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Frequently Asked Questions (FAQ)
about Lakes

Q: What causes the blue-green scum (looks like spilled paint) you see on some lakes?

A:In lakes that are over-enriched with nutrients (i.e., phosphorus and nitrogen), algae tend to prosper and create "algae blooms." One blue-green algal group in particular, Cyanophytes, can be quite bothersome. Blue-greens tend to form large colonies that float on the surface of the water. Some species of blue-greens (i.e., Aphanizomenon) look like grass clippings on the surface of the water whereas more fine-celled forms (i.e, Anabaena) may form large clumps -- both can accumulate on shores, making a real mess. Nuisance algae blooms can be especially pronounced during the "dog days" of summer when the water is warm and the weather is hot and calm. The characteristic paint-like appearance and the noxious odors from blue-green algal blooms result from the decomposition of the algae. Blue-green algae may produce toxins which can cause skin irritation in swimmers.  In some cases, animals (dogs, cattle, and waterfowl typically) that have consumed large amounts of the water containing the toxins have died. General rule: When you see conditions like this, avoid swimming or allowing your pets in the water until it clears up.

We can't eliminate blue-green algae from a lake -- they are an inherent part of the overall algal community in the lake. What we really want to do is control their overall intensity and the frequency of the blooms. Since we can't control the water temperature, the best thing we can do is to reduce the amount of nutrients getting into the lake. This can best be accomplished by reducing the amount of phosphorus and nitrogen from man-made sources such as lawn fertilizer, and runoff from cities, cultivated fields, feedlots and a myriad of other sources. Though a reduction of nuisance algal blooms will not be immediate, it is the best long-term solution to minimizing the frequency and intensity of algal blooms.

Q: What is the green floating stuff in my lake that looks like fluffy clouds or cotton candy?

A: It's probably filamentous algae, sometimes called "moss" or "pond scum." This is a common and troublesome aquatic plant that forms dense, hair-like mats. Filamentous algae form on near-shore bottom sediments or submerged objects in ponds and lakes where there is good transparency and where sunlight reaches the bottom of the pond or lake. As the algae grow, it produces oxygen that gets trapped in the entangled strands of algae. This entrapped oxygen makes the algae buoyant and causes it to rise to the surface of the pond or lake. Some of the more common forms of filamentous algae can be identified by their texture, although microscopic examination is usually required for exact recognition. Cladophora feels "cottony", while Spirogyra is bright green and very slimy to the touch. Pithophora (or "horse hair") has a very coarse texture like horse hair or steel wool. The best method for homeowners to remove filamentous algae is to rake out the floating clumps and limit the nutrients that reach the water from your property. You can compost these piles or use them in your garden as mulch. Remember, chemical control of aquatic plants does require a permit from the Minnesota Department of Natural Resources.

Q: What causes foam to gather on the shoreline?

A: In most instances, the "foam" we see on the surface of our lakes and streams is natural. It's created when air is mixed with natural organic compounds, such as decomposing plants and animals. The mixing or agitation in lakes is commonly caused by wind and wave action; in streams, it may result from the water flowing through a stretch of rapids or over a dam. As with other things that float on the surface, the foam will often collect on a downwind or downstream shore, and although it may not be aesthetically pleasing it is natural and harmless.

Extreme case of shoreline foam Photo: Robert Korth, UWEX/UW, Stevens Point

Typical case of shoreline foam

Q: Where can I find more information about the threat exotic species pose to Minnesota's lakes?

A: An "exotic" can be either a plant (e.g. Eurasian watermilfoil - Myriophyllum spicatum) or animal (e.g. zebra mussels - Dreissena polymorpha [pictured]) that is not native to lakes or streams in Minnesota. These exotics are introduced to either accidentally or intentionally. The Minnesota Department of Natural Resources (DNR) has an Invasives Species Program whose purpose is to curb the spread and minimize the harmful effects of nonnative species.

Q: What causes that yellowish powder or dust in my lake water?

A: The powder you are seeing is most likely pine pollen. It is very common to see this material in late spring to summer. After becoming waterlogged, the pollen sinks to the bottom. This is a natural event and shouldn't affect the overall water quality of your lake.

Q: What causes that "root beer" color in some lakes?

A:Sometimes described as root beer, coffee, tea, or bog stain, such coloring is natural for many waters throughout Minnesota; however, it tends to occur more frequently in northern Minnesota. The coloration is not harmful and results from incompletely dissolved organic materials, sometimes referred to as tannins, which come from the decomposition of wetland plants in the watershed of the lake. We often find that the shade of the water color is proportional to the quantity of wetlands in the watershed. Since the predominant land type in the watersheds of northern Minnesota lakes is a mixture of forests and wetlands, the lake water can have very dark coloration in that region of the state. When coloration is very dark, it can limit water transparency.

Q: What causes swimmers itch?

A: Swimmer's itch is caused by a blood fluke common in waterfowl. The itching is caused when the fluke, while in the immature stage of its life cycle (referred to as "cercaria"), mistakes you for its primary host -- waterfowl -- and burrows into your skin. Aside from the annoying itching, it's harmless to humans. Learn more about swimmer's itch and how to avoid getting it at the Minnesota DNR's Web site. 

Q: How can I get rid of all the weeds by my dock?

A: Aquatic plant communities provide critical fish and wildlife habitat. Native emergent plants are natural shoreline stabilizers because they reduce wave action that can erode lakeshore, and they also provide important cover, nesting and feeding areas for fish and wildlife. Muskgrass (Chara spp.), bushy pondweeds (Najas spp.), and broad-leaf pondweeds (Potamogeton spp.) provide shade, shelter and foraging opportunities for fish. The taller, denser foliage of coontail (Ceratophyllum demersum) and northern watermilfoil (Myriophyllum sibiricum) provide excellent habitat for aquatic invertebrates. Bushy pondweeds are among the most important plants for waterfowl, which consume the stems and seeds of the plant. Aquatic plant stems may also be grazed by muskrat, deer and beaver. A healthy diversity of native aquatic plants can also help prevent exotic species from becoming established in the lake. Because aquatic plants play such an important role in the aquatic ecology of the lake, removal of these plants should be done in a manner that limits the disturbance to the overall plant community. Removal of aquatic plants may require a permit from the Minnesota Department of Natural Resources. Find out more information at the DNR's Aquatic Plant Management Program Web page.

Q: How does a lot of boat traffic affect the water quality of a lake?

A: The only study we know of that investigates the impact of boat traffic on water quality is conducted in 1996 by the Wisconsin Department of Natural Resources. Titled "Impacts of Motor Boats on Water Quality in Wisconsin Lakes," it looked mainly at traffic issues, and did not address the impact on water quality of the oil and gas used in boat motors. Some general conclusions from this report were: 1) Boat counts on weekends doubled or tripled on average for most weekends; 2) water clarity was temporarily reduced due to increased turbidity by nearly 10% on weekends; 3) shallow lakes and near-shore areas are more affected than deeper lakes; and 4) boat traffic may stimulate algal growth in lakes containing soft-water sediments. The study suggested that impacts on aquatic plant communities and shoreline erosion need to be studied further.

Another Wisconsin study looking at the effects of motor boats on submerged plants, "Effects of Motor Boats on Submerged Aquatic Macrophytes," was printed in the Journal of Lake and Reservoir Management 13(1):1-12, 1997. This paper concludes that motor boat traffic does reduce plant biomass primarily through direct cutting and scouring of sediments.

A 1998 study by the Minnesota DNR compares boating trends in north central Minnesota from 1985-1998. This study looks at trends in total number of boats, boating perceptions and attitudes, and the use of public accesses. You can read about it in more detail at the Minnesota's DNR's Web site.

More Information or Assistance

If you have questions that weren't answered above, contact Matt Lindon, 651-757-2530, or Kelly O’Hara, 651-757-2622.

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