eutrophication

Eutrophication is the process by which a body of water becomes overly enriched with nutrients, especially nitrogen and phosphorus, leading to excessive growth of algae and aquatic plants. The word comes from Greek roots meaning "well nourished," but in water systems that extra nourishment often causes serious ecological harm.

Historically, eutrophication was often a slow natural process that took place over long periods as nutrients gradually built up in lakes and other waters. Human activity has greatly sped it up, especially through agriculture, urbanization, and wastewater discharge. Fertilizers used in farming can wash into waterways, carrying excess nitrogen and phosphorus. Sewage and poorly managed wastewater systems can add even more of these nutrients.

One of the first results is an algal bloom, where algae grow very quickly across the water's surface. These blooms make the water murky and reduce the sunlight that underwater plants need. When the algae die, they sink and begin to decompose. That process uses large amounts of oxygen, which can create hypoxic (low-oxygen) or anoxic (no-oxygen) conditions. These "dead zones" can kill or drive away fish, shellfish, and other aquatic organisms, disrupting the food web.

Eutrophication can also change water quality by causing bad smells, cloudy water, and sometimes toxins produced by certain kinds of algae. It is a major environmental problem in freshwater lakes, rivers, estuaries, and coastal seas around the world. Reducing it usually means controlling nutrient pollution through steps such as using less fertilizer, improving wastewater treatment, and restoring wetlands that can filter runoff naturally.