By MAGGIE SCHUETTE-VOSS
A partial fish-kill occurred in Winsted Lake during the winter months, due to lack of dissolved oxygen. As to why the oxygen levels are low, the Department of Natural Resources (DNR) does not have a definitive reason.
Each lake, said Lee Sundmark of the DNR, begins the winter with a dissolved oxygen "budget": the amount of oxygen in the lake before ice forms, and what the fish will have to breathe during the winter.
Sundmark, said the fish-kill could be the result of one, or a combination of factors that affected the oxygen budget.
Dead fish, he said, were found where Dairy Farmers of America (formerly Mid-America Dairymen) discharge warm water into the lake. Sundmark said the creamery has developed a baffle to cool the water and bring up the oxygen levels before it is discharged.
Despite this effort, the discharged water is warmer than the lake water. Sundmark said the fish could have been attracted to the warmer, open, water where they congregated in such high numbers that dissovled oxygen was diminished and metabolic by-products contributed to cause a partial fish kill. The by-products are released naturally when fish become stressed.
Although four aerators are used in winter to increase oxygen levels, Sundmark said that does not guarantee winter-kill will be prevented.
"In most cases aeration just provides a longer period between partial or extinsive winter kills," he said. "Instead of every two to three years, it might occur every five or six years."
The position and/or place of the aerators may not be allowing them to bring in to the lake as much oxygen as they could, Sundmark said.
Sundmark explained the aerators main function is to create open water and mix in the oxygenated surface water. Aerators should be set up so the lake can take advantage of the northwest winds and create more open water.
"The machine provides some dissolved oxygen, but most comes from air exchange between the air/water interface and lake mixing," he said.
Initially, Sundmark said, the aerators were not running a maxium efficiency. "They need to be directed outward," he said. Positioning the aerators to work at their optimum involves trial and error, he said. "We have been working with the lake association and offering suggestions."
Sundmark said he understood that maximizing open water during the winter is a conflict with the city's wish to have a skating rink on the lake.
A third problem is Winsted Lake has few, if any, rooted, aquatic plants to produce oxygen. Without the plants, the lake relies on the air exchange at the air/water interface and alge for its oxygen supply.
Sundmark said Winsted Lake is not in danger of becoming a "dead" lake. "There are still a fair number of crappies and northern pike. This spring we'll set some trap nets and find out through presence/abscense of fish sampled what didn't survive," he said.