While long strands of filamentous algae are unappealing and have little value to the pond ecosystem, some other types of algae can actually provide important benefits to the pond. Planktonic algae are microscopic algae living throughout the water and can sometimes be confused with muddy water. They often cause the water to look green, brown or reddish in color. Plankton algae grow very quickly when conditions are optimum in the summer, often resulting in a "bloom" where the pond water becomes turbid or colored within a day or two. These blooms usually disappear in early fall around the first frost, causing the pond to clear up very quickly - often overnight. While plankton algae blooms occur in response to warm and sunny conditions in the summer, the algae growth is supported by high levels of nutrients in the water (most often phosphorus and nitrogen) that may come from fertilizers, manures, septic systems, urban runoff or animal waste entering the pond.
Plankton algae provide important benefits to the pond ecosystem. They support the base of the food chain in the pond or lake and are fed on by zooplankton (microscopic animals) which, in turn, become food for fish. Thus, ponds with abundant planktonic algae are often able to support larger populations of fish that grow more quickly. Since planktonic algae typically grow in response to nutrients in the water, pond owners in some areas of the U.S. who are interested in raising fish quickly, especially for aquaculture, will actually fertilize their pond to stimulate the growth of plankton algae.
Moderate blooms of most plankton algae are generally beneficial and not a concern for the pond ecosystem, but large blooms can sometimes kill fish later in the summer as the algae decompose and remove oxygen from the water. Of greater concern are blooms that are dominated by blue-green algae (technically cyanobacteria). Unlike many other types of plankton algae, blue-green algae often cause odors, unsightly surface scums (that are not necessarily blue green in color) and, in rare cases, toxins that can injure animals and humans. Pond owners should keep swimmers and animals out of ponds that exhibit the symptoms of a blue-green algae bloom.
Pond owners seeking to control the growth of plankton algae should first look at reducing or redirecting obvious sources of nutrients. This can be accomplished by reducing fertilizer applications near the pond, maintaining septic systems properly, redirecting nutrient-rich runoff away from the pond, and maintaining vegetative buffer strips around the pond. If you fail to address the underlying nutrient causes of planktonic algae growth, you will probably encounter a perpetual need to control algae blooms in the summer. Installation of aeration devices may also be beneficial in reducing planktonic algae growth during the summer by mixing different sections of the pond water and by adding oxygen that helps break down old organic matter.
Various other additives like bacteria, enzyme, or herbicide products can also be used to control algae growth but keep in mind that any herbicide application to a private pond requires a state permit through the PA Fish and Boat Commission and the Department of Environmental Protection.
For more details on plankton algaeincluding their causes, benefits, problems and control, go to the Penn State Extension website.