Prior Lake Green Water Update – What Lakeshore Residents Need to Know

By: Upper Prior Lake_fall 2016_greenSteven Reinders (PLA Board), Jamie Rockney and Kathryn Keller-Miller (Prior Lake-Spring Lake Watershed District)

Late summer algae blooms on Upper Prior Lake created excessively green, unswimmable, and potentially unhealthy lake conditions in 2016 (see photo at right). The underlying cause of this problem is complicated and the following is a summary of why this could be happening and what is being done to address the problem.



 upper vs lower water comparison       filter discs upper (left) vs lower (right)

Lower Prior water on left; Upper Prior water on right                  Upper Prior filtration disc on left, Lower Prior on right


Why is Upper Prior so much greener than Lower Prior Lake?

Phosphorus is the main factor that causes algae growth.  The average concentration of phosphorus in Upper Prior is higher than Lower Prior, which is one of the main reasons for the difference in water quality between the two lakes.

The solid lines, in the graph, below show the average concentrations of phosphorus in the two lakes since 2003.  The dotted lines represent the water quality standards, which are different for each lake (more explanation in the following section). If phosphorus concentrations continuously exceed the standards, the lake is considered impaired, or polluted, by the Minnesota Pollution Control Agency. Upper Prior exceeds the water quality standard more often than not, while Lower Prior has stayed well below the standard for the last 13 years. For more water quality information on Upper Prior Lake, visit the Prior Lake-Spring Lake Watershed District’s website.

Prior Lake phosphorus 2003-2016

Figure. May-September average phosphorus concentration for the years 2003-2016 on Upper Prior Lake (red) and Lower Prior Lake (blue).

The graph above shows there is a slight decrease in phosphorus over the past 14 years.  Why then, was the algae bloom so bad in Upper Prior lake in 2016?  There are many other factors that can impact the algae growth, including: lake depth, carp, vegetation, temperature and weather and boat traffic.

Depth:  In general, a lake that is 15 feet or less in depth is considered a shallow lake and a lake deeper than 15 feet is classified as a deep lake. Thus, Upper Prior is considered a shallow lake and Lower Prior a deep lake.  Shallow lakes generally have higher concentrations of phosphorus for many reasons, so the water quality standards for deep and shallow lakes are set at different levels.  It is not “fair” to expect the water quality of a shallow lake to be as good as a deep lake.  For more information on the challenges of managing shallow lakes, read the article Shallow and Deep Lakes: Determining Successful Management Options by Cooke, Lombardo, and Brant.

Water layers circulate more easily and frequently in shallow lakes. This brings phosphorous contained in the sediments at the bottom of the lake to the upper water layers where they can feed algae blooms.  This is especially true if other factors are helping to churn up the phosphorous and bring it to the surface.  Water layers in deeper lakes tend to circulate less, leaving the sediments and phosphorous on the bottom.

Lake depth and stratification

Lake depth & mixing: shallow lakes mix easier than deep lakes.

Carp: Carp are an invasive, bottom feeding fish which root up and erode the lake floor sediments, releasing phosphorous and making it available to feed algae blooms.  They also destroy vegetation which takes up phosphorous and keeps sediments in place. The recommended maximum threshold for a lake’s carp population is approximately 100 pounds per acre. Given recent fish surveys, it is estimated that Lower Prior Lake is at or below that level whereas Upper Prior Lake has approximately 3-5 times that amount.

Carp prefer higher oxygen levels in the water. In shallow lakes, higher oxygen levels can extend all the way to the bottom of the lake, making the whole lake hospitable for carp.  In deeper lakes, higher water oxygen levels are found around the shorelines but the deeper waters have lower oxygen levels, limiting the area that carp will inhabit and damage. Carp also excrete one half of their total daily food intake into the lake which provides additional fertilizer for algae blooms.

Vegetation:  Plants compete with algae to take up phosphorous; more plants mean fewer algae blooms and less green water. Based on studies by the Prior Lake-Spring Lake Watershed District (PLSLWD), approximately 10% of Upper Prior has plants growing on the lake bottom compared to about 40% or more in Lower Prior.  According to a study by Canfield, plant coverage of 40% or more is ideal for water clarity.  See the Aquatic Vegetation Density Mapping Report by PLSLWD for more info on vegetation density in our local lakes.

Vegetation plays an important role in stabilizing the lake bottom and reducing phosphorous release. When vegetation is absent, wind, waves and boat traffic can cause lake sediment to become suspended.  Carp play a major role in the lack of vegetation as their bottom feeding habits uproot naturally occurring vegetation.

Water clarity also impacts vegetation growth.  When water clarity is low, sunlight cannot reach plants to provide sunlight for photosynthesis.  85% of Upper Prior lake is less than 15 feet deep, which is the ideal depth for plant growth.  If water clarity increases significantly in Upper Prior Lake, it is possible that plants could grow in 85% of the lake bottom, rather than the current 10%.  Although this would significantly benefit water clarity and habitat for aquatic life, vegetation can be a hindrance to recreation.

Temperature and Weather:  Heavy winds can churn up lake bottom sediment and release phosphorous. This happens more commonly in shallow lakes because the sediments are closer to the surface. As 2016 had some very windy weather, extra phosphorus was probably stirred up from the lake bottom and further fueled algae growth.

Shallow lakes also tend to be warmer than deep lakes which further contributes to algae growth.  The winter of 2015-16 saw a very late Ice-On and an early Ice-Out which allowed the lake to warm up sooner than normal, potentially increasing algae growth.

According to the Minnesota DNR, 2016 was the 5th warmest year on record for Minnesota since 1895 (DNR; Looking Back at 2016). Not only was 2016 very warm, it experienced the 2nd wettest year on record since 1895.The DNR also noted that none of the past 122 years have been both this wet and warm. Heavy rains and storms create mixing conditions in the lake while adding additional phosphorous into the lake from stormwater runoff, providing more phosphorus for algae. High lake levels can also allow cause phosphorous to seep into the lake from shorelines. The 2014 flood saw record rains and high lake levels which may have contributed to the current problem.

Boat Traffic:  Wakes from boats can mix lake sediments and erode shorelines releasing phosphorous. This is also more common in shallow lakes as the sediments are closer to the surface and more easily disrupted.


Where is the Phosphorous Coming From? External Load vs. Internal Load

Phosphorus can either come from sources outside of the lake or from sources within the lake. These sources are often referred to as a lake’s external and internal pollutant loads.

The external load is the phosphorous entering the lake from overland run-off, streams, ditches, storm water, and other sources. The main sources of external loading in Prior Lake are likely lawn and farm fertilizers as well as leaves and other organic materials entering the lake either directly or indirectly from run-off in the watershed. Upper Prior Lake is fed by Spring Lake, which is in turn fed by the drainage from the upper watershed. This funnels water and pollutants like phosphorous from the upper watershed into Spring Lake, which then flows into Upper and Lower Prior Lake and eventually into the Minnesota and Mississippi Rivers which ultimately ends up in the Gulf of Mexico.

Many local city streets and gutters drain directly into Prior Lake. This also allows lawn fertilizers, leaves and other organic material to enter the lake.

The internal load is the phosphorous stored in the lake bottom sediments. This can be from natural sources, such as animal feces or decaying vegetation, or from septic systems that drained into the lake in the past, as well as from other external sources that settle to the lake bottom and accumulated over time. This phosphorus can sit at the bottom of the lake and be relatively inactive, but certain situations, like mixing or low oxygen levels, can release the phosphorus from the bottom of the lake making it available for algae.


What is being done to control the Phosphorus from ENTERING Spring Lake and Upper Prior Lake?

  • In 1998, the Prior Lake-Spring Lake Watershed District (PLSLWD) installed a Ferric Chloride treatment system southwest of Spring Lake where County Ditch 13 drains into Spring Lake (and eventually Upper Prior Lake). Ferric Chloride binds to phosphorus in and sinks to the bottom, making it inert and unusable for algae growth. This treatment facility reduces the phosphorus entering the lake from this ditch by approximately 60%.

ferric chloride treatment site

Ferric chloride treatment site

    • The PLSLWD works with farmers and the Scott Soil and Water Conservation District (SWCD) to install projects that reduce the amount of soil (which contains phosphorus) reaching the streams, ditches and lakes.
    • The PLSLWD and SWCD also work in urban areas to install rain gardens, rain barrels, shoreline restorations, and other projects to filter and reduce the amount of water reaching the lakes. A wetland restoration on the west end of Spring Lake, was recently completed by the PLSLWD, City of Prior Lake, and Scott County, which stores water and decreases phosphorus.
    • The PLSLWD also restores wetlands which create storage for flood waters. A Flood Reduction Study has recently been completed by PLSLWD and the City of Prior Lake to analyze more options for increasing storage in the upper watershed, thereby reducing potential phosphorus contributions from upstream.
    • Many other projects are either underway, or completed, and can be found on the PLSLWD website.

What can be done about the EXISTING phosphorus in Upper Prior Lake

Carp Control: The PLSLWD currently has a carp management project funded by an MPCA grant project. Part of the grant project includes carp removal that includes netting carp on Upper Prior Lake. The first netting in Fall of 2016 was very promising and resulted in the removal of 1.9 tons of carp. Netting efforts will continue over the winter and into 2017. In addition to carp removal, the District is also tracking the carp to identify spring spawning areas and install carp barriers to prevent carp from accessing their spawning sites.

carp seine

For more information on the PLSLWD carp management plan:

Vegetation Management: Aquatic vegetation can improve water clarity because it uses phosphorus in its growth cycle.  However, some plants are invasive and take over native plant communities.  The PLSLWD treats the invasive aquatic plant, Curlyleaf Pondweed, to allow room for natives to grow.

Alum Treatments: Another option to reduce phosphorous levels involves dispersing Aluminum Sulfate (alum) into the lake. The alum binds to the phosphorus rendering it unusable to the algae. Spring Lake (which feeds into Upper Prior) was treated with alum in 2013 with promising results as seen in these before and after photos:

    Spring Lake Before Alum Treatment                Spring Lake After Alum (less than two weeks)

2013 alum treatment_1st day on Spring lake      2013 alum treatment_final day on Spring lake

The treatments typically reduce the phosphorus levels for 10 to 30 years. Unfortunately, high carp population can reduce the longevity of an alum treatment and treating Upper Prior will cost about $900,000.  Carp removal is significantly less expensive. Depending upon the effectiveness of the carp removal program, an alum treatment may not be necessary. The PLSLWD recommends delaying alum treatments until at least Fall of 2018 so the effects of the carp removal can be evaluated.

The Watershed is not the only organization that can fund alum treatments. Lakeshore homeowners, Lake Associations, Cities, Townships, Park Districts, grants and other private contributors can also fund alum treatments.

Other options:   Dredging and aerating the lake were also considered.   After analyzing these options, it was determined an Alum Treatment would be more cost effective if further treatment is needed after carp removal.


What YOU can do to help

  • Stop fertilizing your lawn, or only use fertilizers that have no phosphorus (it’s the law!).
  • Leave as much aquatic vegetation in the lake as possible – plants help increase water clarity.
  • Clean up leaves in the street where storm sewers drain directly into the lake.
  • Don’t mow right up to the lake edge- leave an unmowed strip of vegetation next to the water.
  • Install shoreline buffers, rain gardens, or rain barrels. The Watershed offers funding for some of these projects.
  • Participate in the Carp Fishing Contest, the Prior Lake Cleanup and other events:
  • Share this information with your friends and neighbors who use and enjoy the lake for any and all activities.
  • Communicate (respectfully) your concerns to local government agencies.

Last, but not least, join the Prior Lake Association. We are a volunteer, non-profit organization which serves to keep everyone who uses Prior Lake informed of issues that affect the lake.  We also fund many functions on the lake, such as the July 4th Fireworks.