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. E xcessive 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.