Serpent Lake’s Fight to be Clear Again

The following information was provided to us by Arlen Bowen. The first bit of information is about the fluorescent green algae that we have been seeing in the lake this year. The second part is an article that was submitted to the courier.

Filament Algae

Filament Algae is the bright green growth that looks like strands of hair.  This algae grows on rooted plants that are stressed and dying off.  Two infestations have occurred.  In 2009, the rooted plants in 3 to 15 foot of water were stressed due to less sunlight as the clarity had been dropping.  In 2020, the infestation is due to increased clarity.  Rooted plants are now growing strong in water beyond 15 feet as the lake clarity increases.  The new strong deep plants are forcing marginal rooted plants in shallower water to die.  In late June, patches of rooted plants in shallow water with Filament Algae attached were clearly visible around the lake.   Then, in July, these rooted plants broke-free with the Filament Algae hanging on.  The algae and host plant floated where ever the wind would blow them.  Both plants turned brown and at times gather to form an unsightly mess.  As the lake clarity stabilizes,  the unsightly mess will diminish.

Serpent Lake’s Fight to be Clear Again


A wakeup-call for many living on or near Serpent Lake came on September 29, 2003.  An algae bloom covered 80% of the lake’s north shoreline for the next 5 days.  Old-timers report that was the first-time they saw such a large bloom.  Algae increased to the point where dead algae piled up on the surface up to 70 feet from shore.  This was Serpent Lake’s “red tide”, thought only to happened in warmer water with serious agricultural runoff.  Was Serpent Lake loved to death?  Could that be the underlying cause for the algae bloom?  


Serpent Lake sits at the top of a watershed; no cities upstream adding contaminants and no large farm fields upstream adding runoff.  In 1960, the Girl Scouts counted only 127 docks on Serpent; the lake shore was not crowded.  

In the 1977-1979 timeframe the clarity of the lake was first measured by the Serpent Lake Association (SLA) using a Secchi disk.  This disk would disappear when lowered 20.2 feet out in the middle of the lake in 50 feet of water.   The lake was crystal clear; the lake was number one for walleye fishing for several years; there were jobs available in the area and best of all there were lake lots available to build year-round homes.  

People loved The Serpent.

Serpent is not a typical MN lake with just one town.  Serpent has two towns with storm sewer pipes running into the lake from everywhere.  Cabins and homes ended up being packed along nine miles of shoreline.  By 2005, there were 292 docks.  Cabins became lake homes with 2 car garages, paved driveways, no rain gardens and lawns mowed right down to the lake.  

Crow Wing Soil and Water Conservation District reported that shoreline development had increased to the point where Serpent Lake had a higher impervious surface percentage within 500 feet of shore than 52 other lakes in the county. Sixty lakes were in the study.   

More impervious surface means more runoff, more runoff means more phosphorous, more phosphorous means more algae, more algae means a lower Secchi disk reading.  The 20.2 foot average Secchi of the 1970s decreased year-by-year until it reached its lowest average ever in 2012 of 12.1 feet.       

The Serpent was “loved-to-death”.


Prior to the algae blooms of 2003 and 2004, people recognized the lake needed help.  The City of Deerwood stopped using a small upstream lake, Cranberry, as a full-time lagoon in the 1970’s.  Crow Wing County townships adopted the 1989 MN Shoreland Standard which limiting impervious surface percentage on shoreline property.  

In 2004, SLA started a robust water testing program with standards supported by MPCA and data admissible in a court of law.  Variations in the annual Secchi value were reduced by averaging 12 evenly-spaced readings between June 1 and September 30.

Lakeshore owners spoke out strongly in 2005 to reject a zoning change for the development of a high-density housing complex with a large marina out into the lake 

Aerial photographs by AW Research identified properties with extra heavy algae growth by their shore.  SLA paid to have the photos taken and then supported the testing of all 143 septic systems not tested in the last 5 years.  Lakeshore owners, without being asked, replaced the 30 cesspools found with holding tanks or drain fields.  Phosphorus seepage into the lake was reduced.

In 2005, The state required shoreline fertilizer must be phosphorus-free.   SLA delivered a free bag of it to every lakeshore owner to help sell the idea.  

The City of Crosby and SLA funding helped divert runoff from Cross Ave into two rain gardens in 2009.  Many SLA members adopted buffer zones along their shore to reduce lawn clippings, leaves, and runoff from entering the lake.  Plants in the buffer zone have very long root systems that absorb runoff and capture phosphorous.   

All this effort, over 30 years, combined to slow the year-to-year decline in clarity to one foot every 5.7 years. A big push is needed to get headed back to clearer water.   


In 2012, a two-year study ended that looked at many sources of phosphorus.  A partnership started, supported and funded by SLA, Crow Wing Soil and Water Conservation District (CW SWCD) and MPCA to collect data.  This data was used in the watershed model to quantify individual phosphorus sources.  SLA contributed by recording stream flows in and out of the lake and their phosphorus levels.  The final report identified the top three contaminant sources as runoff from Crosby, runoff from Deerwood and stream flow from Cranberry Lake.  

SLA was named the 2013 “The Crow Wing County’s Conservationist of the Year” award by CW SWCD.

In 2014, using the 2012 study results, a $1.2 million Legacy Fund grant was awarded by BWSR/MPCA to reduce annual phosphorus in Serpent by 78 pounds (20 percent) or the equivalent of 20 tons of algae.  The grant had four goals: reduce Crosby stormwater runoff, reduce Deerwood stormwater runoff, reduce phosphorous levels in Cranberry Lake and upgrade zoning ordinances to reduce stormwater runoff around the entire shoreline. 

In 2016 to 2018, Legacy Funds were spent diverting Crosby and Deerwood runoff into holding ponds and filtration systems.  Cranberry Lake’s total phosphorus concentration was reduced 75% by treating the it with alum.  Three of four governmental units on Serpent Lake adopted new stormwater runoff ordinances. 

The 2017 clarity was 18.3 feet, an increase in clarity of over 6 feet in just 5 years.  Secchi readings, after 2017, had some “degradation” from bike path construction and some “improvement” due to Zebra Mussel filtering out algae.  

Flow out of Serpent Lake each year is only 33% of the lake’s total volume.  This percentage means projects completed in 2018 will contribute to clarity improvements in 2019, 2020 and 2021.  


In 2019, Serpent Lake Secchi disk readings averaged 21.7 feet.  The best ever recorded.  Better than the “bench-mark” years of 1977- 79, when the three-year average was 20.2 feet. 

The City of Crosby received the 2019 “Minnesota Community Conservationist Award” for reducing city runoff into Serpent from 14 acres along SE Second Street.  

Serpent needed many projects, over 40+ years, to show a lake’s decline can be reversed and returned to its former crystal-clear state.  Serpent Lake is one of the first in MN to demonstrate that a serious decline can be reversed.  There is now hope for other lakes dealing with serious algae bloom. 

Serpent Lake has had an unexpected consequence result by reducing phosphorus and algae mass for water clarity.  The Zebra Mussel hatch in May of 2019 had a death rate of over 99%.  The yearlings died from the lack of algae as they grew. This low survival rate was predicted by a large lake study in Ontario Canada where clearer lakes had fewer Zebra Mussels.  

An unexpected blessing… The Serpent is BACK-TO-CLEAR.

Thanks to SLA members, many government officials, CW SWCD, BWSR and finally, the taxpayer’s support of the Legacy Fund.           


The Ontario study also shows that with a clarity increase of another two or three feet, Serpent would have NO Zebra Mussels.  This is within reach.  Consider reducing runoff by building a rain garden or adding a “No-Mow” buffer zone along the shore or using pavers to add (or replace) a driveway or patio.  Remember to use only phosphorus-free lawn fertilizer.     

Let’s keep going and get both: no Zebra Mussels and great clarity.  That would be “VERY unexpected”.  

Attached Pictures:

Data Graph: Secchi Disk (1977 to 2019)           

            Sampling:  Lowering a Secchi Disk into Serpent Lake