Phosphorus is an essential nutrient for plants and animals and is usually present in natural waters attached to sediment, in organic material, and dissolved in the water. There are natural sources of phosphorus, such as certain soils and rocks, but most elevated levels of phosphorus are caused by human activities. These include human, animal, and industrial wastes, as well as runoff from fertilized lawns and cropland. Excess phosphorus in water speeds up plant growth, causes algal blooms, and can result in low dissolved oxygen, or hypoxic, conditions that can lead to the death of certain fish, invertebrates, and other aquatic animals.Plant growth in surface waters is generally limited by the amount of orthophosphate, the dissolved form of phosphorus, present. It is the simplest form of phosphorus found in natural waters and is most available for plants to use. In most waters, orthophosphate is present in very low concentrations. The amount of phosphate dissolved in water is expressed in milligrams per liter of water (mg/L).