Wetlands are particularly good water filters. Due to their position between upland and deep water, wetlands can intercept surface-water runoff from land before it reaches open water. They also filter nutrients that have been removed from upland areas due to water erosion or ground waterflow. This is especially important for nitrogen and phosphorus. As wetlands have been removed, more of these nutrients moved into lakes and rivers, causing excess vegetative and algae growth which results in cultural eutrophication of water bodies.
Cultural eutrophication describes human-generated fertilization of water bodies.
The main sources of phosphates and nitrates for this process are treated sewage and runoff from farms and urban areas. Nitrates and phosphates are the most common limiting factors for organism growth, especially in aquatic ecosystems. Nitrates are supplied in limited quantities by decaying plant material and nitrogen-fixing bacteria, but phosphates must come from bones, organic matter or phosphate rocks. Consequently, the use of phosphate detergents, combined with agricultural or urban runoff, has greatly affected many water bodies by increasing the nutrients available for algae and weed growth. . As water moves through a wetland it leaves behind nutrients that are used by the plants and animals living in the wetland.