Turtles under the ice and a surprise

I had not seen the turtles since September. I looked at the pond often, but the turtles had disappeared. One curious thing is that last year I used to see them every week in November and even December. Sometimes they sunbathed.  One change that I have noticed since last year is that the bottom of the pond is more colonized by algae and plants, offering plenty of places to hide. The other change is that the male, for some reason, is more persistent in his attempts to mate with the female.

Sunday morning, at 10:00, the grass was frozen. The temperature was -3,2ºC and I felt the cold in my ears.

For the first time in the fall, the pond froze. And I saw the turtles, as it happened other times the pond freezes (see "water depth and August temperatures at different levels  (28-11-11)"; "The turtles and the ice (09-02-12)"   "It happened again (10-02-12)").

The water temperature at 10 cm was 3ºC and at 50 cm 6,6ºC.

Under the log

For some reason, sometimes, when the pond freezes, the turtles become active and I see them near the surface. Curiously I already see them in the early morning, when it is still freezing and there is no direct sunlight on the pond. They remain active until the afternoon, long after the ice has melted.
When they are under the ice they move slowly . They are aware of my presence and if I get too close to them, they hide the head and move away.

The first time that I witnessed it  I thought that the turtle needed to breath air and, impulsively, I broke the ice to examine the animal.  Later I found some papers in internet about metabolic depression at low temperature, non pulmonary respiration and the probable anoxia tolerance of this species  (see: Water quality (05-12-11)). There is a very interesting book about the physiology of turtles where there is plenty of information about it

If the water has normal oxygen levels, this species can remain submerged for months without the need to breath air.

The ice can alter the water oxygen levels as it prevents the passage of this gas from the atmosphere to the water. Could it be that this behavior when there is ice on the surface of the pond is related to the oxygen levels? I wonder how long does is take since the ice forms to have an effect in the water oxygen levels. I supose  it depends on the pond characteristics (water movement, oxygen consumption, oxygen generation...).  Regarding the aquatic plants and algae, I wonder how does the winter affect photosyntesis if it does, as this would alter the oxygen levels during the day and the night. Do the algae generate/consume oxygen in winter? They change to a very dull color and it is not until spring when they become green (and I can see bubbles on their surface).

It could also be that it is not related to the oxygen and they do it for some other reason. In any case I don't read this behavior as a signal that something goes wrong. Even on the assumption that the ice was really decreasing the oxygen levels it wouldn't mean that things were going wrong. According to articles, it is believed that hipoxia or anoxia in the ponds were Clemmys hibernate are not something extraordinary and these animals are adapted to this situation.

It is neither something extraordinary that they are active under the ice. Biologysts have seen wild turtles doing it and other turtle kepers have noticed this behavior in their ponds,  different to mine.

I think the cold tolerance of this species is fascinating and it has disrupted all the schemes that I had about what is hibernation in turtles. And once I have become used to seeing them active in winter and I think they can no longer suprise me, this is what I witness some hours later. It is early December at noon, the air temperature is 6ºC and the water temperature at the turtles level is 3,09ºC. There is still some ice melting.

This is cold tolerance, doing it in 3ºC water!...