Also, it seems that the term "turnover" is used to describe more than one phenomenon. Every pond or lake that has a thermocline turns over in the fall when the surface layer cools and sinks, thus mixing with the lower layer; this is normal and usually doesn't kill any fish. But I've also heard the term used to describe a phenomenon that is less common whereby water cooler than the water in the pond is introduced suddenly - i.e. by a cold rain - and somehow displaces just a portion of the water column with deoxygenated water (biologists feel free to chime in here and elaborate/correct). I don't remember the exact specifics and didn't find anything just now on google, but the gist is that only the fish at one certain depth of the pond are affected - but it can kill every fish at that depth in the pond. This happened once years ago to a pond I had worked with previously; I wasn't fortunate enough to be in TN when it happened and thus couldn't have prevented it; dozens of "huge" (pond owner's adjective) shellcracker bit the dust from it.
