I'm surprised you had enough warm water to get them going for this season, but if they did and things froze up for long enough, I would suppose that could have stressed the tadpoles. Regarding the crayfish, I'd be interested in theories. I thought they existed under the water or burrowed all year long.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."