After the ice melted on the pond I carried on with the oxygen measurements to see if levels went down to the pre-ice levels.
And they did.
So now we are back to the oxygen level that’s about right for the present temperature – 5 degrees C – around 12 or 13 milligrams of oxygen in each litre of water (just a little higher than in November probably because the water is a bit colder now, and oxygen dissolves better in cold water than warm water).
When something new and unexpected happens contrary to what most people would have expected, its interesting to see whether it has been reported before. If it has, you can be much more confident that this is something real, and not some kind of fluke or even a simple mistake.
And there is one example reported by American lake ecologists a couple of years ago of the phenomenon – not in a garden pond, but at least we know that it has been seen somewhere else before, which suggests its a real phenomenon.
You can read a summary of the scientific paper describing this work here, though to read the whole thing you will need access the archive of the journal which requires a subscription.