Row, row, row your boat gently down the stream. Merrily, merrily, merrily, merrily, life is but a…hey! What’s all that green gunk hanging off my paddle, and what is that awful smell? Yikes! I think I just saw a Loch Ness monster!
If you’ve explored the lower stretch of Lake St. Croix or many of our inland lakes in late summer, you’ve probably shared a similar experience. Around this time of year, water temperatures are at a peak and algae and aquatic plants grow exponentially. To some extent, the greening of our lakes is a natural, seasonal change, much like the changing colors of tree leaves in the fall. In some lakes, however, high levels of phosphorus amplify this natural process, allowing the algae and plants to grow faster and larger than they would normally.
To the casual observer, one grimy, green, gross lake may be indistinguishable from the next, but there are important differences to look for. For example, duckweed is a beneficial aquatic 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 plants up close, you can actually distinguish little tiny leaves floating and little tiny roots dangling into the water. As the name implies, duckweed is a good food source for ducks and other waterfowl. Much of the green that is currently floating on our lakes are duckweed, though there is plenty of filamentous algae as well.
Filamentous green algae, which is commonly found in lakes, is your stereotypical swamp monster type of algae – gooey, blobby, and greenish brown. It’s a recreational nuisance but not toxic. Chara, a form of filamentous algae found in lakes with good water quality, has long, stringy strands and looks more like a plant without roots.
The biggest concern for human and aquatic health are blue-green algae, which usually look like pea soup or spilled green paint in the water. Blue-green algae exists in all of our lakes and rivers and are a critical part of the aquatic food web, but the algae can quickly multiply into large colonies during the summer, causing blooms that are sometimes toxic to people and animals. The majority of blue-green algae blooms are safe, but it is impossible to tell the difference between a safe bloom and a toxic bloom just from looking at the water. People and animals may develop skin irritation or upper respiratory problems from exposure to harmful algae blooms, and in extreme cases, dogs and other animals have even died after drinking lake water containing these toxins. Algae blooms are also problematic if they grow large enough to cover an entire lake or pond because the algae consume oxygen during the night to fuel their growth when sunlight is not available. As a result, dissolved oxygen levels in the water plummet and fish die.
Two weeks ago, the Ramsey-Washington Metro Watershed District discovered a small blue-green algae bloom in Carver Lake in Woodbury. Though the bloom doesn’t appear to pose a large threat to the lake, the city is still advising beach-goers to swim at their own risk until the algae has disappeared. Blue-green algae blooms have also cropped up in various locations along the St. Croix River in recent years, particularly along the southern stretch between Stillwater and Prescott.
In freshwater systems, phosphorus acts as a limiting nutrient for algae and plant growth, meaning the algae will keep growing until they run out of phosphorus. All lakes need some phosphorus, but some get way more than they need, due to runoff from surrounding neighborhoods, businesses and farms. Minnesota passed a “phosphorus-free” fertilizer act in 2005 (Wisconsin has one too) as one way to keep the nutrient out of lakes. It is now illegal to apply fertilizer containing phosphorus to a lawn unless you are seeding or establishing new sod. Fertilizer isn’t the only problem, however. Yard waste like lawn clippings, leaves and seeds also contain a lot of phosphorus and can easily be washed into storm drains that connect to our lakes and rivers when it rains. One easy way to help keep lakes from going green is to rake and sweep up dirt and yard waste from sidewalks, driveways and streets throughout the year.
Learn more about how to identify algae and other aquatic plants from the Minnesota Department of Natural Resources.