The snow story: a surprising development

The day the snow fell: 6 January 2010

A thick blanket of snow covered our ponds nine days ago.

I assumed that, by cutting out the light, the snow would effectively stop photosythesis in the pond, causing oxygen levels in the ponds to go down.

But it didn’t.

As you can see on the graph below, for the first three days after the snow fell, oxygen levels in both ponds were little changed. The old pond dropped a bit from the 6th to the 7th January, but the new pond oxygen went up a bit.

Pond oxygen levels: 19 December 2009 to 14 January 2010

So I began to wonder: could the small area I’d cleared of snow around the ice holes where I was making my oxygen measurements, could these small holes be lettting enough light in to allow photosynthesis and oxygen production to continue?

The answer was a pretty resounding yes.

I covered the snow holes with a couple of black door mats to cut out the light, the old pond on the 9th January and the new pond on the 10th. To make oxygen measurements I could simply lift the mats off for a few minutes.

The result was dramatic: on both ponds the oxygen dropped, most markedly in the old pond which has abundant underwater mosses. The fall in oxygen levels on the new pond was slower, perhaps because there is much less underwater plant growth in that pond. But the effect was clear in both ponds. With all light excluded, oxygen production appeared to have ceased. I should perhaps add that the mats were not creating an airtight seal over the ice holes!

So clearing the snow around the ice holes – which I’d done without thinking about the consequences – was allowing enough light in for plants and algae to go on making oxygen. When I blocked out that light, oxygen levels in both ponds fell to their lowest values so far measured.

This is a pretty conclusive demonstration of the effect of snow cover: not too surprising really. Perhaps more interesting is the fact that even quite small amounts of light entering the ponds seemed to maintain their oxygen levels.

And there is further, perhaps final, twist to this story. I will report that over the next few days.

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One Response to “The snow story: a surprising development”

  1. Helen Edwards Says:

    Been a long time since I studied photosynthesis so I have 3 questions…
    firstly I have plants growing in my pond that have foliage both in the water and above it (e.g. brooklime). Although these are not classed as oxygenating plants the part above the water obviously continues photosynthesising even when the pond is covered with ice and snow. Can this photosynthesis, occurring above the water, produce oxygen under the ice or is it all released into the air above the water?

    Secondly, the decrease in oxygen when the pond is covered by snow or mats is presumably accompanied by an increase in carbon dioxide (as a result of respiration of pond plants and animals). (This part’s more sketchy…) 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??

    Finally, high CO2 levels can kill fish. Can they also harm other pond life?

    Thanks Jeremy!

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