Jim asks a good question (or six) about oxygen

In a very short e-mail Jim has managed to pack in a lot of pertinent questions! I thought the answer might be of more general interest.

Pity I couldn’t make the answer as short and pithy as the questions – must be a lesson in there somewhere.

Anyway….Jim says

‘A colleague at work forwarded on the results of the Big Pond Thaw survey 2010 and I was very interested in your advice to have shallow ponds (<30cm), as they have higher levels of oxygen.  I thought that shallow water depths tended to be warmer (and certainly more prone to warming) than deeper water and therefore potentially less oxygenated as colder water retains oxygen much better?  This lower oxygenation would therefore be bad for the amphibians.  In addition, I’ve been helping out with toad migrations in our local area and I was informed that toads prefer deep water and therefore are less likely to breed in shallow (<60cm deep) ponds?  Would a pond with a graded depth be the best compromise?’

Dear Jim

You’re right that cold water has more oxygen than warm water, all other things being equal. Theoretically, 100% saturated oxygen concentrations go from about 15 milligrammes of oxygen in each litre of water at 0 C down to about 8 milligrammes per litre at 30 C.

So, in theory, half as much oxygen in really warm water as in cold.

But lots of other factors can affect this.

If there are a lot of plants and algae producing oxygen you can get significantly more oxygen in the water than ‘100% saturation’, and if there’s lots of decaying organic matter, or lots of fish in a small pond, oxygen levels can go down a lot as bacteria and animals use up oxygen. And I expect you’ll be familiar with the idea of summer oxygen swings during algal blooms – the algae produce oxygen like mad in the sun, then use it up during the night.

So to answer your questions specifically:

The first thing to say about shallow ponds is they don’t automatically have more oxygen than deeper ponds. A shallow pond full of leaves will have low oxygen levels because of all the rotting organic matter. But they do have a better chance of generating a lot of oxygen internally if they’ve got plenty of plants (or for that matter algae) because they’re completely in the light.

Plants are a crucial part of the story here because they can completely alter the amount of oxygen in the pond – at least where there aren’t large quantities of organic matter.

Next, you’re right of course that shallow water is warmer and warms up more – but this is natural, not bad.

In fact its one of the plus points of very shallow water – its wonderfully warm. And being well-lit it can also be well-oxygenated.

Also many pond animals – beetles, backswimmers and boatmen, many snails, rat-tailed maggots etc etc – come to the surface to breathe air and others have adaptations to lower oxygen levels. The ‘warm water bad/cold water good’ idea is really something from fish keeping – where warm water does make it more difficult to keep fish, especially ornamental fish, because they will usually be stocked at a very high density and so be under considerable oxygen stress as water warms up.

And finally of course you’re right about toads generally being associated with bigger (and so usually deeper) ponds. But quite why they are is, I think, still a bit of a mystery. The tadpoles do tolerate fish predation so that would make them favour these situations (big ponds are more likely to have fish) but they do also breed in small garden ponds – much less so than frogs of course, but they do, so the pure size of the pond is obviously not a limiting factor.

In our detailed surveys in Abingdon of garden ponds, off the top of my head, 2 out of 30 had breeding toads compared to about 20 out of 30 with breeding frogs. And thinking about it, they’re abundant in one of the most wonderful ponds we know in the New Forest – a famous spot for all sort of extremely endangered freshwater plants and animals – and here you see toad tadpoles swimming about in a few inches of water, and the whole pond barely comes up to your knees.

I think I’ve heard that toad tadpoles also have higher oxygen requirements than common frogs which, given that larger ponds and lakes would often have higher and more stable oxygen levels than smaller ponds, might also be part of the explanation.

More generally, I’m not sure the oxygenation is all that critical for amphibians – of course the tadpoles/newtpoles need oxygen but the ones that live in ponds are probably adapted to the natural levels, and fluctuations, of oxygen that you see in good quality well-vegetated ponds. This is different to rivers and streams where the ‘typical’ animals are those that need oxygen to be constantly high, the natural condition for rivers because of the water movement.

Finally, as you’ll probably have seen for yourself – frog tadpoles love the almost hot water you get at the edge of really shallow ponds and swarm there.

So, to try to wrap all that up – our very shallow ponds at home produce a lot of oxygen, including in cold weather, because they have abundant plant growth (including mosses which are winter green); the small volume means the oxygen can re-charge quickly, and in warm weather there’s so much photosynthesis going on that there’s plenty of oxygen production.

Although we have plenty of leaves in my ‘Old Pond’ we like leaves, and in a shallow pond they don’t seem to use up much oxygen. In deeper (down to 50-60 cm ponds) with lots of leaves there is so much organic material that you’re creating a big oxygen stress – and the lowest oxygen levels in the Abingdon ponds are all very leafy ponds.

Finally our measurements in Abingdon garden ponds showed that there was never more than a degree or so difference in the temperature between the top and bottom of a garden pond – they’re too shallow for the depth to make any difference. The idea that deep water has more oxygen comes from studies of lakes where this, of course, is true and there’s a big enough mass of water for there really to be separate areas of water in the same waterbody – this doesn’t really happen in small ponds.

Last – the 64,000 dollar question – should we have graded depths? I’m not sure that makes all that much difference in small ponds. I think our ponds suggest that shallow works fine; going down to half a metre, or a metre, in a garden-pond-sized pond is going to give steep sides.

In practice, if you can fill a deep steep-sided pond with scrupulously unpolluted water, it’ll be fine – it’ll be great as a habitat but it won’t be quite as good as the pond that also has very gently sloping sides and plenty of shallows. To achieve that in the relatively small space of a garden, usually mean having shallower ponds – although as we’ve still so much to learn about designing small ponds for wildlife people will probably be able to thank up other solutions to the problem.

I think one of the best things about all this is just how much there still is to learn about something apparently so familiar as the garden pond.

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