Algae Blooms in Lakes and Ponds

According to the Colorado Department of Public Health and Environment, harmful algae blooms often have the following characteristics:

• May look like thick pea soup or spilled paint on the water’s surface.
• Can create a thick mat of foam along the shoreline.
• Usually are green or blue-green, although they can be other colors, including red or gold.
• Sometimes are made up of small specks or blobs floating just at or below the water’s surface.

Harmful algae blooms are NOT:

• Long, stringy bright green grass strands that feel either slimy or cottony.
• Mustard yellow (this is probably pollen).

Blue-green algae thrive in warm, nutrient rich water. Water nutrients such as nitrogen and phosphorus help algae grow and support fish and other aquatic life. But too much nitrogen and phosphorus in the water allows blue-green algae to grow quickly and form blooms and scums. Blooms can occur anytime but are most common during hot, sunny weather and in slow-moving water bodies such as lakes.

Swimming is not permitted in any park water bodies in the City except Union Reservoir per Longmont’s Park Regulations. The rule is based on State Health Department requirements because water quality monitoring is not conducted at any park water body except Union Reservoir.

• Union Reservoir is the only City pond where recreational contact, specifically swimming, is allowed.
• McIntosh Lake is not a City pond, but the City has a recreational lease. Carry-on, non-motorized boats only. This includes canoes, kayaks, bellyboats, sailboats and other small boats. This does not include inner tubes.

Current rules and regulations do not prohibit leashed pets from entering a water body in a City park or open space, with the exception of waters at Button Rock Preserve  However, pet owners should always exercise caution and keep pets out of the water if there is any question about the presence or absence of a harmful algae bloom.

Some algae blooms produce toxins that can cause illness in humans, pets and wildlife that come in contact with the algae. Humans can present a range of symptoms. Download the Colorado Department of Public Health and Environment’s Fact Sheet [PDF] about algae blooms to learn more.

The timing, intensity and duration of a blue-green algae bloom will vary from year to year based on nutrient availability, air and water temperatures, sunlight and wind velocity. Since these factors vary from year to year, blooms cannot be accurately predicted.

We can’t eliminate blue-green algae from a lake — they are common in the overall algal community. 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.

These algae prefer slow moving or stagnant water. Flowing streams like the St. Vrain and Left Hand Creek are far less susceptible to a bloom; however, as water levels decrease following spring runoff, stagnant pools may form along the banks of flowing streams. These pools could be susceptible to a bloom and should be avoided if there is any question about the presence of harmful algae.

If you believe that you or your pet has become ill because of harmful algae, seek medical attention. For questions about harmful algae health effects, contact the poison control center. If you believe that a pet or livestock has become ill because of harmful algae, contact your veterinarian immediately.

Download this brochure developed by New York Sea Grant [PDF] to learn more information about how to keep your pets safe.

The City of Longmont has good quality water that meets all State and Federal regulations. It obtains its source water from North St. Vrain Creek, Carter Lake and South St. Vrain Creek. These sources are high quality, protected and monitored extensively. Minimal human impact and low nutrient loading make them less susceptible to blue-green algal blooms. The City of Longmont Water Treatment Plant visually inspects source water on a daily basis and has procedures in place to deal with an algal bloom. Its treatment processes would be able to handle all but the most extreme of blue-green algal blooms. In 2018, the City of Longmont participated in an EPA study that included monitoring for cyanotoxins in its treated water. The cyanotoxins were not detected in any of the samples.

Algae blooms are triggered, in part, by excess nutrients in the water. There are steps you can take at home to help prevent a bloom. Fertilizers used on lawns and yard waste like grass clippings and leaves contain these nutrients. You can reduce your impact by limiting the use of fertilizers to only what you need, adjusting your sprinklers to avoid excess water running off onto the sidewalk or street, and composting leaves and grass clippings or sweeping them back onto landscaping.

Soaps used to wash vehicles can also contain nutrients that algae love. Consider using full or self-service carwashes that recycle or treat the water used. If washing a vehicle at home, limit the amount of soap used and rinse excess onto landscaping instead of into the street where it will eventually reach our creeks, ponds or reservoirs.