Nitrogen aids in the production of plant and animal tissue, while phosphorus plays a role in cellular growth and reproduction. The problem arises when these nutrients appear in excess, creating a significant increase in rate of plant growth often yielding an alga bloom. The process of eutrophication leads to a decreased amount of available sunlight. The plants are left without their necessary light requirements. Nutrients enter waterways through a variety of different sources and mechanisms.
Agricultural runoff represents the greatest single contributor of nitrogen and phosphorous.
Orthophosphates are applied to agricultural land as fertilizer, which is absorbed into the soil, becoming strongly bound to these particles. Other sources of phosphorous include domestic sewage and animal wastes. Low concentrations of phosphorous may leach into the groundwater, eventually entering the streams. Larger amounts are carried be eroded soil particles and runoff. The sources of nitrogen are typically similar to phosphate's origins.