Too cold for oxygen
WILLMAR -- It just got too cold, too fast for fish in Lake Wakanda.
That was the conclusion Bruce Gilbertson, manager of the Minnesota Department of Natural Resources' fisheries office in Spicer, presented to the Kandiyohi County Board of Commissioners Tuesday.
Gilbertson notified the board that the aerator working to keep the lake open to collect oxygen was being shut off.
He said Wednesday in an interview that signs were beginning to point to something happening at the lake, which is a little more than two miles southeast of Willmar.
"Normally, that aeration system has been capable of preventing winterkill in that basin. However, there are a number of factors this year. Wakanda has received a lot of nutrient load, we had a relative drought in the late summer, even though we got rain in the late fall, a lot of that was absorbed by the soil," Gilbertson said. "We came into the winter, by some standards, with a normal ice-up period, but earlier than recent years, then we got snow cover early that blocked sunlight. Rather than producing oxygen, the plants themselves were consuming oxygen, even when they were decaying."
Higher agricultural and storm run-off has been a problem at Lake Wakanda for some time, and has been the subject of many discussions over the years about how to better manage the lake. The higher nutrient load increases the undesirable aquatic vegetation, like algae blooms, and cuts down on sunlight needed during the warmer months to produce oxygen.
The aerator didn't break down and was working, Gilbertson said. It just wasn't able to keep up with the cold, early-winter temperatures. Though the machine does pump air into the lake from the bottom, it's not that process that adds oxygen to the water.
"The lift from the bubbles that comes out of the system, as they rise, they pull deep, relatively warmer water up to the surface, which allows the exchange of oxygen," he said. "But you have to have a significant area open for the exchange of oxygen. The system was not able to keep a large enough area open. Gradually, it went down and reached a point where I felt the levels were low enough where we probably felt that we had lost enough game fish."
So there will be a winterkill not seen at the lake since the mid 1990s. But it's not that bad of a thing. Winterkill is common on shallow, prairie lakes in southern Minnesota, which Wakanda is one of. At only 15 feet at its maximum depth, Wakanda would be susceptible to it without the aerator.
But having a winterkill once and a while can actually benefit a lake's ecosystem.
"It's an attempt to kill off enough undesirable fish," Gilbertson noted. Carp have been a growing problem for Wakanda and steps had been taken to reduce it with a fish-control structure to prevent a mass migration of fish from invading the lake.
Liberalized fishing rules went into effect this week on Lake Wakanda. Those rules allow resident anglers to take fish of any species in any amount by spear, gillnet or angling. Any rough fish may be sold.
However, anglers are reminded that the area around the aerator, which is marked, is too dangerous for anglers and violators can be cited for trespassing.
Once spring returns, Gilbertson said his office will do netting surveys to see which species might have survived. Then they'll stock the lake with walleye fry and hope they survive the next winter.
"There really won't be any fishing that would be easily available this summer," he said of this year's prospects on Lake Wakanda. "The fish barriers are not 100-percent effective, so there will be fish there, depending on the overflow from the spring melt.
"But by the end of the summer of 2010, walleye fry that we will stock this spring will start to reach a catchable size. We'll probably see clearer water and more rooted vegetation than we've seen. But without effective barriers, we will see the effect of carp again."