Why is my pond green?

As rainwater flows over lawns and hard surfaces it picks up nutrients and pollutants that are then captured in stormwater ponds to protect our lakes and streams. Due to the purpose of ponds, it is not surprising for ponds to turn green over time. 

There are three main types of organisms that can make a pond look green. To determine what might be happening in your pond, review the information below:

  1. Is the water green throughout the entire water column? You can tell if this is the case by putting your hand below the water surface and your hand looks green when held several inches deep. This type of green typically does not sit on water surfaces until the hottest days of summer and is very bright green and thicker at shoreline. This type of green is often caused by a blue-green algae.
    blue green algae
  2. If you have a green mat on top of the water that tends to move frequently with winds and rain, then you probably have a healthy population of duckweed. Duckweed is a plant that is a food source for water fowl and actually helps with the water quality of the pond by removing nutrients. There is little you can do to remove the duckweed and it is unadvisable since this plant provides food and habitat for several different species.
  3. Do you have clumps of hair-looking green floating on your pond? If so, it is likely filamentous algae. It will float on the surface, or even cling to plants.
    filamentous algae

The best thing you can do to prevent a green pond due to any of the above organisms is to reduce the amount of nutrients in the water body by practicing environmentally safe lawn care practices. By you and your neighbors sweeping up grass clippings and fertilizers from paved surfaces and making sure erosion and piling of vegetation near the pond isn’t occurring, you will help to eliminate the main food source for these organisms and reduce the pond’s green appearance. For further information about what you might be able to do to help your pond or wetland, download the brochures below.

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1. There is a small metal tag attached to some trees in my yard. What does that mean?
2. I passed a development and saw some silt fence down near a wetland. Who should I contact to get it fixed?
3. What is buckthorn and how can I manage it?
4. We have a pond/stormwater basin in our backyard. We would like to have a dock and a canoe so my kids can fish. Is that possible?
5. Does the City treat my pond for mosquitoes?
6. Why is my pond green?
7. How many trees do I have to plant on my lot with a newly built home?
8. What should I do if I see pollution going into the storm drain or in my pond?
9. What can I do to improve the water quality in Lakeville?
10. I have a group that is interested in volunteering. How do I find out what opportunities are available?
11. I have a tree in my backyard that looks sick. Who can I contact to determine if it is diseased and if there is anything I should do?
12. Where can I purchase native plants?