February 2009 was one of the coldest months in a long time. The pond was frozen for almost the entire month and it snowed frequently. The temperature never got above 2-3 in the day so the snow remained even in small amounts.
I tried to make sure the pond was not fully iced over to allow air to get to the pond. However by mid February the icewas actually so thick that i could only get a tiny hole(just over finger size) by using a stone to chip away about 2-3 inchs of ice. Even though it is best not to break ice with force in winter, i felt it was more important to keep a air flow to the pond for hibernating frogs etc. Snow remained on top of the ice so i doubt there was much if any light getting into the pond. Eventually i couldnt even break the ice with a stone so i just stopped trying.
Luckily it started to thaw out in the last week of the month so there was no major damage.
In the pictures below you can see the ice was covered in snow and that cats and birds had been walking on it. I would have tried it myself but i didnt want to damage the liner or plants by falling in :) it was about 3 inches thick by then though.
In these two pictures there is a thick layer of ice and then snow on the top which has got wet and turned to ice.
This picture was about a week later than the two above but the pond hadnt thawed out at all.