Eutrophication is the excessive enrichment of a water body with nutrients, especially nitrogen and phosphorus, leading to algal blooms and oxygen depletion. It is a major cause of water quality degradation in lakes, reservoirs, and coastal areas worldwide.
Eutrophication occurs when elevated inputs of nitrogen and phosphorus stimulate rapid growth of algae and aquatic plants in lakes, reservoirs, estuaries, and coastal waters. The primary sources of excess nutrients include agricultural runoff containing fertilizers and manure, municipal wastewater effluent, and atmospheric deposition from fossil fuel combustion. When large algal blooms die and decompose, the bacterial decomposition process consumes dissolved oxygen, potentially creating hypoxic or anoxic conditions that cause fish kills and loss of biodiversity. Some algal blooms produce harmful toxins (harmful algal blooms or HABs) that pose direct threats to drinking water supplies, recreational users, and wildlife. Eutrophication can shift aquatic ecosystems from clear-water, macrophyte-dominated states to turbid, phytoplankton-dominated states, a regime shift that can be difficult to reverse. Lake trophic state is classified using indices based on chlorophyll-a concentration, Secchi depth, and total phosphorus. Management strategies include reducing nutrient inputs through best management practices in agriculture, upgrading wastewater treatment to include nutrient removal, and implementing buffer zones along waterways.