Can we turn our cities’ green lakes blue?

by Lawrence A. Baker, Department of Bioproducts and Biosystems Engineering, Kate Carlson, U-Spatial, University of Minnesota


Como Lake in St. Paul attracts visitors from throughout the metro region.

In the heat of the summer, many Minnesotans yearn to “go north” to their favorite lake.  While such trips become memorable vacations, about three-fourths of Minnesotans live in cities with populations larger than 2,500 and 51% of us live within one mile of an urban lakeshore. Hence, the lakes we visit most are urban lakes. These are often centerpieces of our cities, lakes such as Como Lake in St. Paul, the Minneapolis Chain of Lakes; Albert Lea and Fountain Lakes in the City of Albert Lea, and Shagawa Lake in Ely.  Minnesota’s two large lake districts envelope the cities of Alexandria and Brainard.

The problem is that many of our urban lakes are green (eutrophic), not blue. The “green” is caused by an overabundance of algae, which results in reduced water clarity, an increase in undesirable blue-green algae, and depleted oxygen (needed by fish) in the bottom waters. These characteristics degrade the value of lakes for recreation at a time when you most want to be near a lake, or better yet, in one! As a rough guide, the MPCA’s legal criterion for considering a lake in the Metro Region to be legally “nutrient impaired” is about 1.4 m (4 ½ feet), which means you can’t see your toes.


Location of nutrient-impaired urban lakes throughout Minnesota.

Despite a sustained effort to improve urban water quality in Minnesota’s lakes over the past 20 years, we identified 177 nutrient-impaired lakes in or directly abutting Minnesota’s cities in 2018.  Thirty of these lakes are in cities outside the Metro Region. Very few urban lakes once designated as impaired have achieved the status of being “restored”.


Streets often convey nutrients from vegetated landscapes to stormwater drains, which often empty into lakes. Enhanced street sweeping along streets with tree canopies has proven to be an effective way to remove nutrients cheaply.

We can do better in the future.  Over the past 20 years we have acquired vast troves of data on water quality, hydrology, and land cover. New data tools now operate at the scale of square feet, not square miles, and at time scales of minutes, not months. These tools will enable us to reduce pollutants at their source, making pollution reduction cheaper, more effective, and fairer than it has been in the past. This will lead to ways to turn our cities’ lakes from green to blue, improving our nature in the cities.

Link to larger image of impaired lakes graphic