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.