Planktonic Algae

Planktonic algae floats in the water column and can sometimes be confused with muddy water. It causes the water to look green, brown or reddish in color.
Planktonic Algae - Articles


  • Description of Planktonic Algae

  • Microscopic algae that float in the water column
  • Cause the water to look green, brown, or reddish
  • Can sometimes be confused with muddy water
  • Plankton algae grow very quickly when conditions are optimum, often resulting in a "bloom" where the pond water becomes colored within a day or two
  • Blooms of plankton algae usually crash in early fall around the first frost, causing the pond to clear up very quickly (often overnight)
  • Ponds with plankton algae typically have clear water during the winter but cloudy water during summer

Value and Concern to the Pond

  • Planktonic algae are at the base of the food chain in the pond or lake.
  • They are fed on by zooplankton (microscopic animals) which, in turn, become food for fish.
  • Ponds with abundant planktonic algae are often able to support larger populations of fish that grow more quickly.
  • Excessive growth of planktonic algae will cloud the water, making it less aesthetically pleasing.
  • Overabundance of planktonic algae can lead to fish kills in the late summer and fall. This occurs when the large number of algae dies suddenly due to colder air temperatures.


  • Large blooms of planktonic algae grow in response to excessive nutrients (phosphorus and nitrogen) in the pond water from barnyards, crop fields, septic systems, lawns, and golf courses.
  • Control of overabundant algae is best accomplished by reducing or redirecting nutrient sources from the pond. 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 your 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 reduce planktonic algae growth during the summer.

Physical Control

  • Planktonic algae are too small to be effectively controlled with any physical or mechanical methods.

Biological Controls

  • There are no fish that eat enough planktonic algae to make them viable control options.
  • Relatively new products composed of bacteria and/or enzymes are available to control algae growth. The bacteria and enzymes feed on nutrients in the water, making them unavailable for algae growth.
  • Bacteria/enzyme products have specific requirements and can be pricy but very effective.

Chemical Controls

There are numerous aquatic herbicides that can be used to control algae. You can learn more about these herbicides in Management of Aquatic Plants . Here are some tips for properly using an aquatic herbicide to control planktonic algae:

  • Keep in mind that chemical control is often necessary every year or even multiple times during a year.
  • Positively identify the problem in your pond as planktonic algae before proceeding with chemical control.
  • Carefully measure the pond area and/or volume to determine the amount of herbicide needed. Consult the fact sheet titled Pond Facts 4: Measuring Pond Area and Volume  for more information.
  • Make sure that you obtain and submit the required state permit before applying the herbicide. Before applying a herbicide to your pond, you must obtain a state permit from the Pennsylvania Department of Environmental Protection. The two-page application form and instructions for this permit are online at 'Application for Use of an Algaecide, Herbicide, or Fish Control Chemical in Waters of the Commonwealth'.
  • Aquatic herbicides can be purchased from some home and farm supply stores, hardware stores, or various online suppliers. Costs can range from less than $100 to over $1,000 to treat a one-acre pond.
  • Follow the herbicide label carefully. It gives specific instructions on when and how to apply the chemical.
  • Herbicide treatments should be done early in the growing season before the plants cover a large portion of the pond. Treatment of severe infestations may cause a fish kill due to reduced dissolved oxygen.

Additional Resources

For further information and publications on pond management, visit the Penn State Extension website or contact your local extension office.

Prepared by Bryan R. Swistock, senior extension associate, Penn State; and Heather Smiles, fisheries biologist, Pennsylvania Fish and Boat Commission.