Phosphorus is a nutrient essential for biological growth that often acts as the limiting nutrient in freshwater ecosystems. Excess phosphorus in water bodies is a primary driver of eutrophication and algal blooms.
Phosphorus in aquatic systems exists in several forms including dissolved reactive phosphorus (orthophosphate, PO4 3-), dissolved organic phosphorus, and particulate phosphorus bound to sediments. In most freshwater lakes and rivers, phosphorus is the nutrient that limits algal growth, meaning even small increases in phosphorus loading can trigger significant ecological responses. The EPA recommends that total phosphorus concentrations not exceed 0.05 mg/L in streams entering lakes and 0.1 mg/L in streams that do not directly discharge to lakes or reservoirs. Agricultural fertilizers, animal manure, wastewater treatment plant effluent, and detergents are the major anthropogenic sources of phosphorus to waterways. Unlike nitrogen, phosphorus does not have a gaseous phase in its biogeochemical cycle, so it tends to accumulate in soils and sediments and can be remobilized under certain conditions, such as low dissolved oxygen at the sediment-water interface. Advanced wastewater treatment technologies including chemical precipitation, biological phosphorus removal, and membrane filtration can reduce effluent phosphorus concentrations to below 0.1 mg/L. Many states have adopted numeric phosphorus criteria for lakes and rivers to protect against eutrophication.