Phosphorus is an essential element and is needed for growth and other metabolic processes. In freshwater lakes, phosphorus is often found to be the growth limiting nutrient because it occurs in the least amount relative to the needs of plants. Phosphorus can occur in many forms, but it is inorganic phosphate (PO4) that readily is utilized and taken up by plants for growth. Phosphorus does not readily dissolve in water and can form insoluble forms with iron, calcium, and aluminum and precipitate to the bottom of lakes. Under anaerobic conditions or during spring turnover, phosphorus can be released back into the water column.
Phosphorus originates from many sources, both natural and those relating to human activities. Major sources include:
Human, animal, and industrial
waste
Storm
water
Soil
erosion
Excessive
use of fertilizers for crops, lawns, and home gardens
Because phosphorus is often the limiting nutrient in lakes, productivity increases when phosphorus is added to the lake system. An excessive amount of phosphorus can result in cultural eutrophication, an increased amount of plant growth that results in declining water quality. Cultural eutrophication is almost always associated with the activities of people in the watershed and differs from the natural process of eutrophication or lake aging.
Symptoms of cultural eutrophication include algal blooms and/or massive amounts of weedy aquatic plants. The growth of these plants interfere with recreational activities, such as swimming and boating and often result in anaerobic conditions due to the death and decomposition of the plant material.
Concentrations for total phosphorus vary greatly, however, a high quality lake typically has a concentration below 10 micrograms per liter at spring (lake) turnover.
