The Prior Lake-Spring Lake Watershed District has received numerous telephone calls and emails this spring reporting more “slime” in the water than normal. This “slime” is actually called filamentous algae. Admittedly, it does look unpleasant, but there are no health concerns with contact.
Filamentous algae are common and can fluctuate in abundance depending mostly on climate conditions and nutrient levels. Because of the current low water levels and abnormally warm air temperatures, conditions are ripe for producing algae.
Filamentous algae starts by growing under the surface of the water on rocks and plants, but will often float to the surface and create a mat resembling wet wool. Sometimes you can even see gas bubbles under the algae making it look especially concerning, but that’s normal for these algae. Filamentous algae may even float to the surface and then sink back down… and repeat the process. Depending on the age or condition of the algae, it can be a bright-green, lime-green, or dark-green. The algae have long strings, or “filaments” when picked up. Here are several images of the non-toxic filamentous algae on Jeffers Pond in June 2021:
Filamentous algae is sometimes confused with blue-green algae, or Cyanobacteria. It is important to know the difference because blue-green algae may contain a toxin (microcystin) that can make people sick or even kill dogs and livestock. Therefore, caution should be exercised to deter animals from drinking water with blue-green algae. No humans have been known to die from it, but it could accidentally get splashed into a person’s mouth if swimming in it, potentially causing illness.
Normally blue-green algae grows in the “dog-days” of summer, but it is possible to have spring or fall blooms, if the conditions are right. With the abnormally hot conditions lately, algae blooms are expected earlier in the season, including blue-green algae.
Blue green algae can be described as looking like pea soup or spilled green paint floating on the surface. It does not have long strands or float in mats like filamentous algae. Instead, it is mixed into the water column or floating in globs that aren’t connected by strands. Here are several images of blue-green algae found in Upper Prior Lake in 2016 and 2018:
Algae is a natural part of the ecosystem and an important part of the food chain. Lake water cannot be expected to be crystal clear all the time. The District does not treat algal blooms and there are currently no short-term solutions to fix algal blooms in lakes. Instead, the District focuses efforts on reducing the amount of nutrients reaching lakes and streams that support algae blooms. Once a bloom occurs, the only option is to wait for the weather to change, such as significant rainfall, wind shifts, or cooler temperatures, to disrupt the algae’s growth. While labor intensive, filamentous algae can be removed with raking. Other algae cannot be removed physically as it is mixed in with the water. If you suspect blue-green algae, follow the motto, “When in doubt, best keep out.”