It is a plant nutrient needed for growth, and a fundamental element in the metabolic reactions of plants and animals. Algal blooms are a classic symptom of cultural eutrophication. Most of the eutrophication occurring today is human caused. Sewage from wastewater treatment plants and septic tanks is one source of phosphorus in rivers. Animal waste containing phosphorus sometimes finds its way into rivers and lakes in the runoff from feedlots and barnyards.
Soil erosion can also contribute phosphorus to rivers.
The removal of natural vegetation for farming or construction, for example, exposes soil to the eroding action of rain and melting snow. Soil particles washed into waterways contribute more phosphorus and fertilizers used for crops, lawns, and home gardens usually contain phosphorus. When used in excess, much of the phosphorus in these fertilizers eventually finds its way into lakes and rivers. Draining swamps and marshes for farmland or shopping malls releases nutrients like phosphorus that have remained dormant in years of accumulated organic deposits. Also, drained wetlands no longer function as filters of silt and phosphorus, allowing more runoff and phosphorus to enter waterways.