eutrophication

Eutrophication is the enrichment of an aquatic ecosystem with nutrients, especially nitrogen and phosphorus, leading to increased primary production. While a natural process over geological timescales, most contemporary eutrophication is driven by human activities that raise nutrient inputs to water bodies.

Nutrient enrichment stimulates blooms of phytoplankton and algae. These blooms can reduce light penetration, alter food webs, and promote dense surface scums. When the living material dies, microbial decomposition consumes oxygen, often creating hypoxic or anoxic conditions that stress or kill fish and other aerobic organisms. In some cases, harmful algal blooms produce toxins that affect wildlife and human health. Over time, the system may shift from a clear-water, macrophyte-dominated state to turbid, algae-dominated conditions, a process that can be difficult to reverse.

Common sources include agricultural runoff (fertilizers, manures), urban and suburban runoff, untreated or inadequately treated wastewater, and industrial discharges. Detergents containing phosphorus historically contributed to eutrophication, though many regions have reduced such inputs.

Eutrophication affects freshwater lakes, rivers, estuaries, and coastal seas. Notable examples include the nutrient-rich Mississippi River basin contributing to the Gulf of Mexico dead zone, and parts of the Baltic Sea and certain North American and European lakes.

Management focuses on reducing nutrient inputs and restoring ecological balance. Approaches include upgrading wastewater treatment, adopting best management practices in agriculture, restoring wetlands and riparian buffers, and, in some cases, physical or biological restoration to remove nutrients or reestablish desired species assemblages.