Welcome to AA, your pond is beautiful!
I have a few questions:
-How deep is the Pond?
-Is it rocky or muddy on the bottom?
You are pretty far north, even in southern ontario i am a little more south than you. We both get crazy winters most likely. My friend used to have turtles in her pond and she had to bring them in. If you are going to leave them out you MUST have a deep mud bed on the bottom of your pond, and the pond should be over four feet, preferably deeper. They like to go way below the frost line.
I would reccommend that you bring them in, but
IMHO, turtles do not belong in ponds which also contain fish, enless you want to sacrifice your fish to satisfy your turtles hunger. So i would say bring them in, but thats just one persons opinion.