Turnover happens in many temperate lakes, and in most cases, is considered a good thing. It helps to mix the water column which distributes the dissolved stuff that keeps things alive and thriving. I think the key is whether or not a thermocline develops. In shallow water, the water stays pretty much mixed so the thermocline doesn't form. When a thermocline does form, it acts as a density barrier. The water on top, stays on top and continues to heat up during the summer. Once the surface water starts to cool off in the fall and winter, it's density increases to the point that it overcomes the thermocline barrier and sinks to the bottom. That in turn forces the deeper (now relatively warmer water) up. Pretty much mixing the pot.
For some reason, I've always been fascinated by the miracle of nature that is water. It's the only thing I know of that actually becomes less dense when it becomes a solid. If it wasn't for that simple fact, lakes would freeze from the bottom up, which means many of the fish as we know them wouldn't exist. Maybe not something that is a big deal for you guys down south where the water never freezes, but I'm reminded every year up here when the lakes ice up. It really is a miracle that someone up there figured that out ahead of time and made things the way they are.
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