How do phosphorus and nitrogen contribute to eutrophication in the Great Lakes?

In the Great Lakes, phosphorus and nitrogen act as essential plant nutrients that drive eutrophication, a process of nutrient enrichment that accelerates the aging of an aquatic ecosystem. While eutrophication is a natural, slow process, human activities—such as the discharge of sewage, agricultural runoff, and the use of fertilizers—have significantly increased the concentrations of these nutrients, a phenomenon known as cultural eutrophication.123

These nutrients stimulate the rapid growth of algae and microscopic organisms, leading to dense algal blooms. As these blooms die, microorganisms consume large amounts of dissolved oxygen during the decomposition process. This depletion of oxygen, or hypoxia, creates conditions that are harmful or fatal to fish and other aquatic life.13

In the Great Lakes, these nutrient concentrations are highest near urban centers, bays, and harbors. Because phosphorus and nitrogen are often the limiting factors in lake productivity, they are primary targets for pollution control efforts aimed at curbing the degradation of these vital freshwater resources.214