Reduction in phosphorus input to a lake is often the target of a lake or watershed management program. There are two principal pathways that supply phosphorus to lakes. The first is from external sources, including runoff from the watershed, point sources such as sewage treatment plants, and atmospheric input. Controlling the external nutrient sources is the main reason why lake management programs include and emphasize proper management of the lake's watershed. Runoff from fertilized fields or lawns, livestock areas, urban and paved areas, and areas of soil erosion all carry phosphorus into the tributaries and eventually into the lake.
Additional phosphorus is added from septic tanks, as well as municipal or industrial discharges.
Some phosphorus is also carried from distant sources in rain and snow, and enters the lake directly through precipitation. The internal supply of phosphorus to the plants in a lake is much harder to measure and control the phosphorus from external sources. Nearly all lakes have at least part of the bottom covered by soft mud or silt, which is usually very rich in phosphorus. In shallow water, these sediments encourage the growth of rooted plants such as milfoil or pond lilies. Phosphorus compounds are part of a special chemical cycle in lake bottoms that is mediated by dissolved oxygen.