The Phosphorus Cycle

Phosphorus plays a major role in biological metabolism. In comparison to other macronutrients required by biota, phosphorus is the least abundant and commonly is the first element to limit biological productivity in most freshwaters. As such, phosphorus is strongly related to phytoplankton biomass and production rate, as well as to hypolimnetic oxygen concentration and other trophic status indicators. Recent studies of phosphorus cycling in the trophogenic zone have demonstrated that the exchange of phosphorus between its various forms is often rapid and involves numerous complex pathways. Sedimentation of particulate phosphorus is the principal loss mechanism in lakes along with loss via outflow. Depending on redox conditions at the lake bottom, phosphorus can be returned to the hypolimnetic waters. As the major control on trophic status of most lakes, control of the inputs of phosphorus from external sources is a key management activity preventing or reducing eutrophication.