The recovery of Upper Prior Lake hinged on a few simple radio tags.
The lake, like many other waters in the Metro, had been infested and overrun by invasive carp, which were rooting out the lake bottom and turning the 400-acre lake into a murky stew of algae and bacteria. Each time the local watershed district tried to capture and remove the elusive carp, hiding somewhere in depths of up to 40 feet, their nets came up empty.
So, the lake’s managers tried something new.
In 2016, a team surgically implanted radio tags beneath the armor-like scales of a small number of the fish. The radio signals exposed the carp’s patterns: where the fish congregated just before breeding, when they came to shore and where they schooled together under the ice in winter. The next time the Prior Lake-Spring Lake Watershed District put their nets in the water, they did not come up empty.
Over the past 10 years, Upper Prior Lake has been brought back to life. Once beset by nutrient pollution, toxic algae blooms and water so murky that almost no native plants could grow, the lake is now, by most measurements, healthy.
This summer, the Minnesota Pollution Control Agency recommended removing Upper Prior from its impaired waters list for nutrients.
Hundreds of other lakes in the Metro and around Minnesota are still on the nutrient impairment list, an ignoble designation for waters that are so rich in phosphorous, algae and bacteria that they are often unsafe for swimming. The success at Upper Prior in Scott County shows strategies that not only worked, but worked relatively quickly.
“In today’s world it’s so important to see that we can get there,” said Emily Dick, the water resources manager for the Watershed District. “This is a Metro Lake with an intense urban area around it and then rural areas next to that. But we can get there.”
In addition to carp removal, the Watershed District, city of Prior Lake, Scott County and the state have chemically treated the water, sprayed invasive weeds and added wetlands back to the landscape in order to drastically cut the amount of nutrient polluting the lake. Since 2010, they have spent about $9 million restoring the lake.
Upper Prior Lake was first added to the nutrient impaired list in 2002. The lake is part of a small chain in the center of the city of Prior Lake where the water flows north. It collects water from Spring Lake, just to its south, and empties into Lower Prior Lake downstream.
Efforts to restore Upper Prior had been underway for a few years, but gained steam in the mid-2010s.
Managers focused on two pillars of nutrient management: First to remove the excessive amount of phosphorous that had already built up inside the lake over decades from farm manure, fertilizers, lawn clippings, roadways and all the other human development surrounding the lake. And then to restore buffers, wetlands and other natural filters to keep new sources of phosphorous from entering.
Carp, though, were a critical and difficult-to-solve part of the problem. The fish, which typically weigh between 7 and 10 pounds, are bottom feeders that continuously root up the lakebed while hunting for food.
That hurts the lake in two ways, said Jeff Anderson, the water resources coordinator for the Watershed District. It stirs up nutrients and sediments that had settled on the bottom and releases them back into the water where they can feed algae and cyanobacteria. And it uproots native vegetation, preventing plants that could help keep the lake healthy from growing, he said.
In the early 2010s, water managers hired commercial fishing operations a total of four times to scoop up the carp in large seine nets. They failed each time.
Then several state and federal agencies, including the Minnesota Board of Water and Soil Resources, Department of Natural Resources and the EPA all worked with locals to implant the radio tags.
Armed with the knowledge of where the fish were typically hiding and migrating on both Upper Prior and Spring Lake, fishing crews pulled up a total of 34 tons of carp in their next attempt. They’ve continued catching and removing carp each year. Since 2016, the watershed district has pulled out nearly 100,000 pounds of the fish, or roughly 12,000 individual carp, Anderson said.
That’s reduced the carp population to a sustainable level, where native plants have a chance to grow, he said.
Before carp removal, aquatic plants were only found growing on 9% of the entire floor of Upper Prior. After removal that had jumped to 34%.
With the carp reduced, the Watershed District started treating the lake with aluminum sulfate, better known as alum. The chemical is sprayed into the water out of a boat and binds to the phosphorous in the water column, sinking it down to the lake bottom.
“We look at alum as the last nail in the coffin that addresses the sins of the past,” Dick said.
The district received about $400,000 from the state’s Clean Water Fund to spray Upper Prior with alum twice, once in 2020 and again in the spring of 2026.
Alum and carp removal are estimated to have removed more than 1,400 pounds of phosphorous each year from Upper Prior, putting a significant dent in the lake’s nutrient problem.
City, county and watershed district officials have also been working to keep new sources of phosphorous from entering the lake.
The local governments received a $400,000 Clean Water Fund grant from the state in 2014 to restore a wetland near Upper Prior and install an iron-enhanced sand filter. Street sweeping efforts were enhanced and property owners have installed rain gardens, said Dick, the district’s water resources manager.
“It’s taken years of work, and definitely needs a holistic approach,” she said. “It takes all of these pieces, and the support of the people who live around the lake. We have nutrient problems because of systemic changes to the way we use our land and water. So, to fix it, it needs a systemic response.”
Reporter
Greg Stanley is an environmental reporter for the Minnesota Star Tribune. He has previously covered water issues, development and politics in Florida's Everglades and in northern Illinois.
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