I guess it wouldn't be too worrisome in those temps but on lakes in northern Iowa the ice could be nearly 2' thick and you'd still find a soft spot in places. Not sure why unless there might be a natural spring below the surface. Ice skating is one of the few things I miss about winter, always had a great time skating and watching the little ones in their first try. Cool video, beautiful scenery.
Years ago when going duck/geese hunting following an early freeze, there often would be an open spot of water in the pond on which we were hunting. Watching the birds come in to land on the water was more fun than shooting them. They'd hit the water only to hit the ice a few seconds after touchdown and go skidding across the pond on their butt, squawking loudly, flapping their wings and feet all splayed out.
