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Watershed and in-lake conditions to blame for Lake Mitchell's algae woes

Green algae flows along the western bank of Lake Mitchell just north of the amphitheater in 2016. (Matt Gade/Republic)

A major food source for algae growth nearly doubled in Lake Mitchell in 2017, and a study unveiled this week helped pinpoint the causes.

A report from Omaha-based water quality specialists Fyra Engineering, which cost the city of Mitchell $73,725, shows an average of 889 parts per billion (ppb) in Lake Mitchell this year. On average from 1991 to 2017, the level sat at approximately 474 ppb.

The data isn't a revelation, considering the bright blue-green algae that covered the shoreline of Mitchell's lake throughout the summer. But the 54-page report does come with some answers, as well as some unknowns, about the source of Lake Mitchell's algae problems.

Charles Ikenberry, of Fyra Engineering, told a group of Lake Mitchell stakeholders on Tuesday that it's difficult to tell if high dissolved phosphorus loads from the lake's watershed were caused by legacy agriculture practices or the native soil phosphorus content. What is clear, Ikenberry said, is that phosphorus already trapped in the sediment at the lake floor is approximately as much of a problem as the amount coming in from the watershed.

"At times, under the right conditions, that sediment (at the bottom of the lake) becomes available and is redistributed or released up into the water column, and that really is bad news for water quality in Lake Mitchell," Ikenberry said Tuesday.

With the issues at Lake Mitchell caused in part by phosphorus loads from both inside and outside of the lake, Fyra pitched a trifecta of plans to improve water quality. Those plans ranged as high as $81 million, although grant funding is expected to cover the majority of the costs.

Fyra also offered a range of possible solutions that could be used to improve lake conditions, particularly during the summer months.

The report is now in the hands of city officials and committees, who will work with Fyra to refine the plan before a public presentation. And Mike Sotak, of Fyra Engineering, acknowledged that selling the project can be as challenging as putting together a plan.

"The engineering's the engineering of it," Sotak said after Tuesday's meeting. "Although you've got to have the right skillsets to be able to do the work, doing the work is fairly straightforward. What we have to do ahead of this is predict the sociological aspect of the acceptance of the technical material."

And Sotak is hoping those who see the value of a clean lake in Mitchell outnumber the dissenters.

"There's never been a lake project in history that pleased 100 percent of the people," Sotak said. "This is going to be an expensive project, and there is no way that everybody is going to support that kind of expenditure of funds, there's just no way."