Green days on a summer lake – and that’s not a good thing

Last week I was at Carver Lake in Woodbury setting up a water pollution mystery game. The park has 4-miles of single-track mountain bike trails and a brand-new bike park for kids to practice their skills riding over rollers, ramps, and rocks. There are also paved walking trails, playgrounds, and a swimming beach. Unfortunately, however, the beach at Carver Lake in Woodbury has been closed for most of the summer due to a blue-green algae bloom.

Take a ride on the trails at Carver Lake and learn why algae blooms have closed the beach.

Green Water:

To the casual observer, green water is usually considered an indication of poor water quality. However, that is not always the case. 

Sometimes a lake appears to be green because it is covered in duckweed, a beneficial native plant that floats on top of the water. From a distance, a duckweed covered pond or lake may look green and slimy, but if you look at the water up close, you can actually distinguish little tiny leaves floating on the water’s surface and tiny roots dangling down into the water. As the name implies, duckweed is a good food source for ducks and other waterfowl.

Not all green is a sign of bad water quality. On the left, the water is filled with blue-green and filamentous algae. In the center, the water is clear but covered in duckweed. On the right, the water is clear but has aquatic plants growing below the surface of the water.

Filamentous green algae can make swimming and boating unpleasant but is not harmful to humans and wildlife. The filaments often tangle together to form stringy, hair-like strands or slimy, green mats that float on the water’s surface. Filamentous algae are a natural part of Minnesota lakes and provide cover for small aquatic insects and animals that fish eat. Chara, a form of filamentous algae found in lakes with good water quality, has long, stringy strands and looks like a plant without roots. The life cycle of filamentous algae is 30 to 60 days.

The biggest concern for human and aquatic health is blue-green algae, which usually looks like pea soup or spilled green paint in the water. Small amounts of blue-green algae exist in all of our lakes and rivers and are an essential component of the natural food web. When a lake has too many nutrients, however, these algae can quickly multiply into large colonies and create blooms that are sometimes toxic to people and animals. If an algae bloom grows large enough, it can even kill fish by depriving the water of dissolved oxygen.

Algae bloom along the St Croix River at the Kinnickinnic Beach (2007)

Stormwater pollution:

Algae blooms in Carver Lake and other urban lakes are usually caused by too much phosphorus flowing into the water from sidewalks, streets, and parking lots. This runoff is known as stormwater pollution.

Local cities work in partnership with watershed districts and private landowners to complete stormwater retrofit projects to reduce runoff pollution. Projects in Woodbury have included narrowing residential streets, installing raingardens along roadways to capture and infiltrate runoff, and re-designing existing stormwater ponds to filter out more phosphorus. These large-scale projects are effective, but also costly.

In Woodbury, some city streets have been retrofitted to reduce stormwater runoff. Raingardens, like the one at this corner, catch runoff from the street and soak it into the ground.

There are also several ways that community residents can help to reduce stormwater pollution. Yard waste like lawn clippings, leaves, and seeds contains high levels of phosphorus that can be washed into our lakes via storm drains when it rains. Help to keep our lakes blue by sweeping dirt and yard waste off of your sidewalk, driveway and curb-line throughout the year and adopting your local storm drain: If you use lawn fertilizer, be careful not to over-apply and sweep up any granules that spill onto the sidewalk or driveway. 

If you live on one of the lakes, maintain a buffer of native plants along your shoreline, inspect your septic system regularly, and consider building raingardens to capture and filter stormwater runoff from your house and driveway. For advice and grant assistance, sign up for a free site visit at