In a waterbody, P is required for algae growth. In most waterbodies, the limited supply of P limits algae growth. Large amounts of P promote rapid growth of large amounts of algae or algae "blooms." This excess algae damages aquatic habitats by reducing sunlight penetration, reducing dissolved oxygen levels and promoting the growth of a toxic form of microscopic marine life. In short, it can result in fish kills, reduce aesthetic value and could make the water less desirable for livestock watering. An example of actions taken to prevent P enrichment of surface water is the lowering of phosphate levels in detergents. The P in manure and litter is also a concern. Many states have taken steps to limit the amount of manure P applied to all fields. In Tennessee, the goal is to limit the amount of manure P applied to fields with a high potential for run-off.
"W094-Tennessee Phosphorus Risk Index," The University of Tennessee Agricultural Extension Service, , http://trace.tennessee.edu/utk_agexani/63