The blooms, reducing the amount of light penetrating the lake, also reduce oxygen production by photosynthesis and can cause severe oxygen depletion when they die and decompose. Phosphorus has been shown to be a major nutrient in controlling the growth of algae. Since algae require all the basic nutrients for growth, their growth can be controlled by limiting any one of the nutrients. Phosphorus is the nutrient over which we have the greatest control. The entry of phosphorus into Lake Erie is usually categorized as being from point sources or non-point sources.
Point sources are pipes, culverts or similar single points from which phosphorus is released.
Non-point sources are just the opposite. Here the phosphorus enters lakes and streams from large, poorly defined areas such as agricultural fields, highways and parking lots. Point sources of phosphorus arise from the discharge of municipal and industrial wastewater. Phosphorus enters Lake Erie in many different forms, not all of which are usable by phytoplankton. It is also known that detergents, sewage and agricultural fertilizers are major sources of phosphorus in a form capable of stimulating the growth of algae.