I don't have any experience with alligator snappers outdoors but I do with other large pond turtles. I used to live in Chicago so I know what you are talking about, Mike. Here in Tampa it only gets into the 30s for about one or two weeks a year. When that happens my cooters moved into the bathtub. Even with a good heater the water temp still scared me. I assume you are building an in-ground pond, in which case the ground should insulate the water somewhat, but I would suggest building and insulated cover for your pond for the fall and heating it with a large titanium heater. Play winter by ear. If the water gets too cold then go back to the drawing board.
I am moving to Denver next month so I will have to figure out a whole new strategy out there. It's always a learning process and the source of a lot of fun as well as annoyance.
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