The amount of P in a lake depends on land use.  There is a strong disparity between land types in how water flows and how nutrient transport takes place. Faster water causes more erosion and picks up nutrients, while slower water in the forest can actually deposit sediment, allowing runoff to percolate down.
	In addition, higher populations and septic use provides a greater source of P loading in a lake.
	Lake bottom sediments also retain quantities of phosphorus based on water chemical conditions.
	Finally direct water inputs into the lake provide a source of phosphorus.