Lake Biology: Turning Over?

I’m trying to find out the validity of an expression that is frequently cited when mid-western lakes have a great deal of plant material floating on the surface, usually during summer months. The expression goes “the lake must be turning over” as a way of explaining the phenomenon. It seems to imply some activity on the lake’s bottom of a biological nature. Does anyone know if there is some truth to this or is it an old wives tale and is all wet?

I don’t know much about this topic (and I will leave it to others to give us the Straight Dope) but I have heard of it. I thought, however, it was more of a physical phenomenon than biological, i.e., more to do with temperature differences than biomass.

IIRC, there was a lake in East Africa, near a volcanic zone, that had a great deal of dissolved carbon dioxide in it. When it “turned over” the CO2 was released into the local atmosphere and blanketed the nearby villages, asphyxiating the villagers. This was maybe 4 or 5 years ago and I read about it in Scientific American – so the source is good, if my memory is! I’ll see if I can’t find some info on the web.


Strangers have the best candy.

I actually don’t know exactly what the expression refers to, but I’ll give it a shot.

What I do know, is that lakes tend to be very temperature-stratified in North America. That is, the top of the lake gets heated by the sun, and the warm water stays at the top. So most of the year, the lake has two layers: the warm epilimnion which is on top, and the cold hypolimnion which is at the bottom. The layers are separated by a surprisingly thin layer, known either as the limnion or the thermocline. Once I went swimming in a shallow lake and I’m pretty sure I felt this. I paddled around on the surface, which was nice and warm. When my body became vertical, my feet and lower legs were plugged into what felt like icewater.

For most of the year, the two layers mix very little, but in the summer time, many lakes are heated to the bottom, and the stratification is destroyed. The lake mixes, and the ecology changes temporarily.

My guess is, that when the stratification is destroyed, plant matter which might have been anchored to the bottom gets moved around by the mixing of the lake. This might even be a method of propagating itself, since I think lake plants can “re-plant” themselves fairly easily, or at least plant their offspring.

Thanks, jazz!

Other than moving the deadly lakes to the wrong side of the continent my memory wasn’t too bad. I must not be as old as my wife tells me I am!

Strangers have the best candy.

you’re welcome Pluto

on an aside, I LOVE your sig line.

If you ever stop using it, can I have it?

Thank you Pluto, Boris and Jazz for your insight. It does seem that this is a physical rather than a biological event. After learning about the killer lakes in western Africa releasing CO2 gas, I’ll be taking SCUBA gear whenever I venture near a lake.

Why would you go to a lake without SCUBA gear?

“You can’t run away forever; but there’s nothing wrong with getting a good head start.” — Jim Steinman

Dennis Matheson —
Hike, Dive, Ski, Climb —

Petosky stone hunting?


What are petosky stones?

Oh yes, for all you boaters, skiiers and fishers out there… The red flag with the white stripe (USA/North America/Caribbean) or the blue and white flag (pretty much everywhere else) means a diver is underwater there. It is not a slalom flag or an indication of a fishing hole. Just last month I watched a jet ski run directly over a dive flag. Let’s give it about a 50 foot range, ok? Thanks…

“You can’t run away forever; but there’s nothing wrong with getting a good head start.” — Jim Steinman

Dennis Matheson —
Hike, Dive, Ski, Climb —

Just to get back on topic for a moment, a lake ‘turning over’ causes the algal bloom by bringing up nutrients from the bottom. (It happens every summer back in the prairies, where I’m from.) All sorts of nitrogen-rich material settles out, and isn’t used up because there’s not much living down there. Then when the good stuff rises into the sunlight, the small amount of algae that’s always there starts reproducing like crazy.

The old reservoir near my home town used to go stagnant and suddenly thicken up (and stink!) every year when it turned over. We were lucky - some lakes around there had red algae instead, and smelled worse.

Bob the Random Expert
“If we don’t have the answer, we’ll make one up.”

I always considered a lake to be turning over in the fall. The warm water on top cools down and then the layer on the bottom is warmer. The warmer water rises to the top. The contents of the bottom are in the rising water.

Algea blooms and excessive plant growth in the summer are a fertilizer run off problem. That’s also why phosphates were removed from your detergents.