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sparky27

What is lake turnover?

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I have heard of lakes turning over in the fall. can anyone explaine what exactly that is?

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Turnover has to do with water temps, as well as a bit of chemistry. Water is most heaviest at 39degrees and as temperature increases or decreases from 39degrees, it becomes increasingly lighter. So when the temperature of the lake water gets to that 39 mark the water at the surface "sinks"

causing it to mix with the water at the bottom. The lake then settles at 39 degrees for a while water colder than 39 degree's being lighter then cools untill the lake freezes.

In the spring everything is reversed. Temp warms till it hits 39 everything mixes then the surface warms.

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Actually, and I just asked a bunch of questions on this in the other thread, the surface water temp doesn't have to get to 39 to sink. It just has to get cooler than the water right below it. Then it will be less dense and will sink, swapping with water below it, which also then cools and becomes denser and so on. The DNR seems to subscribe to the 39 degree theory, and maybe that is correct - lakes are continuously "turning" until 39. But I think common belief is that it is mostly completed by about 55 or so. I think we all could use a refresher course...

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Hi Sparky -

Copying this from a post I made in another forum on the site, (cuz I'm too lazy to type it over again):

Turnover is really an unpredictable deal, with tons of variables - how 'hard' or pronounced the thermocline is, how rapidly water temperatures fall, how high water temps got over the summer...wind and rain, etc.

Basically though, as you probably know, most deeper lakes stratify during the summer. A thermocline - a band of rapidly decreasing water temps - forms, with water above it that gradually increases in temerature as you get closer to the surface. Below the thermocline, the water is relatively uniform in temperature. The thermocline acts almost like a lid on the lake basin. On most lakes, there's little to no oxygen below the thermocline. Since there's no photosynthesis beyond the level of sunlight penetration, there's no oxygen being created, and what is there is used up by the process of decay going on with any organic matter that falls through the thermocline. So below that layer, it's a no oxygen environment, except on lakes that are spring fed.

When the water cools in fall, the surface water cools to a temperature below that of the water underneath it, and sinks, displacing the water below it. When this displacement is fast enough and with enough volume, it breaks up the thermocline and the mixing process can bring up stagnant water from below the thermocline that gets re-oxygenated.

The effects and severity of turnover can really vary. Sometimes, when the process is very gradual, or when there was a weak thermocline to begin with, it's barely noticeable. Other times, often when the process is accelerated by high winds and/or a rapid drop in surface temps, it can pretty profound. The water gets very murky with lots of gunk floating everywhere, and the whole lake can smell bad. I can remember one fall a few years back when I pulled into the public access, opened the door, took one whiff, shut the door and drove off - the turnover smell was overpowering. When there's a rapid turnover, it really seems to put most species of fish off for a few days. But it's pretty temporary.

A long winded explanation that's still probably only part of the story, but that's turnover in a nutshell... Hope that helps.

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Quote:
Turnover is really an unpredictable deal, with tons of variables - how 'hard' or pronounced the thermocline is, how rapidly water temperatures fall, how high water temps got over the summer...wind and rain, etc.

Quote:
Sometimes, when the process is very gradual, or when there was a weak thermocline to begin with, it's barely noticeable.

As Rob said later on in his post, turnover can be a very gradual process. Looking at the upcoming weather forecast, that may be just what we are looking at. Relatively stable conditions, lacking a strong wind "blow", and cool evening temps with mild days. We shall see....

Lots of this stuff gets far beyond my level of scientific interpretation and planning, so if you're somewhat like me and tied to a schedule of some sorts I'd still encourage you to head out and read the conditions of the lake you're on.

I'm likely looking at metro waters this weekend and still banking on pre-turn conditions.

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a recent musky publication i was reading talked about using a temperature gauge to measure the temps at different water levels down to 50+ feet and then fishing the depth that has the ideal temperature. I was wondering if anyone had any experience using such a devise or if they know of some guides that use it. The reason i bring it up is because I suppose it could be useful in determining the exact time that the turnover is occuring.

im sure many of you probably read the article. just thought it was kind of interesting, any thoughts?

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Vexilar has something I think called the deptherm that tells what depth the device was lowered too and the temp.

There is a cool device that has a digital readout saying depth of the unit along with dissolved oxygen and multiple other things like pH and conductance and so on. Only problem is I think its in the multiple thousands of dollars range.

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I have a depth raider on my downriggers and it tells the water temp and the speed the unit is moving at that depth. Its costs about $500 but it is very important on Lake Superior. Often times your boat speed will be 2.3 mph but your lure speed 100 feet down is only 1.5 because of current. Also you can watch the temps change as your going along it may 43.5 f at 80 f.o.w and troll 2 miles and the temp may drop to 39.7 f at the same depth. Becuase there is not much structure on Superior fishing the right temp zone is very important. But I dont think a depth raider would help you much with musky fishing

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