Eutrophication is the process of enriching an ecosystem with nutrients, primarily nitrogen and phosphorous. It can occur naturally in aging lakes. Humans can also introduce these nutrients into an ecosystem, at which point it is called artificial eutrophication.
Fertilizers used in farming can be washed into nearby water bodies, enhancing their nutrient levels. Eutrophication leads to extensive growth and reproduction of algae in the form of algal blooms. These can both benefit and, from time to time, harm the ecosystem. The goal of all eutrophication, natural and artificial, is a balanced and healthy nutrient level in a lake or other ecosystem.