For a pond owner, sunny summer days can mean fishing with kids, canoe trips, and picnics under a shady tree by the water. However, those hot summer days also lead to pond owners calling the Extension office asking: “Help! Why is my pond water suddenly green?”
The abundance of microscopic algae in our water will color pond water, turning it from clear to often green (or sometimes other colors like red, brown, or brilliant blue!). Planktonic algae form the base of a pond’s food chain and can support healthy levels of oxygen in the pond water for fish and other aquatic life. However, excess nutrients in a pond, paired with warm, summertime temperatures, create the ideal conditions for the rapid, dense growth of algae, called a “bloom.” Algal blooms may cause dramatic and marked changes to the water’s color and clarity, lead to dissolved oxygen issues as algae break down, create unpleasant odors, and lead to other water quality concerns. Certain types of algae, often referred to as harmful algal blooms, can also release toxins into the water, which can harm fish, pets, livestock, and people.
To determine what type of algae is present in your pond, read HGIC 1889, Submitting an Algae Sample for Identification for more information.
So, when is algae too much of a good thing? Algae blooms are a problem when they interfere with the use of a pond or contribute to water quality problems. Measuring the clarity of your pond can help indicate if a bloom is excessive. Slowly lower a secchi disk, or even a can attached to the end of a marked pole, into the pond and note where it disappears from your view. Ideally, you should be able to see down more than 18 inches into the pond. If you can’t, it may mean that you have a very dense algae bloom and excess nutrients present in your pond.
Nutrient management in your landscape is an important step in algae bloom prevention. Take some basic steps to help prevent nutrients from being introduced into your pond by using fertilizer responsibly and only as needed (read HGIC 1652 Soil Testing for more information on soil testing), managing pet and livestock waste, stabilizing shorelines with vegetated buffers, and keeping leaf and lawn clippings out of the pond.
If you do have a dense algae bloom, control options may include the use of a combination of mechanical, biological, and chemical actions to help manage. Use the following Home and Garden Information factsheets for more information:
Or, check out the Land Grant press article “Pond Weeds: Causes, Prevention, and Treatment.”