Why the water under the ice splits into layers
When a pond freezes over, the ice caps the surface and cuts the water off from the air. Oxygen already dissolved in the water gets used up by decaying plants and animals, and it cannot be replenished from the atmosphere, so a low-oxygen zone develops. At the same time, temperature forms its own gradient: the water is coldest near the ice and warmer with depth. These developing oxygen and temperature gradients are the backdrop for everything frogs do in winter [1].
The result is a layered pond rather than a uniform one. A frog sitting in one spot faces a trade-off, because the water that is best for one need is not necessarily best for the other. The studies describe frogs moving within stratified layers, which implies the layers are distinct enough to be chosen between [2].
The frog's winter trade-off: cold saves energy, oxygen keeps it alive
Cold water slows a frog's metabolism, which stretches its stored energy through a long winter. But cold water can also be low in oxygen, and a frog submerged under ice cannot surface to breathe air. So the animal has to balance two pressures at once: stay cold enough to conserve fuel, and stay in water oxygen-rich enough to survive. The research frames this directly as balancing hypoxia (too little oxygen) and hypothermia (too cold) [2].
Frogs appear to handle this by behavior rather than by staying put. In cold-submerged conditions they respond to oxygen and temperature gradients by shifting position, effectively choosing among the stratified layers [1][2]. The exact cue they use to sense low oxygen at the skin is not settled; one study notes it is not known how the amphibians detect the lack of oxygen at the skin, though the behavioral response itself is observed [1].
Coping with a thin oxygen boundary, and when winterkill happens
Ranid frogs (the common pond frogs) overwintering underwater face low temperature often accompanied by low dissolved oxygen, and they have had to develop ways of coping with the thin low-oxygen boundary layers that form around them [3]. This is a physiological as well as behavioral challenge: the animal must keep extracting what oxygen is available while its metabolism is slowed by cold.
Mortality is the outcome when the balance cannot be held. The physiological ecology work on ranid frogs addresses overwintering behavior and mortality under these cold, low-oxygen conditions [3]. In practical terms, a pond that loses oxygen faster than frogs can find a tolerable layer, or one where the usable cold, oxygenated band shrinks, is where die-offs become likely. The studies support this link between the layered environment and survival, but they do not pin down a single mortality rate that applies across ponds.
What the evidence does not yet settle
Two things stay open. First, the sensing question: how frogs detect low oxygen at the skin is described as unknown, so the trigger behind their movements is inferred from behavior rather than explained [1]. Second, the numbers: the work connects cold, low-oxygen overwintering to mortality [3], but the evidence here does not give a clean figure for how many amphibians die in a given winter. What the research does establish is the mechanism of the problem, a layered pond that forces a trade-off, and the behavioral strategy frogs use to manage it.
Sources used in this answer
Behavioural oxy-regulation by cold-submerged frogs in heterogeneous oxygen environments
Cold-submerged frogs respond behaviorally to developing oxygen and temperature gradients in ice-covered overwintering environments, though how they detect low oxygen at the skin is not known.
Balancing hypoxia and hypothermia in cold-submerged frogs
Frogs overwintering under ice balance hypoxia against hypothermia, moving within stratified temperature and oxygen layers to manage metabolic demands.
Physiological ecology of aquatic overwintering in ranid frogs
Ranid frogs overwintering underwater cope with low temperature and low dissolved oxygen, including thin hypoxic boundary layers, and this physiology is tied to overwintering behavior and mortality.