Helen asks…

Helen: Could plants growing at the edge of the pond (e.g. brooklime), with foliage both in the water and above it, be adding oxygen under the ice or is it all released into the air above the water?

Jeremy: I think there would be two ways oxygen produced by the Brooklime might get under the water.

First: many marginal water plants pump oxygen to their roots, and this oxygen can get out into the sediment that the roots are growing in. What I don’t know is whether the oxygen produced like this is enough to get from the mud into the water. I suspect not if the sediment is very muddy and has lots of oxygen-using decay going on. But it’s not a area I have first hand experience of.

I don’t know whether Brooklime specifically takes oxygen to its roots, but I wouldn’t be surprised if it did.

The second way the Brooklime could get oxygen into the water is by ordinary photosynthesis going on in the submerged leaves which I think could probably go on producing oxygen too.

Helen: The decrease in oxygen when the pond is covered by snow or mats is presumably accompanied by an increase in carbon dioxide. Is this also the case when the ponds are covered only with ice because although plants continue producing oxygen, the CO2 they produce at night gets trapped??

Jeremy: I haven’t measured carbon dioxide in my ponds (or any other for that matter!) so don’t have direct experience to go on here.

But like you I would have assumed that carbon dioxide would go up as oxygen went down. However, just looking at some old American information from shallow lakes with winter fish kills due to icing over, it seems that carbon dioxide was pretty much unchanged as oxygen went down. So maybe it won’t be as straighforward as we might intially assume.

Helen: High CO2 levels can kill fish. Can they also harm other pond life?

I don’t know whether increasing carbon dioxide concentration would affect the other animals. I suspect they’d succumb to lack of oxygen first – though again I’ve no first hand experience of this and I haven’t found any information about it specifically.

I did see one study that showed Daphnia, at the end of the winter, to be less abundant in lakes with fish winterkill, but then the Daphnia went on to be more abundant later, presumably because there were fewer fish to eat them. Whether the Daphnia were knocked back overwinter by CO2 or lack of oxygen I don’t know.


One Response to “Helen asks…”

  1. Helen Edwards Says:

    Thanks Jeremy. Would definitely be interesting to monitor CO2 levels in the future.

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