By John Cross
Free Press Staff Writer
LAKE CRYSTAL — The four-wheel-drive pickup lurched and bounced across the hard-packed, snowy moonscape of Loon Lake toward an aeration system marked by a small patch of open water.
Coming to a stop, Craig Berberich, a technician from the Minnesota Department of Natural Resource’s Waterville Hatchery grabbed a four-inch hand ice auger and a device used to measure the amount of dissolved oxygen in the water and set off across the ice.
Fifty yards from the open water along a perimeter of thin ice signs, he drilled through a winter’s worth of ice and dropped the meter’s sensor down to the bottom of the hole.
The digital readout momentarily read 3 parts per million and then began to fall past 2 ppm, then 1 ppm, finally stabilizing at .9 ppm, a figure he recorded on his chart.
He then lowered it all the way to the lake bottom and watched the number settle at .3 ppm.
A few minutes earlier, he had checked oxygen levels closer to the public access a half-mile distant.
The readings recorded there were .4 ppm at the surface, .3 ppm three feet down, and finally, .2 ppm at the bottom.
Since mid-January, Berberich and other fisheries personnel have been monitoring dissolved oxygen levels on area lakes prone to suffering winterkill.
Winterkill is a condition that results in fish suffocating as heavy snow cover shuts down the oxygen-infusing photosynthetic process in aquatic plants or decaying plants on the lake bottom deplete oxygen levels.
Depending on how gradual the oxygen level falls, most fish species can tolerate oxygen levels as low as 1 ppm with some species tolerating less then half that.
By contrast, summertime oxygen levels in lakes typically are in the 10-12 ppm range. Cold water can reach even higher levels. Indeed, as recently as mid-December, levels on Loon Lake were as high as 17.2 ppm.
Perhaps a measure of the severity of this winter, in the ensuing 10 weeks, levels now have fallen to critically low points.
Even though some of Loon Lake’s levels were extremely low, Berberich was unwilling to a chalk the lake up as a winterkill victim just yet.
“It’s not good,” he said. “It’s possible there could be something happening out there. But if there are some field tile out there running into the lake, they can be fish refuges. But we won’t know until the ice goes out.”
Earlier, he monitored levels on nearby Crystal Lake where he found levels near the aeration system of .8 ppm at the surface, and .5 ppm at depths of three feet and six feet.
Last year, that lake suffered a partial winterkill, the most visible victims being large walleyes.
Subsequent test nettings in the spring revealed good crappie populations, an indication that the winterkill was less severe than first believed, since that species is particularly intolerant of low oxygen levels.
“What we really need right now is three or four days of 40 degree temperatures and we’d be out of the woods,” he said as the pickup bounced across the snow back to the access. “We haven’t had anything like that so far this year.”
