Important messages for today:

• Recent precipitation has caused increases in lake levels
• The ground is saturated, and ponds and wetlands are near capacity
• Precipitation for the year is significantly above average and further excess rainfall could lead to drainage problems and lake flooding
• City Public Works staff has installed “Save our Shores” signs at the Prior Lake and Spring Lake boat launches, asking boaters to voluntarily travel at slow no-wake speeds

The Prior Lake area received another 1.5 to 2 inches of rainfall yesterday (May 27), bringing the total for the year to about 15 inches. We have received about 41 inches of precipitation over the past year (i.e. over the last 365 days). The average annual precipitation for our area is about 31 inches. The past 7 years have all seen above-average precipitation. It is wet out there. Our local lakes are at risk for flooding if the wet weather pattern continues into the summer months.

Prior Lake’s elevation is now 903.63 feet and has leveled off since yesterday’s rainfall. Spring Lake is at 911.88 and rising. We anticipate a secondary rise in Prior Lake elevation due to Spring Lake inputs. If Prior Lake rises past its Ordinary High Water (OHW) elevation of 903.9 feet, a slow no-wake restriction will be enacted. Spring Lake is still about a foot below its OHW. Sandbags and sand will be made available to residents if either lake exceeds its OHW.

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Multiple lakeshore owners reported abnormal lake conditions during and after ice melt of 2019 on Spring and Upper Prior Lake. Most reports were from Spring Lake and included reddish-brown water and pinkish-red patches or “blobs.”  Upper Prior Lake reports mostly consisted of brown-colored water.

The brown material was coming up from cracks in the ice prior to ice out on Spring Lake.  After ice-out, a brown scum formed on the surface when the lake was calm.  During windy conditions, the scum mixed in the water and turned the lake water a brown color.  A resident that lives on the south shore of Spring Lake sent pictures on April 23, concerned about the brown scum and stating the “whole lake has a brown hue.”  Conditions have gotten better, but as of May 7, a lakeshore resident on the north shore of Spring Lake stated, “The water in the lake is still brown everywhere.” 

Upper Prior Lake residents were also inquiring as to why the water was so brown. Kim Silvernagel, a lakeshore resident on the west side of Upper Prior Lake for over 19 years and a member of the PLSLWD Citizen Advisory Committee has never seen the lake this brown before and said the lake is “typically is so clear this time of year.”

What is it?

Several lake experts from the metro were consulted, but nobody could tell exactly what it was from the pictures.  It was perplexing.  The Watershed brought a sample of the brown scum from Spring Lake to the Minnesota Pollution Control Agency (MPCA) on April 25^th.  Matt Lindon, Research Scientist at the MPCA analyzed the sample and concluded that “Given the reports, photos, and microscope assessment, it looks like multiple surface water conditions have been going on in the lake in recent months.”  However, the dominant substance was found to be “Centric filamentous diatoms, likely a species of Melosira, a commonly found diatom in the Midwest.”  In other words, it was an abnormal amount of a normal substance.  Diatoms are a brown alga that is a commonly found in springtime.

After more reports of brown water came in from Upper Prior Lake, a sample was collected from Upper Prior lake on May 6 to see if it was the same condition as Spring Lake.  “The sample had very little algae in it,” according to Lindon.  From what he could find, it was mostly diatoms (brown algae) which “are common types of algae and typically not associated with problem lake conditions in Minnesota.”

There were also some pinkish-red patches that were reported during ice-out on Spring Lake.  A sample of these were not collected, but from pictures, Lindon believes “the bright reddish-purple conditions in March and early April may have been colonies of bacteria that can grow under certain conditions”. 

Is it safe?

The sample indicates the substance is a common diatom, which is harmless.  However, the District did not sample the entire lake and can never guarantee the lake is safe to be in.  The samples that were analyzed were only one liter from the entire lake. 

Lakes are dynamic and conditions change every minute and are different everywhere across the lake.  The District hasn’t received any complaints of people or dogs getting sick from being in the water this spring.  The District recommends sticking to the MPCA adage, “When in doubt, best keep out.”   

Why did this happen?

We can’t tell what exactly caused these conditions, however there are some theories which may have contributed to this:

• The severe winter and lake ice conditions
• More available iron and/or organic matter in the
  water column
• The large fish kill on Spring Lake in fall of
  2018 could have contributed to changing the chemistry of the lake

“The expansive surface scum seen in late April appears to be partly a dense diatom bloom and could have little to no relations to prior referenced events on the lake.  It is not uncommon to have a diatom bloom in Minnesota lakes in the spring.  Spring Lake is a fertile lake prone to bacteria and algal events,” stated Lindon.

It’s possible we may never know exactly what caused this, especially if the conditions don’t happen again.  Lakes are a dynamic system with hundreds of variables changing constantly. 

What is the District going to do?

The District will continue to collect observations and note the conditions.  Hopefully this was just a one-time thing and it will not continue this way for the summer or future years.  If conditions persist, more sampling and analysis will occur.