Luke
In the autumn the ponds are drained and empty, then it snows and they fill up with snow. When it warms up the snow melts and fills them up with pure fresh snow water. There are virtually no ponds covered in ice with water beneath.
The snow this winter has been perhaps 2m deep on average.
In the most recent pics what may look like ice is actually the snow melting around the edges leaving a 'cone' of snow coming up from the bottom into the centre of the pond.
Mark