wide range of relative wetland areas and numbers of wetlands across different landscape parts of the two investigated water districts. The distance of waterborne transport from source to receiving water, and the existence of large lakes along this transport distance emerge instead as key landscape features for the large-scale nutrient retention in a landscape. These results apply to different types of nutrient sources, including point, agricultural and other diffuse sources, and to both nitrogen and phosphorous transport and retention. These large-scale results must be considered in policy and management for efficient protection and restoration of good water quality, as well as in assessments and accounts of wetland ecosystem services on landscape scales. In general, such improved understanding of the large-scale nutrient and pollutant retention effects of different features and their distribution in a landscape is needed to better inform decision making aimed at reducing eutrophication and pollution of aquatic ecosystems.

Figure: Retention of nitrogen for various sub-catchments in the North and South Baltic water management districts. Note that nitrogen retention is generally small in catchments located near the coast, while it is much higher in catchments further inland. The catchments with the highest retention tend to lie upstream of major lakes or the farthest inland.
Figure: Retention of nitrogen for various sub-catchments in the North and South Baltic water management districts. Note that nitrogen retention is generally small in catchments located near the coast, while it is much higher in catchments further inland. The catchments with the highest retention tend to lie upstream of major lakes or the farthest inland.