Pond oxygen levels over Christmas

My two ponds have been frozen now for nearly two weeks.

With the rain of the last couple of days the ice is beginning to melt at the edges, but when I made the latest measurements yesterday evening ice still covered most of both ponds.

Oxygen in the old pond is still well above normal levels still, mirroring the pattern of last year. It should go down as the ice melts.

The oxygen level in the new pond is also interesting: it has stayed pretty constant throughout the freeze. Probably there are not enough plants in the water to raise it very much.

Quite why levels rose in this pond yesterday I don’t know: one possibility is that with the day’s heavy rain, and the slight melting around the edges, better oxygenated water entered the pond. But that’s just an educated guess.

The one thing that hasn’t happened in these two ponds is that oxygen has gone down during the icy weather.

Of course, these ponds are a little different to most others: shallower, clearer and with more plants.

The next thing to explore is what happens in more typical garden ponds when they ice over. Our measurements show these often have quite low oxygen levels.

How these ponds respond to icing over will be something to test in the next cold spell.

A pondy Happy Christmas

To all pond lovers………

A very Merry Christmas and a Happy New Year.

Ho, ho, ho.

No evidence? No problem!

Another unfounded suggestion about iced-up ponds comes my way, from a reputable organisation.

The authors get off to a good start, suggesting that there’s no need to make a hole in the ice.

But then they spoil it a bit. The reason they propose you don’t need to break the ice is because…

‘If there is a deep section, the pond will contain enough oxygen to keep amphibians alive over winter.’

Oops. Our recent detailed measurements in Abingdon garden ponds suggest that simply having a deep section is no guarantee that there will be oxygen in the water.

The results of the Abingdon survey – which we’re working on now – show that deeper garden ponds, when they accumulate large quantities of leaves in the bottom, often have very little, and sometimes no, oxygen.

It looks as though the standard stepped pond shape – with a sump dug down to 50 or 60 cm (sometimes more) – creates an environment where, as leaves build up, oxygen demand greatly exceeds what can be supplied by algae and plants.

So unless your pond has the right general conditions for producing oxygen – which probably means shallow, clear water, plenty of submerged plants, along with planktonic and bottom dwelling algae…

…having a deep water area is no guarantee that there will be oxygen for amphibians, or anything else.

Icy ponds: the oxygen yesterday

The oxygen continued to rise slowly under the ice yesterday in the old pond.

In the new pond, which has few plants underwater, oxygen went down very slightly.

It’ll be interesting to see what happens over the next few days.

Should I break the ice?

This winter’s cold weather naturally prompts many people to ask: ‘Should I break the ice on my pond?’.

Most advisors say you should make a hole to let oxygen in or ‘toxic gases’ out.

So last year, when I measured what actually happened in my pond when it iced over, it was a bit of a surprise to find that oxygen levels nearly doubled when the pond was frozen.

Far from causing a shortage of oxygen, ice cover made oxygen superabundant.

This was because photosynthesis continued under the ice but the oxygen could no longer escape to the atmosphere. So far from threatening wildlife, ice cover made certain there was no oxygen shortage.

So now this year’s freeze is a good opportunity to see whether the same thing happens again: the only difference is, this time I’ve got two ponds!

So what is happening?

Both ponds have been frozen now for 5 days – I began measuring oxygen on the third day.

The picture shows oxygen levels slowly rising in the old pond and staying pretty much constant in the new pond.

Why the difference? I’m not completely sure but there are dense growths of mosses growing under the water in the old pond. The new pond so far has only the algae growing on the bottom sediments and in the water.

So far neither pond is showing any signs of oxygen depletion following ice cover.