Beneficial freshwater algae
Algae are a diverse group of organisms that occur in various shapes and sizes and have different ecological roles. Thousands of species of algae occur world-wide in both fresh and marine waters. Many species of freshwater algae float in the water, but others are attached to submerged rocks or aquatic plants. Most freshwater species are inconspicuous and do not create problems for humans. There are only a dozen or so, so-called "bad actors" that are considered problem-causing algae.
Algae typically serve as an important and welcome part of a lake or pond ecosystem. They form the base of the food chain and are a vital component of lakes. Algae provide a source of food, energy, and shelter for zooplankton (tiny water animals), fish, and other lake organisms. They can play a crucial role in the ability of a ecosystem to absorb nutrients and heavy metals.
The most commonly encountered groups of freshwater algae are green algae, diatoms, and blue-green algae (more correctly known as cyanobacteria). A large and varied group called green algae are the likely ancestors of terrestrial plants. Green algae contain bright, grass-green pigments, and are more abundant than all the other groups. The cells of green algae may occur singly, as spherical colonies, or as filaments. Sometimes filamentous green algae can create problems when it grows in "cotton candy" type clouds in the water. Generally most green algae are highly palatable and a good food source for zooplankton.
Diatoms appear as yellow-green or yellow-brown algae that occur singly or more rarely in colonies. The cell wall comprises two separate valves or shells formed of silica (a major component of glass). The two shells fit together as do the two halves of a petri dish. Because of the silica valves, diatoms often occur in beautiful shapes when viewed under the microscope. Diatoms reproduce through cellular division and also sexually. Each time an existing diatom divides, the silica valves get smaller. Over time, individual cells of a diatom population become smaller and smaller. Luckily for diatoms, their sexually produced offspring are able to secrete entirely new cell walls. World-wide, diatoms provide a major food resource for zooplankton and also produce atmospheric oxygen (http://www.ucmp.berkeley.edu/chromista/diatoms/diatomlh.html). Some marine diatoms can produce a toxin called domoic acid. Domoic acid can accumulate in shellfish and poison humans http://hjs.geol.uib.no/diatoms/Hazards/index.html-ssi). Freshwater diatoms do not produce this toxin. In Washington, diatoms are often the first algae to bloom in early spring. See photographs of diatoms at: http://www.bgsu.edu/departments/biology/facilities/algae_link.html
Blue-green algae (cyanobacteria) contain green, blue, and often red pigments. Blue-greens create problems when their excessive growth produces algae blooms, and few people view them as beneficial organisms in a lake environment. Blue-green algae are discussed elsewhere on Ecology's website.
Other types of algae found in lakes include: Euglenoids, dinoflagullates, brown algae, stoneworts/brittleworts, and desmids. This list is not inclusive of all the kinds of algae that are found in freshwater. Most freshwater algae do not cause problems in lakes. Because they provide a food source for zooplankton, they tend to be rapidly consumed and rarely cause the prolonged blooms that can occur with blue-green algae.