This past week was the warmest week of the summer so far this season in the Bemidji area. Surface water temperatures exceeded 70 degrees for the second time this summer, which is the point where the lakes begin to change into summer patterns.
Algae blooms increase as water temperatures rise, which eventually reduces visibility in the lakes. The lake water begins to stratify by temperature with the warmest water on the surface and the coldest water on the bottom.
Lakes with a significant amount of water deeper than 40 feet will develop a thermocline while shallow lakes will periodically get turned-over by high winds during the summer.
Once a thermocline forms, it prevents the water above the thermocline from mixing with the water below the thermocline. The water below the thermocline has a finite amount of oxygen, which is reduced by fish respiration and decomposing plant and animal matter.
Some lakes have enough water volume below the thermocline that oxygen levels never drop below viable levels for the fish. Other lakes begin to run low on oxygen below the thermocline during the summer, which eventually forces most fish to go above the thermocline to reach higher oxygen levels.
Portions of larger lakes may be deep enough to develop a thermocline while shallower parts of the same lake may be too shallow for a thermocline.
There are many examples of this type of lake in the Bemidji area. The shallow bays and main lake portion of Leech Lake has no thermocline while Walker Bay, Agency Bay, Kabekona Bay and Shingobee Bay are all deep enough to have a thermocline.
Lake of the Woods has a thermocline in Whitefish Bay and Clearwater Bay while most of the rest of the lake, including Big Traverse Bay on the border of Minnesota, will not have a thermocline.
Lake Winnibigoshish is another lake that does not have a thermocline during the summer while Cutfoot Sioux connected to Lake Winnibigoshish does have a thermocline.
Deeper lakes like Bemidji, Cass Lake, Pike Bay, Plantagenet and most of the smaller lakes in the Bemidji area with a significant portion of the basin deeper than 40 feet will also develop a thermocline during the summer.
Some of the other changes that occur in the lakes once surface water temperatures exceed 70 degrees include location changes for many fish species and changes in activity levels for fish as their metabolism increases in the warm water.
Swimmers itch is another warm-water phenomenon, with the parasite from ducks and geese most active in warm water. The parasites are concentrated along the shoreline of many lakes, which is where people usually do most of their swimming.
Muskie fishing usually improves as water temperatures increase. Algae blooms in the lakes reduce the visibility in the water, which increases the chances muskies will "make a mistake" and eat anglers' lures like they were the real thing.
Anglers' presentations also change as water temperatures increase. Faster presentations with spinners, crankbaits and other fast presentations begin to work better, with the fish more likely to chase baits in the warmer water.
Some lakes are large enough and deep enough that they never run low on oxygen below the thermocline. Lake Superior would be a perfect example of this kind of lake, along with many of the Canadian shield type lakes. Many of these lakes are "two-story" lakes, with warm water species living above the thermocline and cold water species like lake trout living below the thermocline.
Most lakes in the Bemidji area do not have enough water volume below the thermocline to support trout or other cold water species all summer and will eventually have reduced oxygen levels in deep water, especially during a hot summer.
Deep water springs are another factor in lakes during the summer. Cold water springs located below the thermocline are like a magnet to the fish, especially when the rest of the lake is running low on oxygen below the thermocline.
The changes in the lakes won't happen all at once. The challenge for anglers is to be able to keep track of the fish as the patterns change.
PAUL A. NELSON runs the Bemidji Area Lakes Guide Service. He can be contacted at firstname.lastname@example.org