Wetlands remove nutrients and sediments from water as plants, bacteria, and physical processes interact.

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Healthy wetland ecosystems are commonly seen as natural water filtration systems. Wetlands can remove sediments and nutrients from the surrounding soil or water, as part of the natural cycling that these elements do between land, water, and air. Nutrients like nitrogen and phosphorous, for example, are taken from the water by bacteria and wetland plants that consume these nutrients as they grow. Physical processes like filtering and sedimentation (particles settling out of the water) can also remove nutrients and particles from the water. These biological and physical processes interact with many other factors, such as temperature and land structure, to affect a wetland’s overall function.

For instance, dense communities of wetland plants slow down water flow, which gives more time for solid particles to settle out and nutrients to be consumed by plants and bacteria. In addition, the leaves, stems, and roots of wetland plants provide a large surface area on which bacteria and other microbes can attach. Certain wetland bacteria consume nitrate (an ion containing nitrogen) in the water and convert it into nitrogen gas, which is released into the atmosphere. This process of denitrification tends to be the way that most nitrogen is removed from the water in wetlands. The plants take up some nutrients, but this is temporary storage as the nutrients are released again when the plants die and decompose. Nonetheless, the presence of plants and their interaction with other organisms in the ecosystem facilitate the wetland’s ability to clean water flowing through.

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“As human activities continue to alter the global nitrogen cycle, the ability to predict the impact of increased nitrogen loading to freshwater systems is becoming more and more important. Nitrogen retention is of particular interest because it is through its combined processes (denitrification, nitrogen sedimentation and uptake by aquatic plants) that local and downstream nitrogen concentrations are reduced…We show that wetlands retain the highest proportion of total nitrogen loading, followed by lakes and then rivers. The differences in the proportion of N retained among systems is explained almost entirely by differences in water discharge. Denitrification is the primary mechanism of nitrogen retention, followed by nitrogen sedimentation and uptake by aquatic plants.” (Saunders and Kalff 2001:205)

Journal article
Nitrogen retention in wetlands, lakes and riversHydrobiologia, 443: 205-212January 1, 2001
Saunders DL; Kalff J

“Through-flow wetlands, defined here as wetlands through which water flows on its way to streams or other water bodies, are often situated at points of ground water discharge, serially along a stream’s flow path, or parallel alongside streams and rivers. With their characteristically shallow depths and high rates of biological productivity, such wetlands typically function as sinks for excess nutrients in through-flowing water (Zedler 2003), which can be significant at the landscape level in watersheds where large quantities of water move through wetlands.” (O’Brien et al. 2012:221)

Journal article
Nitrogen transformations in a through-flow wetland revealed using whole-ecosystem pulsed 15N additionsLimnology and Oceanography, 57(1): 221-234January 16, 2012
O'Brien JM; Hamilton SK; Kinsman-Costello LE; Lennon JT; Ostrom NE

Journal article
Regional and global concerns over wetlands and water qualityTrends in Ecology & Evolution, 21(2): 96-103February 1, 2006
Verhoeven JTA; Arheimer B; Yin C; Hefting MM

Journal article
Do macrophytes play a role in constructed treatment wetlands?Water Science and Technology, 35(5): 11-17January 1, 1997
Brix H

Journal article
How effective are created or restored freshwater wetlands for nitrogen and phosphorus removal? A systematic review.Environmental Evidence, 5: 1-26May 9, 2016
Land M; Granéli W; Grimvall A; Hoffmann CC; Mitsch WJ; Tonderski KS; Verhoeven JTA

Journal article
How much biodiversity is enough?Agroforestry Systems, 45: 23-41March 1, 1999
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