Both phosphorus (P) and nitrogen (N) are essential to the plants and animals that make up the food web. These two ts can exist in a number of different forms including organic nitrogen, ammonia, nitrates, nitrite, organic phosphorus and ortho-phosphates. Many of these forms are water soluble and thus immediately available to aquatic plants and algae. Monitoring and measuring P and N levels are important steps for assessing the health of a waterbody.
In most freshwater ecosystems, N is abundant and P is in short supply., Plants do not need a lot of P, so even a modest increase in P can, under the right conditions, set off a whole chain of undesirable events. Excessive plant growth, unsightly algae blooms, low levels of dissolved oxygen, and even the death of certain aquatic animals can be caused by increased levels of phosphorus.
P entering lakes and rivers can be broadly divided into two pathways: external and internal. The external load originates in stormwater runoff, septic tank or sewer seepages, aerial deposition and other external sources. The internal load originates from the P trapped in the lake sediments. Leaves and other detritus that sink to the bottom of the lake are the sources of organic P. P trapped in the sediments may have little or no effect on the day-to-day growth of algae and plants, but it can have deep long-term effects. For example, when the levels of dissolved oxygen are low, P can be released from the sediments. Sediment disturbance (i.e. wave action, fish nesting, and zooplankton migration) can also contribute to the release of P from the sediments.