Ice and oxygen: the end of the story (for now)

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.

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Ice increases oxygen levels in my garden pond

Oxygen levels in my pond roughly doubled when ice covered the pond over the last few days in January. The November values show the typical winter levels. On each day, am indicates measurements made in the morning; pm measurements made in the afternoon (morning is usually lower than afternoon). In January - the line on the right - when ice covered the pond, oxygen levels roughly doubled.

The last few days of ice cover on my pond has revealed something both interesting and surprising about oxygen in garden ponds.

It has shown that ice cover can increase the amount of oxygen in the water – the exact opposite of what most advice on the web suggests will happen.

The graph above shows the oxygen level before ice cover a couple of weeks ago – around 10 milligrammes of oxygen in every litre of water – and how it has roughly doubled after about 10 days of ice cover.

How does this happen?

It’s all down to the ice. Photosynthesis goes on as long as its light and there are plants to do the photosynthesis. In my pond there are algae growing on the pond bottom, quite extensive beds of mosses under the water, and microscopic, free-floating, planktonic algae in the water. The water is also clear, letting light through to the plants, so there’s plenty of photosynthesis producing oxygen.

Normally the excess oxygen simply leaks into the atmosphere – but with a cover of the ice the oxygen is trapped – it can’t get out of the pond and the water becomes supersaturated with oxygen.

Of course, if you have a small pond with a lot of decaying organic matter in the bottom its still possible that there be de-oygenation of the water. But I think this is the first time that anyone has ever noticed that ice can increase the oxygen levels in a garden pond.

More dodgy adv-Ice

I scanned a few websites to see what they were saying about  ice. Here are a few things, which mostly aren’t true:

‘An ice-capped pond is prevented from enjoying the normal gas exchanges between the water and atmosphere‘.

As yesterdays measurements on my pond show, exchange with the atmosphere is not necessary for keeping high oxygen levels. So, although its true that ice will stop the exchange of gases, this doesn’t reallly matter.

‘The biggest enemies of your pond and its inhabitants are algae in the summer, fallen leaves in autumn and ice in winter‘.

Three myths for the price of one!

‘….freezing can threaten the survival of aquatic animals as it prevents oxygen diffusion from the air into the water. Holes should be opened in the ice to allow oxygen into the pond, and for toxic gases to escape. A good way to do this is to place a bucket of hot water onto the ice.’

More of the same. I rather doubt that a small hole in the ice would actually make much difference to the amount of oxygen getting into the pond anyway because diffusion of oxygen into still water is an extremely slow process.

‘Another important reason not to let your pond ice-over is because the oxygen levels in the pond water can drop to dangerous levels.’

Well, as we saw yesterday, oxygen levels actually rose to unprecedented levels during ice cover. It will be very intgeresting to find out how general this phenomenon is.

And to finish with, one from the Environment Agency of England and Wales:

When winter arrives, your main concern is going to be the pond freezing over. In really harsh conditions, a couple of weeks of freezing weather will cause the pond to freeze up to a depth of 15cm. If this happens, the fragile ecosystem of the pond can change dramatically. At least one opening should be made in the ice as there is a danger of oxygen levels dropping if the pond is completely frozen over.

And some more added later: from a book called ‘The Pond Book’ (not ours at Pond Conservation, I hasten to add!)

‘Ice is a poor conductor of heat’ – well we did that one the other day. Ice is actually a good conductor of heat.

But on the positive side – ‘…if a snowfall occurs light will be blocked out of the pond…‘ is true and really can lead to a drop in dissolved oxygen concentrations as photosynthesis, which produces the oxygen under the water, is stopped.

I’d be interested to hear from anyone who has 15 cm of ice on their pond – perhaps in the north of England, or Scotland (it doesn’t count if you live somewhere really cold!). Come on guys – time for some feedback!

Should I break the ice on my pond?

My pond today with about 1 inch of ice

UPDATE: Ice cover doubles pond oxygen level. See latest.

It’s a common piece of advice that you should break the ice on your pond.

The reason usually given is to let more oxygen in or let toxic gases out. Here’s a typical example from a major conservation organisation’s wildlife pond handbook:

‘Air holes should be opened every day during long freezing spells. These allow oxygen to diffuse into the water from the air and allow gases to escape from the pond.’

My pond has had ice cover now for the last 8 days, so I thought it was time to measure the dissolved oxygen concentration, to see if I needed to ‘allow oxygen to diffuse into the water from the air’.

The dissolved oxygen concentration this afternoon was 17.9 milligrammes of oxygen in each litre of water – this is in fact considerably more than 100% saturated – actually, its the second highest value I’ve ever measured in the pond, only slightly below the peak in summer when algae were producing oxygen like mad.

It’s considerably higher than the last time I measured the oxygen concentration a few days ago – roughly double.

I imagine that the ice is trapping the oxygen that is being produced under the water by algae and mosses.

There is no shortage of oxygen: indeed, the ice seems to have increased its concentration – the direct opposite of what standard advice tells us.

Mind you, you might want to break the ice to give the birds somewhere to drink.

Ice keeps the pond warm

Pond and air temperatures so far this week

Though we’ve had a run of four or five cold days, followed by a warmer day today, the temperature in the pond has barely changed.

Under water there’s been little more than one degree difference between the coldest night and warmest day.

Ice keeps your pond warm

My pond, today

At night it’s getting down to around -2 C now, cold enough for ice to form on the pond.

But under the surface things are staying a nice constant 4 C.

The ice is keeping the heat in.

By the way – look how clear the water is now.