This has nothing to do with human biology – if your professor thinks it relates to high elevations, they’re not thinking.

First of all, water holds much less oxygen then the atmosphere, and it holds more when cold. Oxygen also diffuses slowly through bodies of water, so there’s no clear depth distribution (it’s higher right at the surface and around plants, but otherwise distribution can be uneven, depending on currents, weather, all sorts of factors.

In the autumn of temperate zones, ponds "turn over": as oxygen-rich surface water cools, it becomes more dense and sinks; deeper water replaces it, picks up oxygen from the air, cools and sinks; when the whole pond is at 4 degrees C, the turnover stops (below that temp, water becomes less dense as it moves toward freezing). This means that at winter, before ice formation, oxygen levels are pretty much the same everywhere in the pond.

I very much doubt that’s the answer your professor is expecting, but it’s the answer.

Well I can’t give a very good answer, but I used to have a fish pond (relatively deep) and the goldfish almost always stayed at the bottom – unless they came up to eat… maybe that is of some help?

My guess:
If the water at the top is the hottest (simple chemistry), then the top of the water should hold the least amount of oxygen. (typical solubility laws for most gases)

But the top is also in contact with oxygen rich air.