In the deeper, less oxygenated regions, the water milfoil can grow while totally submerged, thanks to its porous roots. Fed domestic sewage, cypress domes remove most of the harmful nitrates and phosphates, binding them up in mud and tree roots. Scientists caution that wetlands, vital as pollution traps, could suffer from overloading of contaminants. More often than not the wastewater reaching the marshes is tainted with salts or trace elements such as selenium, a leachate resembling sulfur that becomes more concentrated as it moves up the food chain. Some years ago, the selenium buildup got so bad at the Kesterson National Wildlife Refuge that massive die-offs of migratory birds occurred.
Wetlands are home to many species of birds.
Wetland birds are determined by the kind of habitat and the seasonal movement of migrating species. These birds depend on wetland habitats for their survival. Most nest within wetlands, a few such as the Great Blue Heron nest elsewhere but feed extensively in wetlands, and others such as the Wood Duck nest away from wetlands but rear their young in marshes and fens. These birds use wetland habitats extensively for breeding or feeding, as well as other types of habitat. If wetlands are destroyed, the population of these birds would decline.