Eutrophication is the enrichment of an aquatic system by addition of nutrients. It is primarily caused by leached phosphorus or nitrogen containing compounds into lakes, rivers, bays, or other semi-enclosed waters. Some algae and blue-green bacteria thrive on the nutrients and a population explosion, known as an algal bloom, occurs. Such "explosive" growth is usually unsustainable in natural systems. Perhaps because a nutrient or some other factor becomes limiting, the populations in "bloom" crash (death of numerous individuals occurs).

An adverse impact of eutrophication on aquatic systems is depletion of oxygen. Oxygen is required by all respiring plants and animals in an aquatic environment and it is replenished in daylight by photosynthesizing plants and algae. Under eutrophic conditions, dissolved oxygen is reduced by the dense population, and additional oxygen is taken up by microorganisms feeding on dead algae. When dissolved oxygen levels decline, especially at night when there is no photosynthesis, fish and other animals in the water may suffocate. In extreme cases, anaerobic conditions ensue, promoting growth of bacteria such as Clostridium botulinum that produces toxins deadly to birds and mammals.

Eutrophication is a exploited in many man-made aquaculture schemes.