This month's hot days that seemed to flow one from another without much of a reprieve at night have begun to alter the ecosystems of many Minnesota lakes and more days with temperatures near 90 degrees are in the forecast for this week.
The water temperatures in Lake Minnetonka are a couple of degrees above normal for mid-July, said Telly Mamayek, Minnehaha Creek Watershed District's communications director. Excessive heat can cause fish kills, though none have yet been reported.
Tanager Bay, which is shallower than other bays in the lake, was at nearly 84 degrees last Monday, July 16, she said, while Wayzata Bay was less than 82 degrees.
The warmer water temperatures create an environment where less oxygen is stored in the water, which disturbs the food web and alters the food groups that are available for other organisms, said Kelly Dooley, MCWD water quality specialist.
Algae also likes the warmer temperatures, spurring its growth, which uses more oxygen and blocks light to the plants on the bottom of the lake, she said.
But this happens every year, said Dooley. Usually the warmer water temperatures only last a couple of weeks, and then they begin to drop for fall preparation for winter.
She said they usually hear about fish kills, but none have been reported throughout the watershed.
"It [the warm water temperatures] would only be alarming if these temperatures started in May or June and stayed all summer," said Dooley. That would not leave enough time for the lake to recover.
This year, the lake is a few degrees warmer, but the lake is reacting the same, she said.
The average temperature of 26 bays around the lake have varied over the last five years, with this year being the highest.
In 2008, the average water temperature at this time of year was 76.68 degrees, and it dropped to 73.83 degrees in 2009.
Starting in 2010, the average temperature has climbed a couple of degrees each year from 77.83 degrees, to 79.92 degrees in 2011, to 81.43 degrees this year.
Though no fish kills have been reported in Lake Minnetonka, the Minnesota Department of Natural Resources received reports of dead northern pike from homeowners on Bass Lake in Plymouth.
The above 95 degree days with nights hovering around 80 degrees create extremely warm water temperatures for the northern pike, said Daryl Ellison, DNR Metro West Fisheries area supervisor.
Optimum oxygen levels for fish is between 5 ppm and 6 ppm, and when it is depleted to 3 ppm, the fish do not stay in that area for very long. They search out cooler temperatures with higher amounts of oxygen, which is getting harder to find, he said.
"There is no doubt the [northern] pike are stressed and looking for cooler temperatures," he said.
Earlier this month, some shallower lakes reached a surface temperature of 90 degrees, said Ellison, which caused some fish kill.
Even though Lake Minnetonka's ecosystem is reacting normally for this time of year, Dooley said there is more algae. The MCWD monitors the amount of algae, but not the specific kinds. Usually, warmer water means a different mix, with more blue-green algae, she said.
The cooler weather last week helped, but if the heat continues, changes in the lake's food web will continue.