Duckweed and oxygen

Oxygen levels in my three ponds over the last few days. Values are milligrammes of oxygen in each litre of water. Each vertical bar covers a 24 hour period. The low values in the Old Pond are pretty low - at this time of the year 12-15 milligrammes per litre would be about fully saturated. Click the graph for a better quality image.

Everyone says that duckweeds reduce the amount of oxygen in the water.

So I thought it would be interesting to see how the dense cover of duckweeds was affecting the Old Pond.

For the last few days I’ve measured dissolved oxygen levels in my three ponds: the Old Pond, which is covered in duckweed, the New Pond, which is in full sun, and Katy’s Pond, which is shaded and rarely gets the sun.

The results are interesting.

All the ponds go up and down a lot – naturally oxygen levels are highest in the evening and lowest in the morning.

The New Pond is always  highest and the Old Pond, covered in duckweed, is lowest. Katy’s pond – with mosses and in the shade – is in the middle.

What’s also interesting is that the New Pond currently has no submerged plants. It’s oxygen is produced – and used – entirely by microscopic algae.

So oxygen is behaving as we would expect. But what effect is this having on the animals in the ponds? This is harder to say as we can’t really compare the ponds because they have completely different animal communities.

But what about in the Old Pond where’s there’s least oxygen? Here the range of animals is pretty much the same as we’ve seen throughout the year – including at those times when there was much more oxygen in the water. It’s harder to say whether numbers are greater or fewer – I haven’t counted things carefully enough to say anything about this.

So far the only really big differences from earlier is the complete lack of backswimmers and more mosquito larvae than before (see the post on scaring mosquitos off).

At the moment the low level of oxygen in the Old Pond doesn’t seem to be affecting the animal community too much – it will be interesting to see how things develop over the autumn and winter.

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4 Responses to “Duckweed and oxygen”

  1. John Says:

    Out of interest do you know if Duckweed, or other nutrient greedy plants, can be used as a way to take nutrients out of ponds by letting it grow then removing the bulk of it over and over? Having two dogs and a young niece and nephew who all use the garden brings to mind many nutrient dumping possibilities so I would love to have a backup for retrieving a pristine pond. ;)

  2. jonspond Says:

    I used to use fast growing algae to filter my marine fish tank – so theoretically it could work with duckweed – I think that a recent article on this blog highlighted the limiting factors which would drive a bloom – if you have all the nutrients in the right amounts then the invasive weeds will grow harvesting this will remove nutrients but this would be a lot of hard work

    It really depends on how clean the pond is at the start – I am sure that using rainwater rather than tap water would also dilute any nutrients from a ‘polluted’ pond.

  3. Barbara Johnson Says:

    My pond is covered by duckweed so I am wondering if this would increase frog winterkill? This autumn I am removing a lot of the duckweed as I thought this might help reduce frog mortality this coming winter?

  4. Jeremy Biggs Says:

    Hi Barbera

    If your duckweed is reducing oxygen levels it might well have that effect – I don’t suppose anyone knows for sure.

    Removing the duckweed to improve the situation makes logical sense but there’s quite a few what if’s – and I suppose the bottom line is you’d need to get a bit of a chain of evidence to really convince yourself it did any good (Good Lord – he’ll be wanting me to do science next!).

    So is the pond already low in dissolved oxygen, does removing duckweed then lead to an increase in oxygen (there might be other factors which stop that happening) and then – do you have a pond which stays oxygenated under ice – enough light, enough plants or algae?

    Quite a few ducks (err, frogs?) to line up! But it’s certainly plausible – though at this stage I wouldn’t want to be held to it!

    Jeremy

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