Eutrophication is the process in which an increase in a lakes nutrients changes the organisms that live there. This process can occur naturally or by the actions of people. For example, fertilizers from residential areas can flow into lakes and increase their levels of nitrogen and phosphorus. Algae are tiny organisms that live in water and get energy from the sun. An increase in nutrients causes a tremendous increase in the amount of algae. This is called an algal bloom. You may have seen algal blooms on ponds covered with a layer of green algae. As the algae die, their decay uses up the oxygen in the lake. This can cause the deaths of other organisms, such as fish, that depend on the oxygen in the water. The photograph on the cover of this card shows a lake covered with a layer of green algae.