The Nine Mile Creek Watershed District (District) and City of Bloomington are currently in the process of implementing a multi-stepped approach to improve the health of Normandale Lake. To date, the projects that have been implemented have included a lake drawdown (2018-2019), aluminum (alum) treatment (2019), and herbicide treatment (2020). Herbicide treatments may continue yearly in Normandale Lake, depending on the results of aquatic plant surveys, through 2024.
Beyond the projects already implemented, the District is working with Carp Solutions to determine a carp management strategy for Normandale Lake. In late summer/early fall 2020 the District will assess a baited box net removal strategy for carp. The District will also assess the need to improve oxygen conditions in the lake, after evaluating monitoring data and following the completion of herbicide treatments on the lake.
Project Goals and Objectives
The goals of the Normandale Lake Project are to improve water quality and the ecological health of Normandale Lake. The District and City of Bloomington have implemented several strategies in Normandale Lake to help achieve these goals.
One strategy is reducing the amount of curly-leaf pondweed, an aquatic invasive plant, in the lake through a lake drawdown and follow-up herbicide treatment. Reducing the amount of the invasive plant will help protect and improve the native aquatic plant community and reduce the amount of phosphorus that is released into the water when the invasive plant dies off and decays in mid-summer. Another strategy to improve water quality is reducing the amount of phosphorus that is released from the lake bottom sediments, a process called “internal loading”, by conducting an alum treatment.
Other management strategies under consideration to further improve water quality and the ecological health of Normandale Lake include managing carp and improving oxygen conditions in the lake.
Beyond these strategies that are focused on in-lake management, another important strategy to improve water quality and the ecological health of Normandale Lake is to reduce the amount of phosphorus flowing into the lake from Nine Mile Creek. The District has been implementing programs and projects upstream of Normandale Lake to reduce the amount of phosphorus in Nine Mile Creek, including stormwater management requirements for development and redeveloping properties, stream bank stabilization, and upstream lake improvement projects, along with education programs like adopt-a-drain.
Normandale Lake: A Shallow, Urban Lake
Normandale Lake was created as part of a flood control project in the late 1970s by the District and City of Bloomington in an area of natural marsh land. A permit for the construction of the lake was issued by the Army Corps of Engineers, which limits some management options for Normandale Lake, such as aquatic vegetation harvesting and dredging on the western half of the lake.
The western half of the lake is shallower than the eastern portion. Due to western half of Normandale being shallower, lake users may notice more plants and filamentous algae being caught on emergent plants on this side of the lake.
Normandale Lake is a shallow lake (three-foot average depth), which means aquatic plants naturally grow throughout the lake. A robust, native aquatic plant population is part of a healthy shallow lake dynamic.
Abundant curly-leaf pondweed, an aggressive invasive aquatic plant, was threatening native plant diversity in Normandale Lake. Curly-leaf pondweed’s unique life cycle differs from native plant species in that it starts growing earlier in the season and then dies off early in the summer. Phosphorus is released into the water as the plants decay, fueling algal growth and degrading lake water quality. In addition, decay of curly-leaf pondweed can cause low oxygen conditions. Low plant diversity, along with low dissolved oxygen levels, posed concerns for Normandale Lake’s aquatic communities. The Normandale Lake project seeks to reduce the amount of curly-leaf pondweed in Normandale Lake and protect or improve the native plant community.
Normandale Lake receives water from a drainage area of over 34 square miles, including water from six different cities. The water flows into the lake through Nine Mile Creek and stormwater pipes. Phosphorous from sources such as grass clippings, tree litter, and sediment from paved surfaces and erosion travels with this water. This is called external phosphorus loading. Phosphorus is also released from the lake bottom sediments, called internal loading. These sources of phosphorus fuel algae growth in the lake. Normandale Lake is not currently on the state’s impaired waters list but does frequently have phosphorus levels that exceed state standards (>60 µg/L). The Normandale Lake project seeks to reduce internal phosphorus loading, which will help meet state standards for total phosphorus in shallow lakes. Historically, Normandale Lake has met the chlorophyll-a and Secchi disc depth standards for shallow lakes.
Shallow Lake Expectations
If you picture a Minnesota Lake in your head, it is likely a crystal clear, vegetation-free swimming lake. Unfortunately, that is not the reality for most shallow lakes, especially in urban settings. Watch this video to learn more about expectations for shallow lakes and how they differ from deep water lakes.
Algae in Normandale Lake
Algae are an important part of our aquatic systems, as they produce oxygen through photosynthesis and are food for other aquatic species. However, in abundance, algae can create nuisance conditions and may degrade the health of the waterbody. Algae get their nutrients directly from the water, so the amount of algae in a waterbody is dependent on the amount of nutrients, such as phosphorus, in the water.
There are two primary types of algae that we see in our lakes and streams: planktonic algae and filamentous algae. Planktonic algae (free floating) are microscopic plants that live throughout the water column. Planktonic algae reduce the clarity of the water and can cause the water to look murky and green. Filamentous algae are colonies of microscopic plants that link together and form visible strings, or filaments, of algae. This type of algae grows in shallow, clear lakes where sunlight can easily reach the bottom of the lake. Filamentous algae begin their growth on the bottom of the lake. When sunlight and warmth result in a dense growth, they produce large amounts of oxygen that forms bubbles that become trapped between the tangled filaments. The bubbles cause the filaments to become buoyant and float to the surface. Disturbance of the mats of filamentous algae by wind or rain events may remove the bubbles from the filaments and cause them to temporarily sink to the bottom. At the bottom, they again produce large amounts of oxygen that become trapped between the tangled filaments, causing them to rise back to the surface.
Once filamentous algae float to the surface of lakes, they form globs of algae or thick mats. The floating mats can be unsightly. When mats of filamentous algae grow to the extent that they cover large areas of a pond or lake surface, they can be more problematic.
One of the goals of the recent management activities is to reduce the amount of phosphorus in Normandale Lake, and thereby reduce the amount of algae in the lake. However, it is important to recognize that there will always be moderate levels of algae in Normandale Lake, with Nine Mile Creek flowing through the lake and serving as a daily source of nutrients, such as phosphorous.
What is all that green stuff in the lake?
Use these fact sheets to learn more about what is growing in our lakes:
The Normandale Lake Project involved multiple management activities to address concerns associated with a prevalence of curly-leaf pondweed in Normandale Lake and the release of phosphorus from lake-bottom sediments (internal loading).
A drawdown is one way to control curly-leaf pondweed and to a lesser extent internal phosphorus release from sediment. Curly-leaf pondweed, an invasive aquatic plant, primarily reproduces through vegetative propagules called turions. Turions are produced in late spring, remain dormant in sediment through the summer, and germinate under cooler water conditions in the fall. However, a winter freeze of the lake sediment bed (while the lake is empty of water) can kill the turions, thus stopping the curly-leaf pondweed’s reproductive cycle. Managing the level of curly-leaf pondweed will help protect and improve the native aquatic plant community in Normandale Lake.
How was the drawdown accomplished?
To draw down Normandale Lake, temporary pumps and an existing bypass pipe were used in late-summer 2018. Once the lake was partially drawn down, the District installed a larger, permanent bypass pipe to maintain the lake drawdown and decrease potential impacts of rainfall or snowmelt events during the drawdown period. The new pipe was installed on the north side of the existing lake outlet structure. When open, the pipe conveyed water from the lake, under the embankment and directly into Nine Mile Creek downstream of the existing outlet. Once the larger pipe was installed in 2018, the pumping stopped and the larger pipe was used to maintain the drawdown. In March 2019, following the drawdown and winter freeze of the lake bottom sediment, the bypass pipes were closed and the lake refilled. The permanent bypass pipe can be used again in the future should the lake need to be drawn down. However, there are no plans at this time to conduct another lake draw down.
Lake Drawdown Resources:
Curly-leaf Pondweed Herbicide Treatment
An herbicide treatment is used to kill undesirable plants. The herbicide treatments on Normandale Lake will target invasive curly-leaf pondweed that remains following the drawdown.
Managing the level of curly-leaf pondweed will help protect and improve the native aquatic plant community in Normandale Lake. The District will assess the need for an herbicide treatment yearly, based on plant surveys. Current project plans include up to five years of treatment (through 2024), after which the effectiveness and long-term sustainability of the curly-leaf pondweed management strategy will be re-evaluated.
The first herbicide treatment occurred on May 8, 2020. A whole-lake treatment was planned for in 2020 but based on the limited amount of curly-leaf pondweed found during spring 2020 plant surveys only a partial-lake herbicide treatment was needed.
How does an herbicide treatment work?
After a Minnesota Department of Natural Resources permit is approved, herbicide is applied by a licensed contractor from a treatment boat. Based on the spring 2020 plant survey done on Normandale Lake and upstream areas, an herbicide called diquat was applied to select areas in Normandale Lake and in one upstream area that contained curly-leaf pondweed. Herbicide treatments are done in early spring when the water temperature in the lake is generally between 50-60o F. This is when the herbicide is most effective and limits damage to native plants, which start growing later than curly-leaf pondweed.
Herbicide Treatment Resources:
Alum treatments help control internal phosphorous loading in lakes, reducing the phosphorus available to fuel growth of algae, including filamentous algae. The Normandale Lake alum treatment was conducted on May 7-8, 2019, after the lake was refilled from the drawdown.
How does an alum treatment work?
When aluminum is applied to a lake as a buffered solution of alum, it forms an insoluble aluminum hydroxide floc that settles to the lake bottom. The aluminum binds with phosphorus in the sediment to prevent it from recycling back into the water column.
Alum Treatment Resources: Alum Treatment Fact Sheet (PDF)
What else is being done to improve the health of Normandale Lake?
Carp are an invasive species of fish that cause water quality problems due to their bottom feeding behavior. They stir up lake bottom sediments, reducing water clarity and releasing phosphorus. They also uproot aquatic vegetation, destroying habitat for waterfowl and aquatic communities. A 2018 fish survey indicated that carp were at a level in Normandale Lake where management of the population is recommended for water quality purposes.
A follow-up fish survey was completed in 2019 to assess carp levels and the native fish population following the drawdown. A large number of young of the year carp were found in the lake, indicating successful carp recruitment (survival to the juvenile stage) following the drawdown.
In order to better understand carp movement and how to manage the population, the District worked with Carp Solutions in 2019 to radio-tagged 12 carp for tracking. Tracking information shows carp leaving Normandale Lake, indicating the carp are using areas outside of Normandale Lake as nurseries. The District worked with Carp Solutions in 2020 to continue monitoring the carp population and to determine the best management methods for the carp population in the lake. As part of the work in 2020, two box nets were put into Normandale Lake. A box net is a square net with mesh bottom and mesh sides lined with weighted line around each side causing it to lay flat on the bottom of the lake. Carp are trained to aggregate in the area of the box net with cracked corn (Bajer et al. 2010). The carp can then be selectively and effectively removed using the box nets at the baited sites. Using this method, approximately 5,000 carp were removed from Normandale Lake in 2020.
Fisheries Management Resources:
Improving Oxygen Conditions in the Lake
Improving oxygen conditions in Normandale Lake is another management strategy under consideration to further improve water quality and ecological health. Aquatic plants are an important part of a healthy shallow lake, providing valuable habitat for fish and other aquatic communities and helping to improve water clarity. A 2017 study conducted by the District found that aquatic plants and filamentous algae play an important role in removing phosphorus from the water in Normandale Lake. However, dense and widespread growth of aquatic plants and filamentous algae can also inhibit oxygen transfer at the water surface, contributing to low oxygen levels that can stress fish populations and increase the potential for release of phosphorus from lake bottom sediments. Two management activities considered to improve oxygen conditions in Normandale Lake include selective harvesting of the aquatic plants in a portion of the lake and installation of an oxygenation system. These management activities will be considered further after the effectiveness of the lake drawdown, alum treatment, and successive years of herbicide treatment are evaluated.
Reducing Upstream Phosphorus Sources
Normandale Lake receives water from a drainage area of over 34 square miles, including water from six different cities. The water flows into the lake through Nine Mile Creek and stormwater pipes. Phosphorous from sources such as grass clippings, tree litter, and sediment from paved surfaces and erosion travels with this water. This is called external phosphorus loading.
Reductions in external loading to Normandale Lake will be achieved through stream bank stabilization projects, continued implementation of the District’s permitting program, implementation of management strategies for upstream lakes, and construction of stormwater best management practices in the watershed tributary to Normandale Lake. The District has undertaken a number of upstream projects to reduce external loading and continues to prioritize and assess projects based on its 2017-2027 Water Management Plan for implementation.
The District uses an adaptive management approach for its projects. This allows the District to react to changing conditions, while also remaining mindful of the long-term goals for the resource. Adaptive management is an ongoing, systematic approach for natural resource management. It emphasizes identifying and predicting the outcome of management options, implementing management practices, monitoring the outcome(s), and incorporating what is learned into ongoing or future management decisions. The lake management practices considered as part of the Normandale Lake project are part of an adaptive management approach to improving the water quality and ecological health of Normandale Lake.
The District is implementing a comprehensive targeted monitoring program to assess the effectiveness of the Normandale Lake Project management activities as they are implemented. This will allow the District to evaluate the ongoing need for additional or repeat management activities. The District will conduct monitoring activities to measure water quality, particularly phosphorus, the health of the aquatic plant communities in Normandale Lake, and to measure the amount of curly-leaf pondweed in the lake.
Water Quality: The Minnesota Pollution Control Agency (MPCA) has developed standards for lakes (i.e., State water quality standards) that include numerical goals for phosphorus, chlorophyll-a and Secchi disc transparency. Normandale Lake has been monitored for these parameters by the District and the Metropolitan Council’s Citizen-Assisted Monitoring Program (CAMP) during numerous years from 1990-2019. The historic summer average (June – September) monitoring results for total phosphorus, chlorophyll-a and Secchi disc transparency are summarized in graphs below.
While water quality data were collected by the District in 2018, the data were not included in the comparison with other years since results from the September monitoring event were influenced by the lake drawdown that was underway.
As the lake ecosystem continues to stabilize after the management activities, the District will continue to monitor water quality in Normandale Lake to assess the short and long-term impacts from the management activities on water quality in the lake.
In 2019, the most recent year of analyzed water quality data, the summer average total phosphorus concentration did not meet the Minnesota State Water Quality Standard for shallow lakes in the North Central Hardwood Forest Ecoregion published in Minnesota Rules 7050 (Minn. R. Ch. 7050.0222 Subp 4). The 2018 Engineer’s Report for the Normandale Lake water quality improvement project concluded that stormwater from the large watershed tributary to Normandale Lake, much of which is untreated prior to reaching Nine Mile Creek, contributes significant phosphorus loading to the lake. According to the MDNR, 2019 was the wettest year on record for the weather station located at the Minneapolis St. Paul International Airport. In 2019, a total of 43.17 inches of precipitation was documented by the weather station, which has measured precipitation for more than 50 years. Wet weather during 2019 resulted in higher phosphorus loads to Normandale Lake. The 2019 summer average total phosphorus concentration of 63 μg/L was very close to meeting the State criteria for shallow lakes of 60 μg/L despite the higher phosphorus loading to the lake caused by the wet weather. In 2019 both chlorophyll-a concentrations and Secchi disc depth met the state standard.
Health of Aquatic Plant Community: Eutrophication can have harmful effects on a lake, including reductions in the quantity and diversity of aquatic plants. To help assess the biological condition of a lake plant community, the Minnesota Department of Natural Resources (MDNR) has developed the Lake Plant Eutrophication Index of Biological Integrity (IBI). The plant IBI includes two metrics: (1) the number of species in a lake; and (2) the “quality” of the species, as measured by the floristic quality index (FQI). In Normandale Lake, the District is working to protect or improve the health of the native aquatic plant community.
As the Normandale Lake ecosystem continues to stabilize following the drawdown, alum treatment, and herbicide treatment, the District will continue to monitor the health of the aquatic plant communities to assess the short and long-term impacts from the project.
Curly-leaf pondweed reduction: The District has conducted aquatic plant surveys in Normandale Lake in recent years to better understand the extent of curly-leaf pondweed in the lake and evaluate the effectiveness of lake management activities in reducing the extent of the invasive plant. Data from the plant surveys has been used to compare the frequency of curly-leaf pondweed occurrence and the amount of curly-leaf pondweed (measured as biomass) throughout the lake pre- and post- lake drawdown.
A curly-leaf pondweed turion survey was conducted on Normandale Lake on November 2, 2019, following the summer growing season. The goals of the survey were to determine the level of curly-leaf pondweed turions within the lake’s substrate following the lake drawdown in the winter of 2018-2019 that resulted in the freezing of much of the lake’s substrate. Results from the fall 2019 curly-leaf pondweed turion survey found that curly-leaf pondweed turions were found at only 19 of 50 survey points (38% coverage), and, collectively, there were just 36 live turions present in the samples. In addition, from the size and structure of the viable turions that were found, they likely originated from smaller-than-normal plants during the 2019 growing season (the curly-leaf pondweed plants had a late growth start in 2019 due to the drawdown) and that most larger turions were killed by the drawdown or used up (germinated) during the 2019 growing season.
Summary of aquatic plant and curly-leaf pondweed data
In 2018, the District began implementation of a water quality improvement project for Normandale Lake. A drawdown of the lake was completed in fall of 2018 to expose the lake bed to a winter freeze and freeze out curly-leaf pondweed. A comparison of pre- and post-project aquatic plant data to assess changes after completion of a drawdown and alum treatment documented an overall improvement in the plant community in 2019. The drawdown reduced the frequency and biomass of curly-leaf pondweed, the targeted aquatic invasive species. The frequency of three native species increased in 2019, as compared to previously monitored years, and the quality of the plant community, measured by floristic quality index, improved. Herbicide treatment of the remaining curly-leaf pondweed occurred in 2020, with additional herbicide treatments to be completed in subsequent years, as needed, to reduce curly-leaf pondweed.
As the lake ecosystem continues to stabilize from the drawdown, alum treatment, and herbicide treatment, the District will continue to measure curly-leaf pondweed reduction and monitor the health of the aquatic plant communities to assess the short and long-term impacts from the project. Monitoring data from 2020 will be posted when analyzed and summarized.
More information: 2019 Nine Mile Creek Watershed District Water Quality Monitoring Report
Education and Outreach
Several community meetings have been held regarding the project, including on August 22, 2017, May 15 and May 24, 2018, March 11, 2020, and February, 18, 2021. An Alum Treatment Demonstration Day was held on May 8, 2019. A public hearing was held for the project on June 12, 2018.
Normandale Lake community meeting presentation (February 18, 2021)
Education and Outreach Resources
February 18, 2021 Community Meeting Presentation: Normandale Project_February 2021 Community Meeting Presentation (PDF)
March 11, 2020 Community Meeting Presentation: Normandale Project_March 2020 Community Meeting (PDF)
Information Boards: Normandale Lake Project Information Boards (PDF)
The City of Bloomington and the District partnered to create outreach videos about the project.
Upcoming water quality project to clean up Normandale Lake
Documents found throughout the Normandale project page are compiled below for easy access.
Alum Treatment Resource:
Herbicide Treatment Resources:
Plant and Algae Resources:
Watershed District Guiding Document:
Contact Erica, Program and Project Manager, at 952-358-2276.