As the City of Lakes, Minneapolis is known for its ample and beautiful lakes. Without a dedicated effort to preserve and protect these urban waters, our lakes would not be the valuable resources that they are today. The Minneapolis Park & Recreation Board’s (MPRB) lake water quality monitoring program was implemented in 1991 as part of a diagnostic study for the Chain of Lakes Clean Water Partnership. We currently monitor 13 lakes within the City of Minneapolis:

Long-term stormwater management is one key to keeping lake water quality suitable for recreation and for sustaining plants and wildlife. MPRB is seeking dedicated funding to invest in stormwater management and protect Minneapolis lakes.

Lake Conditions

We make some monitoring data available throughout the season so you have an idea of lake conditions:

  • Updated Secchi disk readings indicate the current clarity of each lake.
  • We monitor beaches during the summer to determine if there are any health risks to swimmers. Weekly data indicates:
    • Open/closed status of the beaches
    • E. coli levels
    • Microcystin levels
    • Current water temperature

Blue-Green Algae

Algae are tiny, harmless organisms that are natural and common in freshwater. Blue-green algae, which is present in lakes throughout Minnesota, are not algae at all, but a type of bacteria called cyanobacteria. In the right conditions they can grow quickly to form “blooms”. Sometimes blue-green algae can produce cyanotoxins that can make humans and animals sick. Before getting in the water, always take an overall look at water conditions and do not enter the water if you see blue-green algae. Blue-green algae blooms and scum can move with the wind, so blooms will shift around the lake depending on weather conditions.

Learn how to recognize and respond to blue-green algae.

Despite its name, blue-green algae may not always be blue-green. It may also be reddish-purple or brown. Blue-green algae causes the water to be murky.

When environmental conditions are just right, blue-green algae can grow very quickly. Most species are buoyant and will float to the surface, where they form scum layers or floating mats.

  • Do not swallow or go into water that looks like "pea soup", green or blue paint, or that has a scum layer or puffy blobs floating on the surface.
  • Keep children and pets out of blue-green algae scum.

For more information about blue-green algae Minnesota Department of Health: Harmful Algal Blooms (HABs)

Filamentous  Algae

Filamentous algae are single cell organisms that grow in long strands, or filaments. These filaments can intertwine to form thick mats which float to the surface of lakes and ponds. Filamentous algae can often be confused with blue-green algae, a cyanobacteria that can be harmful. Filamentous algae is not harmful and does not produce toxins. Learn more about filamentous algae and how to tell it apart from blue-green algae.

Aquatic Invasive Species

Aquatic invasive species are often the most visible sign of change in lakes, and many problem species are very difficult to remove or even limit. We carefully monitor these species in cooperation with various state agencies and work to limit their spread where possible.

About Secchi Disk Readings

secchi diagram

To measure water transparency with a Secchi disk, the disk is lowered from the shaded side of a boat until it cannot be seen. The depth of the water is recorded at the point where the disk reappears upon raising it from its original depth beyond visibility. It is important to remember that the Secchi measurement is a simple, approximate measurement of water clarity and can be influenced by various factors such as time of day, reader’s eyesight, water color and suspended particles in the water. Therefore, Secchi disk readings should be used as a comparative tool to determine trends between lakes and over time.

  • Increased water clarity may be due to reduced nutrient inputs, seasonal algal cycles, decreased sediment inputs, and zooplankton grazing on algae.
  • Decreased water clarity may be due to large algal blooms, increased soil erosion inputs, wind circulation of sediments and nutrients, stormwater inputs, or a decrease in zooplankton.

Secchi readings for the lakes are done during the regular lake sampling schedule, with some lakes sampled every other year. Current Secchi readings taken during lake sampling can be found on the Lake Water Quality Map.

Lake Aesthetic and User Recreation Index (LAURI)

CitywideLAURI

The Lake Aesthetic and User Recreation Index (LAURI) was designed to give recreational users a source of information about conditions affecting their use of city lakes. The goal is to have an accurate, science based and easily understandable recreational indicator for the public. The LAURI has five indices:

Public Health Index (E. coli measured at public swimming beaches)

The score for each lake is based on the geometric mean number of Escherichia coli (E. coli) collected by the beach monitoring program (Table 1-1).  Lakes with more than one beach were averaged together.  EPA and Minnesota guidelines state that beaches should not exceed a geometric mean of 126 organisms per 100 mL during a 30-day time period.  Lower numbers of organisms indicate less risk of illnesses for lake users.

Table 1-1.  Scoring for the public health portion of LAURI.

E. coli, (CFU/100mL)* Score
<2 (ND) 10
2-10 9
11-20 8
21-35 7
36-50 6
51-65 5
66-80 4
81-100 3
101-125 2
>126 1

 

 

 

 

 

 

 

 

 

 

 

* The value used is the running geometric mean for the year, averaged for all the beaches on a lake.

Water Quality Index (water clarity/Secchi depth)

Water clarity is a good integrator of various parameters affecting the quality of a lake. The average Secchi transparency reading from all the data collected during the growing season is used. The lakes are separated into deep lakes and shallow lakes using criteria developed by the Minnesota Pollution Control Agency (MPCA). A shallow lake is defined as 80% littoral (< 15 feet deep). Bde Maka Ska, Cedar, Harriet, and Wirth are considered deep lakes. Loring, Isles, Hiawatha, Nokomis, and Powderhorn are considered shallow lakes. Greater secchi depth indicates clearer water.  LAURI scoring is shown below in Table 1-2.

Table 1-2.  Scoring for the water quality portion of LAURI.

Secchi Depth (m) Deep Lake Score Shallow Lake Score
0-0.5 1 2
0.6-1 2 4
1.1-1.5 3 6
1.6-2.0 4 8
2.1-2.5 5 10
2.6-3.0 6
3.1-3.5 7
3.6-4.0 8
4.1-4.5 9
>4.6 10

Habitat Quality (aquatic plant and fish diversity)

LAURI assessments of habitat quality are based on detailed Minneapolis Park and Recreation Board (MPRB) surveys and DNR fish sampling. Scoring is based on presence of aquatic plants, density of plants, and amount of coverage (Table 1-3). Lakes with more aquatic plants receive higher habitat quality index scores.  Points are also awarded for more diverse fish populations. Each fish score is based on the most recent available DNR fish survey done for a lake. The score from the aquatic plant and fish surveys are averaged for the LAURI.

Table 1-3. Scoring for the habitat portion of LAURI.

Macrophyte
Species
Score Density Score Coverage
>15 ft.
Score # Fish
Species
Score
0 0 Low 0 0-25 2 <=6 2
1-4 3 Low-Med 3 25-50 4 7-8 4
4-8 6 Medium 6 50-75 7 9-11 6
8-15 8 Med-High 8 75-100 10 12-14 8
>15 10 High 10 >=15 10

Recreational Access (availability and ease of public access)

The lakes are also scored for the quantity and quality of recreational access to the water. The recreational score considers the number of fishing docks, beaches, boat launches, intra lake connections, boat rentals, canoe racks and sail boat buoys, boardwalks, picnic areas and concessions at a lake, Table 1-4. While aquatic plants are a necessary part of a healthy lake ecosystem they can also interfere with recreational uses of the lake.  Lakes also receive points for invasive plant growth management.

Table 1-4. Scoring for the recreational access portion of LAURI.

Recreational Opportunities # RecOps at Site Total # RecOps +
Aquatic Plant Mgmt
Final Score
Fishing Dock 0 1
Beach 1 2
Boat Launches 2 3
Intra Lake Connection 3 4
Boat Rental 4 5
Boat Storage 5 6
Boardwalk 6 7
Picnic Area 7-8 8
Concessions 8-9 9
Aquatic Mgmt = yes (+4) >10 10
Aquatic Mgmt = no (+0)
Total # RecOps at site +
Aquatic Plant Mgmt

Aesthetic Considerations (color and odor of water, garbage and debris)

The lakes are scored for water color, odor, and debris based on an assessment done from shore, dock, or boat, Table 1-5.  Higher numbers indicate better aesthetics.  The scores are averaged over the season.  Aesthetics can be difficult to evaluate because they are qualitative and dependent on individual experience.

Table 1-5.  Scoring for the aesthetic portion of LAURI.

Color Score Odor Score Debris Score
Clear 10 None/Natural 10 None 10
Lt. Brown or
Green
8 Musty-Faint 8 Natural 9
Bright Green 5 Musty-Strong 6 Foam 8
Milky White 4 Sewage/
Fishy/
Garbage-Faint
5 Piles of Milfoil
(>3)
7
Brown/
Reddish/
Purple
2 Sewage/
Fishy/
Garbage-Strong
2 Trash: Fixed
(>3)
4
Gray/Black 0 Anaerobic/Septic 0 Trash: Floating
(>3)
3
Many Dead Fish
(>5)
2
Green Scum 2
Oil Film 1
Sewage Solids 0

View LAURI or Water Clarity Information for:

Long-term monitoring is important because lakes can change from year to year due to wet or dry weather and we can only see trends when we put years of data together. Analysis of this data tells us if the water quality is improving or not.

View the MPRB Water Resources Report for detailed annual monitoring data, data collection methods, and more.