Algae are simple, green aquatic plants that have no roots, leaves, or flowers. Algae and other phytoplankton are important because they form the basis for many aquatic food webs. They also produce most of the oxygen present in Earth's atmosphere. However, too many algae growing too quickly can wreak havoc on an ecosystem.
Algae make their food through photosynthesis, a process that involves capturing light energy from the sun and converting it to carbohydrates; photosynthesis requires carbon dioxide and water, and releases oxygen. When algae metabolize the carbohydrates through respiration, they get chemical energy to fuel growth, reproduction, and other life processes.
An "algal bloom" is a rapid rise of the algae population in a body of water. Algae blooms are usually caused by moderate increases in water temperature and excess nutrients—primarily phosphorus and nitrogen from crop fertilizers, animal wastes, or sewage facilities. As such, algal blooms can be unintended consequences of some of the methods we use for growing food or processing waste.
The rapid increase of algae can have a drastic effect on an aquatic ecosystem. The algae cover the surface of the water, blocking sunlight from beneficial underwater plants that provide food and shelter for fish and other animals. When the algae proliferate, they start to deplete the nutrient supply and die off. This, in turn, causes a reduction of dissolved oxygen as the dead algae decompose, a process that uses oxygen. Many fish and other aquatic organisms die when the oxygen level lowers.
Courtesy of the Center for Ecoliteracy