Invasive plants, hot days contributed to the problem.
A beleaguered White Bear Lake, already suffering from continuing low water and a surge in invasive plants, now has hit a possible record-high water temperature.
The latest reading, on July 17, recorded the lake at 86 degrees, 12 degrees higher than June and 2 degrees warmer than the same date last year.
"It's probably the highest reading I've ever taken," said Mike Parenteau, a director for the White Bear Lake Conservation District. "Really warm."
Scientists say that White Bear Lake's temperature -- and similar excessive warmth in other metro-area waters -- has resulted from sustained heat in the 90s this summer. But there's also evidence that widespread growth of the invasive Eurasian watermilfoil in White Bear Lake helped raise water temperatures and in turn drove fish to greater depths.
"It's not a good situation, but it's nature," said Jerry Johnson, east metro fisheries manager for the Minnesota Department of Natural Resources (DNR). "I don't think we have to worry about any fish kills relative to thermal problems. It just means we have to fish in different ways and different areas."
Another DNR scientist, Chip Welling, said Friday that declining White Bear Lake water levels invited rapid growth of watermilfoil on the lake's surface since it was discovered in 1988. The worse the floating tangled mats of vegetation became, the higher the lake's temperature rose because it restricted movement that would blend warmer water with cooler water below it.
White Bear Lake fell to a record low depth in October 2010 at 919.43 feet above sea level. The level on July 17 -- the same day the 86 degrees was recorded -- was 920.45 feet. That's more than 6 feet lower than the all-time high in 1943.
Scientists concur that White Bear Lake's small watershed, lacking any substantial inflow, has caused the low water. The lake has retreated from docks and left expanses of unsightly weeds surrounding the lake's perimeter.
"The lake is healthy, very clear," Parenteau said. "What's left is healthy. We would like to have some more water."
Watermilfoil became such a problem on the lake's surface earlier this summer that it would stop boats trying to cross it, he said. The conservation district fought back on July 11 with a chemical treatment that withered the watermilfoil, he said, and improvements are already apparent.
Funding came from a state grant and "we followed DNR treatment procedure all the way," Parenteau said.
Welling, the DNR's Eurasian watermilfoil coordinator, said the invasive nuisance shouldn't be confused with native plants that promote water clarity and provide valuable fish habitat.
"It's important to know what we're dealing with on any given lake when we get reports of matter vegetation," said Welling. "We do want to be careful that we don't do more harm than good."
Higher temperatures in White Bear Lake will frustrate anglers because for many species of fish, the need for food diminishes in warm water, Johnson said. "The fishery is still good," he said, with a DNR stocking program for walleyes and muskies and a healthy smallmouth bass population.
The hotter the surface water gets, the more fish will move into the depths of the lake, he said. White Bear Lake, deeper than most in the metro area, is 83 feet deep in places.
Steve Heiskary, a scientist at the Minnesota Pollution Control Agency, said there's no direct link between pollutants and warm lakes, but higher temperatures can stimulate the pesky blue-green algae that contain natural poisons, cause odors and suck oxygen from the water. Most Minnesota lakes experienced a temperature increase this summer, he said, but White Bear Lake still stands out.
"Around the state you generally don't see too many temperatures that warm," he said.
Kevin Giles 651-925-5037 Twitter: @stribgiles