Researchers experimenting with cleaning up area lakes by removing carp were thrilled this summer with their success at Chanhassen's Lake Susan -- until they began to see a surprising side-effect:

The water had become so clear that the sunlight passed through it and warmed the lake bottom, igniting an algae bloom that turned the water pea green.

When University of Minnesota researchers removed more than 3,000 carp from the lake last winter, their goal was to clean up its muddy waters. The bottom-feeding fish constantly stir up sediment by rooting through the mud looking for food.

"The surprise was we achieved levels of clarity that went way beyond anything we imagined," said Paul Haik, coordinator of the west suburban Riley-Purgatory Bluff Creek Watershed District.

"The response of the plant community was so sudden and so dramatic that we were stunned.''

Researchers are taking the development in stride, as just another challenge in cleaning up lakes.

The next step is to figure out how to encourage native plants to grow after carp are removed.

Most clean lakes have an underwater meadow of native plants that provide habitat for game fish and keep algae and invasive species in check, said David Austin, task manager for lake restoration at CHM2Hill, a consulting company that acts as the Watershed District's engineers.

Without native plants to absorb nutrients, a lake is likely to turn green with algae as Lake Susan did, Austin said.

Five plant species will be transplanted to Lake Susan from nearby Lake Ann. Some of the transplants will be caged off underwater to keep the carp and muskrats from disturbing them, Austin said.

"For plant restoration in these shallow lakes, the science isn't all in yet. We are still learning how to do this,'' Austin said. "This will be a pioneering effort at how to bring the native plants back and keep the invasives at bay.''

'A work in progress'

The ongoing study by the University of Minnesota was commissioned by the district to improve water quality in carp-infested lakes in Eden Prairie and Chanhassen and to establish guidelines that could be used on other lakes.

In January, university researchers and commercial fishermen cut a hole in the ice at Lake Susan and used a 2,000-foot skein net to scoop out the carp. They figure they removed 78 percent of the carp that had populated the 100-acre lake.

With fewer carp churning up the lake bottom, water clarity doubled in depth during April, May, June and into July, said university biologist Peter Sorensen, who is leading the carp research project.

But by early August, all that extra sunlight cutting through the clear water had warmed the lake bottom enough to release nutrients that prompted the algae bloom.

"The benefit was all early in the season,'' Sorensen said. "Success was not total. It's a work in progress. We are learning.''

Ray Newman, an expert on invasive plants, will oversee the plant restoration work at Lake Susan. He and Sorensen are both professors in the university's Department of Fisheries, Wildlife and Conservation Biology.

Other lakes could benefit

Two-thirds of all Minnesota lakes, and all metro-area lakes, are infested with common carp, according to Sorensen. They can dig up to a foot into the muck at the bottom of a lake, uprooting good plants and stirring phosphorous on the lake floor back into the water, where it fertilizes algae and weeds. Their own excretions further foul the water.

Until carp are controlled, it's unlikely water quality will improve, Sorensen said.

Although a second summer's results will be needed to be sure that the removal of carp, and not this summer's drought, improved water clarity in Lake Susan, Sorensen said, "I think it was highly likely it was the carp."

One especially encouraging finding is that young carp have not shown up to replace the older fish removed last winter, Sorensen said. Lake Susan carp spawn in an adjoining wetland, and the research team blocked the young fish from reaching the lake, he said.

The positive water clarity results achieved without removing 100 percent of the carp from the lake suggests there is a tipping point -- a certain concentration of carp -- beyond which lake water quality deteriorates.

To try to define that tipping point, the research team will remove another 10 percent of the lake's carp this coming winter, to see if benefits are the same or better next spring.

Laurie Blake • 612-673-1711