Fish Lake in Maple Grove will be removed from Minnesota’s vulnerable waters list
Maple Grove Fish Lake, a popular outdoor refuge for fishing, swimming, water skiing and wakeboarding, will be removed from the state’s vulnerable waters list after years of work to reduce its phosphorus contamination.
“It’s really hard to get a lake off the vulnerable water list,” said Brian Flash, senior water resources manager for the Three Rivers Park District.
In 2008, the Minnesota Pollution Control Agency (MPCA) listed Fish Lake as having excessive nutrient deficiencies. The phosphorus-laden lake caused unwanted algae blooms and was plagued by low clarity. It was not always suitable for swimming and wading.
One study linked the problem to the lake’s already polluted floor, Flash said. He said a much greater amount of phosphorus was lurking in the lake sediments than was re-entering the lake from the surrounding watershed. Seasonal water movement, wind and waves will recycle the nutrients, making them available for algae to feed on.
The dilemma? The crew treated the lake with alum, a non-toxic compound that settles to the bottom and forms a cover over the sediment. Alum traps the phosphorus, so it’s not available for algae to feed on, Flash said. After separate alum treatments in 2017 and 2019, the applications reduced the lake’s phosphorus load by 1,400 pounds, far more than the minimum requirement of 310 pounds, he said. Average water clarity improved from 1 meter to 3 metres.
There is now enough alum in the lake to meet water quality goals for another 20 years, Flash said. When the MPCA updates its statewide list of vulnerable waters later this year, Fish Lake will not be on it, he said.
The work plan was implemented with a budget of $375,471, with funding from a group of partners that included the Minnesota Soil and Water Resources Board, Three Rivers Park District, the city of Maple Grove, Hennepin County and the Elm Creek Watershed Management Commission, Flash said.
He noted that the phosphorus reduction did not address another problem in the lake: invasive Eurasian watermelon plants. As the lake water filtered, increased sun penetration led to increased plant growth, including the expansion of non-native milfoil. He said lake biologists will study a treatment plan to control milfoil to protect and improve the diversity of native plants.
According to the Department of Natural Resources, Fish Lake is one of the few lakes in the metro area capable of producing northern pike longer than 34 inches. Managed by DNR fisheries for northern and largemouth bass. It also contains a population of walleye, which can be caught, and which are stocked in odd years. The lake is 49 feet deep and surrounded by over 5 miles of shoreline, including a large public boat access at the south end, within Fish Lake Provincial Park.
As of November 2023, 2,798 bodies of water in Minnesota have been listed by the MPCA for impairment. Last year, 27 listings were removed to improve water quality. The list is required by the federal Clean Water Act.