Treatment for curlyleaf pondweed (a non-native, invasive aquatic plant) will be completed on Upper Prior, Lower Prior, and Spring lakes on Thursday, May 19th. It is recommended that you wait three days after treatment to use lake water to irrigate lawns or gardens.

The District completed aquatic plant surveys this spring that identified several areas with high concentrations of curlyleaf pondweed for treatment. Treatment areas are numbered and marked on the maps below in orange for Prior Lake and purple for Spring Lake.

Why treat? As is common with non-native, invasive species, curlyleaf pondweed is one of the first things to grow in the spring. Unfortunately, this gives it a head start and allows the curlyleaf to crowd out native aquatic plants that are good for the lake. Treating the curlyleaf gives native plants a chance to grow. Another problem that curlyleaf presents is its early die-off, which typically occurs in mid-summer. This die-off provides an additional source of phosphorus during a time of year when algael blooms are already more common, potentially leading to stronger blooms.

According to the US Drought Monitor, the Prior Lake-Spring Lake Watershed District area is currently experiencing moderate drought. Possible impacts for this drought category include damage to crops and pastures, low water in lakes and streams, developing or imminent water shortages, and water use restrictions. Our local lakes, streams, ponds, and wetlands are all being impacted by the lack of precipitation.

Low water levels are not completely abnormal in the Prior Lake area.  The following graphs and images show fluctuations in water levels on Prior Lake since 1906.  Figure 1 displays the minimum lake level that was reported on each year there was data. Keep in mind, not all years have data and some years only have one reading, especially on years prior to 1970. The lowest level of 883.60’ was recorded in 1938, nearly 18’ lower than the lowest 2021 level of 901.34.

Figure 1: Prior Lake levels have been recorded since 1906. 

Prior Lake in 2016: Elevation 902.5′

Prior Lake in 1937: Elevation 886.0′

Not only is the drought impacting water levels, but excessively hot temperatures are contributing towards algae growth and low dissolved oxygen levels, which can lead to fish kills.  Dead and dying fish have been observed recently on Little Prior and Lower Prior lakes.

Little Prior Lake Fish Kill on 7/29/2021

Low water levels and hot temperatures are believed to have contributed to this fish kill on Little Prior Lake observed on 7/29/2021.  Prior Lake-Spring Lake Watershed District staff reported dissolved oxygen (DO) levels ranging between 2.32 mg/l at the surface and 0.11 mg/l at the bottom of the lake that day.  Fish generally get stressed out when DO is lower than 4 mg/L, though different species have different tolerances.  Dead fish observed include panfish, largemouth bass, and northern pike.

To report fish kills, please call the state duty officer at 651-649-5451 and email Jaime Rockney at the Prior Lake-Spring Lake Watershed District at jrockney@plslwd.org.

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.”