An oxygen stressed pond

Ready for a makeover: tap water, hard edges and a poor depth profile. We can change the first two, but maybe the deep water will be OK staying deep if we hoik out the surplus leaves - it's impossible to appreciate just what a lot of leaves there were in the pond from this picture.

Update: seeing a reference to an earlier post (here – Pink Ponds), this pond also had some purple bacteria – a sure (though natural) sign of oxygen stress and high amounts of organic matter.

In the Abingdon garden pond survey last year it quickly became clear that ponds with a lot of leaves in (I mean, full to the top) usually had very low oxygen levels.

And because of this they had pretty limited animal communities with only those creatures that specialise on low oxygen environments: Common Water Slaters (Asellus aquaticus), American Freshwater Shrimps (Crangonyx pseudogracilis), the odd leech and sometimes flatworms.

And yesterday we saw another such pond at a well-known agricultural college where we’re hoping to do a pond makeover – taking a typical garden pond and turning it into something better for wildlife.

The pond was thick with fallen leaves and had only 1.6 milligrammes of oxygen in each litre of water near the top with virtually none from 20 cm on down – so very low.

It was underneath a small tree and was only a little over two years old – a surprisingly rapid infilling by leaves.

But does this mean you should rigorously exclude  leaves?  Well the jury is still out on that because part of the problem is, I think, with the shape of the pond.

This one is ‘typical’ garden pond shaped: it has steep edges and shelves down to about 50 cm in the middle, creating a sump where dead leaves accumulate and show little sign of breaking down. In a pond with little oxygen (it’s also covered by duckweed, which probably further reduces oxygen production) there’s little chance leaf litter will break down. So leaves acuumulate putting more and more stress on oxygen levels.

Although I’ve no hard evidence yet, I can’t help thinking that in a shallower, better illuminated and, therefore better oxygenated, pond leaves would break down more quickly. And if parts of the pond dried out from time to time, that would certainly speed up the breakdown.

Despite all this leaf litter and the clearly challenging environment we still found a few Smooth Newt larvae – not a lot: no more than half a dozen all told – but they were still there even in this far from ideal environment.

It will be interesting to work out what the best thing is to do to improve the pond.


One Response to “An oxygen stressed pond”

  1. Tristan Hatton-Ellis Says:

    Interesting Jeremy. When I first moved into this house the garden already had a pond. It was a ‘typical’ 1980s sort of pond – preformed plastic liner and choked with vegetation. Next to it was a large ash tree which deposited substantial quantities of leaves every autumn. Only the top 5cm or so was open water – the rest was thick anoxic sludge with purple sulphur bacteria. It also had Australian swamp stonecrop, which is why I decided to fill it in. During the filling in process I rescued 50-odd adult and juvenile palmate newts! The pond was well lit and for the most part shallow, but the volume of leaves was simply too great for the pond to process.

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