The cooling temps of fall have an effect on area lakes. Turnover, a somewhat mysterious processs to many, can leave some anglers wondering where the fish went, and can even cause a quick late-season algae bloom.
Steve Gilbert, DNR fisheries biologist in Woodruff, said our lakes seasonally stratify during the summer.
“The warmer water stays on top, the cooler water falls to the bottom, and so we kind of see this kind of warm [well-oxygenated] layer on the top of the lake, a ... middle mixing layer and then this lower layer in the lake, which is usually cool water, but it has very little oxygen in it,” he said. “It’s actually separated from the surface waters because of its density and temperature gradient.”
This changes when the surface water temperatures cool.
“Come fall, as the water starts to cool on the surface here, that upper layer of water ... starts to sink. And when it sinks it causes a mixing effect, like an internal current in the lake ... it mixes the lake water,” Gilbert said.
“It mixes that lower water layer with the upper water layer and you find that, for the most part, the oxygen and temperature are pretty much uniform from top to bottom for a while in the fall.”
That’s what’s called turnover, Gilbert said. The fairly uniform temps continue until lakes freeze over, then the warmest water is actually found at the bottom.
Winter stratification occasionally occurs on some lakes due to differing oxygen levels, Gilbert said.
“In the spring, as waters warm up, it reverses and it mixes again ... then you get the summertime stratification,” he added.
It’s not an automatic process. It depends on the lake.
“Some lakes don’t stratify at all. Some don’t even turn over. Some of the shallower lakes that are really well-mixed by wind action never have turnover,” Gilbert said.
Usually late September to early October is when Northwoods lakes turn over. Gilbert said it depends on the temperature.
“If it stays warm like it has been, it’ll postpone it. A couple years now, we’ve had some pretty warm falls and it’s dragged out into October,” he noted.
Gilbert said lakes turn over at different times and at different rates depending on the volume of water that has to cool.
“It takes longer for Trout Lake to turn over with its huge volume than, say, Big Muskellunge Lake or Found Lake ... size matters in that regard.”
Sometimes turnover can cause a brief algae bloom.
“During the summer, all the nutrients and zooplankton will fall down through into that lower water level that’s unoxygenated and they sit there, kind of bound-up,” Gilbert said.
“They’re not really exposed to oxygen, so they’re not [subject to] the oxidation processes that go on. Once you mix that, it upwells all the nutrients and material that’s on the bottom.”
The upwell can cause a bloom as these materials act as a fertilizer for the algae.
Gilbert said it doesn’t usually last long because the water is cool and algae is naturally dying-off with the cooler water temps.
Turnover can cause fishing patterns to change.
“There’s a lot of different theories on that,” Gilbert said.
“Some people say that when a lake is going through turnover, the fish don’t bite as readily. I don’t know if I really subscribe to that,” he added.
“Certainly, some people seem to observe that after their lake turns over, that it takes a while for things to settle, and that makes some sense.”
Changes such as the water clouding up, temperature changes, and suddenly well-oxygenated water in parts of the lakes that fish previously weren’t using, and where they may be finding different foods available, might affect fishing.
The fish are still there, though, and crafty anglers eventually find patterns that work.
Craig Turk may be reached at email@example.com
• Fall turnover occurs when surface water temps cool sufficiently, causing the top layer of water to sink. This causes a mixing effect.
• Lakes turn over at different rates depending on the volume of water that has to cool. Some lakes don’t go through the process at all.