Fall Turnover

[Steve Zissou]

As promised, here's a discussion on lake turnover. Feel free to contribute with personal stories.

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As the summer heats up, our lakes undergo a stratification based on density/temperature.

Three zones develop throughout the depth of most lakes:

1. The Epilimnion - Warm waters found on the top of the lake, these waters undergo mixing readily due to higher temperatures and subsequently lower densities.
2. The Thermocline or Mesolimnion - A band of cooler water which mixes, but typically at a slower rate than the waters found on top of it.
3. The Hypolimnion - Deep cold waters that are typically high in dissolved gases such as hydrogen sulphides.

Most anglers know that summer heat means that you have to go deep. Chironimids are an excellent choice for this time of year, and knowing the depth of your lake can actually help you select what chronis to throw on the line based on where you are. This is because some species of chironimid can't survive the deep water conditions of some lakes while other can.

Not to say that you can't hit various chronis in a lake, but only that hitting a chroni hatch with a fly that is convincing can be more beneficial.

As the summer dies off and fall temperatures start to cool the waters on top of our lake, the thermocline begins to thin until it disappears. Essentially, as the waters cool, they sink and this action breaks down the stratigraphy of the lake. This typically occurs around surface water temperatures of 10C, but it can be as low as 4C (The hypolimnion will never get below this temperature, or it will begin to float). Eventually you have a complete destratification of the lake, and wind action on the lake surface will begin to mix the entire water column, all the way down to the bottom.

The effect is the "turning over" of the lake, and the same thing happens in reverse after ice off. The lake is not stratified, and as the temperature goes up, stratification increases.

[btree]

Cool, I just took notes on this in class! But I didnt get this next question answered as well as I had hoped.

So what about lakes that seem to turn over more than just during the spring? Aren't there some lakes in central BC that are known to turn over 3 or 4 times a year, occasionally more? I understand that this has to do with wind?

[ThaSandman]

That usually only occurs in exposed, shallow lakes - they have a large fetch (distance wind can build), but shallow enough for wave action to disturb the thermocline. Also, in areas where the surface temps and hypolimnion temps are close (high elevations and northerly areas), wind can cool and or disturb the strata and cause incidental turnovers. Basically, if it's windy enough, everything goes to hell.....


So my question is, how does fall turnover affect the fishing? If benthic nutrients are being redistributed, does this mean bug populations grow, so the fish are in a feeding frenzy, or do they clam up and wait for stable conditions?

[btree]

Referring to redistribution of nutrients and spikes in insect growth/activity, I'm certain that this would happen in the spring, but not sure about the fall.

[Steve Zissou]

I think your question on turnover was answered, btree, but just in case:

Really, the strata can be disturbed if the lake is shallow, at a high altitude (temperatures fluctuate), the wind is blowing strong...

The real advantage of the turnover is just that it purifies the depths of certain dissolved gases that are noxious or even poisonous to fish such as some sulphur compounds before they go to the depths for the summer.

Often the tell tale sign of a turnover is the smell of rotten eggs from disulphide gases rising to the top.

[johnk]

I think your question on turnover was answered, btree, but just in case:

Really, the strata can be disturbed if the lake is shallow, at a high altitude (temperatures fluctuate), the wind is blowing strong...

The real advantage of the turnover is just that it purifies the depths of certain dissolved gases that are noxious or even poisonous to fish such as some sulphur compounds before they go to the depths for the summer.

Often the tell tale sign of a turnover is the smell of rotten eggs from disulphide gases rising to the top.

Steve,

A couple things that don't add up as far as what I know..........or think I know! Once a lake has stratified there is no oxygen in the hypolimnion where you said it was high in content. Though wind can effect the depth of the epilimnion it does not create subsequent turnovers beyond spring and fall. Water density is at it's greatest at 4C (1000kg/m3) which is the cause for both spring and fall turnover with the reason being density levels fall above and below 4C. I have never witnessed the smelling of sulphur during turnover but have witnessed it as well as yellowing of the water when we've checked O2 readings of lakes that have winterkilled. The meta/mesolimnion and thermocline are low in oxygen which is important for fishermen to know as the fish will be found slightly above the thermocline. In regards to fall turnover, once the lake has destratified the oxygen located in the epilimnion disperses throughout the water column making the lake very oxygen lean for a short period of time causing the fish to become very lethargic due to having to work much harder with their gills extracting air from the water.

I am under the impression that lakes like Tunkwa with a maximum depth of 17', for all intensive purposes, don't turn over as it has an incomplete stratification in the summer with the epilimnion extending to the bottom. There is no hypolimnion and I was just wondering if earlier in the summer there is actually a thermocline which would dissipate as surface temperatures increase throughout the summer and the epilimnion increase the depth of it's band?

[Steve Zissou]

Steve,

A couple things that don't add up as far as what I know..........or think I know! Once a lake has stratified there is no oxygen in the hypolimnion where you said it was high in content. Though wind can effect the depth of the epilimnion it does not create subsequent turnovers beyond spring and fall. Water density is at it's greatest at 4C (1000kg/m3) which is the cause for both spring and fall turnover with the reason being density levels fall above and below 4C. I have never witnessed the smelling of sulphur during turnover but have witnessed it as well as yellowing of the water when we've checked O2 readings of lakes that have winterkilled. The meta/mesolimnion and thermocline are low in oxygen which is important for fishermen to know as the fish will be found slightly above the thermocline. In regards to fall turnover, once the lake has destratified the oxygen located in the epilimnion disperses throughout the water column making the lake very oxygen lean for a short period of time causing the fish to become very lethargic due to having to work much harder with their gills extracting air from the water.

I am under the impression that lakes like Tunkwa with a maximum depth of 17', for all intensive purposes, don't turn over as it has an incomplete stratification in the summer with the epilimnion extending to the bottom. There is no hypolimnion and I was just wondering if earlier in the summer there is actually a thermocline which would dissipate as surface temperatures increase throughout the summer and the epilimnion increase the depth of it's band?

You're right. The Hypolimnion is pretty much devoid of oxygen. I'm not sure why I put that in there. Cold waters typically are higher in DO than warmer waters, maybe I just had a brain fart. Aeration occurs in the surface waters, but because the lower waters aren't mixing as much, or at all, they aren't receiving any aeration.

I've also never really had any experience with lakes that have had multiple turn overs either, but I don't see any reason why it wouldn't be possible in shallower lakes. Essentially, as you noted, in lakes that have incomplete stratification the epilimnion stretches to the floor of the lake, which means that mixing can occur from wind action on a regular basis. That would essentially mean that the lake is in a constant state of turnover, and aeration of the lake would be fairly good.

It wouldn't suprise me to find that these shallower lakes are stratified for the beginning of the season and then as the water warms the stratification breaks down, though that is beyond my knowledge at this point.