Waterbug,
I am curious, too. But, I think that the bottom of the pond would be warmer as the ground temperature should be higher than the air temp. So, depending upon how much water circulation there is, I would expect the bottom to be warmer always. That being due to the insulation factor and the heat from the earth. But, there is one other thing to consider. Which is that when water nears the freezing point, it has a special quality that no other element does. It expands, rather than contracting, that makes it less dense. Which is why ice forms on the surface of a lake or pond instead of the bottom. So at or near the freezing point, the coldest water must be nearer the surface.
I wouldn't think that it would be noticeable at outdoor air temperatures well above freezing (like at 37°F), but maybe it starts setting itself up this way because the bottom of the pond gets the heat from the earth and the surface is exposed to the outdoor air and wind. During the daylight hours, obviously, the sunlight that hits the pond will pass through the water to the bottom and be absorbed by the darker color liner of the pond, thus heating the water at the bottom moreso than at the surface.
If everyone's temperature readings are to be taken as accurate, then there must be an explanation. The facts that I cited above are as close as I can come to explaining this. I cannot say that I am right, but there is a lot of evidence that leans this way. The only water that disagrees with me here is the ocean saltwater. Since saltwater doesn't freeze at the normal temperature, the colder water can and will sink to a lower level, at least up to a point. Without the saline factor of our oceans, the Haline conveyor would not operate and we would re-enter another ice age. That is a theory that some oceanographers and earth science folks hold with. I happen to agree with them.
I don't know if you agree with me, Waterbug, but this is the best reasoning that I can come up with off the cuff. I don't know how else to explain it. I don't think that it is critically important to a pond owner, but I think that everything is worthy of investigation. For someone in a very cold clime, it might be important. What are your own opinions?
Gordy