As mysteriously as it appeared the black hole has vanished.
When the Highway 371 bay of North Long Lake north of Brainerd froze over last fall the area where the black hole had formed a year ago froze over with the rest of the lake. For a time area lakeshore residents monitored the ice over the hole and found it was thinner than surrounding ice.
"It was 11 inches everywhere else and just an inch-and-a-half over the hole," said Al Cibuzar, head of A.W. Research, the water monitoring company that studied the hole last year.
But the hole never re-opened.
Cibuzar said he flew over the hole in an airplane twice before it froze and saw the same unusual thermal patterns he had seen last year. He also said he saw an unusual round hole on the bottom of the lake in the shallows along the north shore of the west bay and three more in the center bay. The holes appeared to be about 8 to 12 feet deep and filled with debris.
"We don't know what caused them to form or if they're associated with the hole," Cibuzar said.
Water studies of North Long taken in the summer and fall twice found high concentrations of phosphorous, Cibuzar said. Normally phosphorous readings range from 20 to 28. But on two occasions the phosphorous content was found to be 56 and 107.
Last winter, a "black hole" created a large area of open water in the Highway 371 bay of North Long Lake. What caused the hole in the ice to open remains a mystery. This winter it has not formed. (Dispatch File Photo)
"Weird," Cibuzar said. "We've never seen fluctuations like that anywhere else in the watershed."
Large flocks of seagulls inhabiting the lake around the time of the tests could have influenced the readings, Cibuzar said.
"Their effluent can drastically affect water quality," Cibuzar said, "especially if the tests were done that day or within a few hours (of when the seagulls were present)."
Snow cover might have eliminated the black hole. Last winter North Long had very little or no snow cover for much of the winter. But this winter an early snowfall blanketed the lake with a protective layer of insulation.
"When the bottom is shaded, plants die," Cibuzar said. "When that happens no heat is created, so the hole froze. There's quite a mass of (muck) there. When conditions are right and the bacteria from that (muck) is high it can keep the thermal hot plate going. That's just one theory. We haven't ruled out ground water."
Last year the hole froze over Feb. 5 and remained frozen until the ice went out on the rest of the lake.
The Thirty Lakes Watershed District has not spent any money studying the hole this year, Cibuzar said. But area residents continue to keep an eye on it.
"We haven't seen any open water, though no ice fishermen have ventured out there either," said Tom Kasmirski, who lives on North Long. "Early in December a couple guys checked (the ice over the hole) a couple times and found it wasn't as thick. But nobody has checked it since."
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