Lake Cornelia and Lake Edina are shallow lakes with poor water quality that are on the state’s impaired waters list for excess nutrients. The Nine Mile Creek Watershed District aims to control nutrient levels (specifically phosphorous) in Lake Cornelia and Lake Edina.
The Nine Mile Creek Watershed District (District) and City of Edina are currently in the process of undertaking a multi-phased project to improve the health of Lake Cornelia and Lake Edina. To date, the projects we have implemented have included annual lake herbicide treatments by the City of Edina to control curly-leaf pondweed (2018-2020) and an in-lake aluminum (alum) treatment by the District in 2020.
Beyond the projects already implemented, the District is working with WSB and Associates to determine a goldfish and carp management strategies for the Lake Cornelia system. The District will also assess the need to improve oxygen conditions in the lake, after evaluating monitoring data.
Project Goals and Objectives
The goals of the Lake Cornelia and Lake Edina Project are to improve water quality and the ecological health of Lake Cornelia and downstream Lake Edina. The District and City of Edina have implemented several strategies in and around Lake Cornelia to help achieve these goals.
One strategy is reducing the amount of curly-leaf pondweed, an aquatic invasive plant, in the lake through annual herbicide treatments by the City of Edina. Reducing the amount of the invasive plant will help protect and improve the native aquatic plant community and reduce the
amount of phosphorus 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 released from the lake bottom sediments, a process called “internal loading”, by conducting an alum treatment.
To address “external” watershed-based loading from stormwater runoff, the District is designing a stormwater filtration project (discussed in the video below). This project will be constructed in the City-owned Rosland Park, and filter out phosphorus from stormwater entering Lake Cornelia. The Rosland Park Stormwater Filtration BMP is scheduled to be constructed in the fall of 2021.
Other management strategies under consideration to further improve water quality and the ecological health of Lake Cornelia and Lake Edina include managing goldfish and carp and improving oxygen conditions in the lake.
Goldfish and Carp Management
Goldfish and 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.
The goldfish population was likely started with the release of pet goldfish into one or more of the lakes within the system. Awareness and public education are needed to prevent this from happening in any small ponds that could perpetuate the invasive goldfish population in the Cornelia chain.
A 2018 fish summary indicated that while there were carp in the Lake Cornelia system, they were not abundant. Goldfish, on the other hand, were plentiful and at a level where management of the population is recommended for water quality purposes.
A follow-up goldfish and carp study was completed in 2020 to assess goldfish and carp levels, track inter-waterbody movement through the Lake Cornelia system and test management/removal methods. Surveyors found a large number of young of the year goldfish in the lake, indicating successful goldfish recruitment (survival to the juvenile stage).
The District will continue to study the goldfish and carp in in the Lake Cornelia system in 2021-2022, to determine the best methods for goldfish management.
Proposed Rosland Park Stormwater BMP Project
Watch the 20 minute video below for an overview of the proposed stormwater best management practice at Rosland Park. This proposed project aims to reduce the amount of phosphorous entering Lake Cornelia from Swimming Pool Pond and the larger watershed. The District’s engineer gave this presentation at the Public Hearing on July 15, 2020.
Curly-leaf Pondweed Herbicide Treatment
An herbicide treatment kills undesirable plants. The City Edina annually conducts herbicide treatments on Lake Cornelia that targets invasive curly-leaf pondweed.
Managing the level of curly-leaf pondweed will help protect and improve the native aquatic plant community in Lake Cornelia and reduce internal phosphorus loading.
How does an herbicide treatment work?
After a Minnesota Department of Natural Resources permit is approved, a licensed contractor applies herbicide from a treatment boat. Based on the spring plant surveys done on Lake Cornelia, an herbicide called Endothall was applied to select areas in Lake Cornelia. 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.
In-lake Alum Treatment
Alum treatments help control phosphorous levels in lakes. Phosphorous is a nutrient that fuels algae growth. Trained contractors apply the alum using specialized equipment and barges; this equipment ensures the precise placement of the material in the lake. On contact with the water, the liquid alum forms a fluffy aluminum hydroxide precipitate called floc. Aluminum hydroxide (the main ingredient of common antacids like Maalox) binds with the phosphorus to form a compound that does not dissolve in water. The bound phosphorus can no longer fuel algae growth. As the floc settles to the bottom, it removes phosphorus and particles from the water, leaving the lake clearer. The floc then forms a thin layer on the bottom of the lake, and binds with phosphorus in the sediment. The result is a reduction in the frequency and intensity of nuisance algal blooms.
HAB Aquatic Solutions applied alum at Lake Cornelia on May 21, 2020.
Studies and Reports
Contact Randy, District Administrator, at 952-835-2078.