When the surface of a lake freezes over, it fundamentally changes the aquatic environment below. This isolates the water from the atmosphere, posing a significant challenge to the fish and other organisms that remain active. Despite the common perception that the entire body of water must freeze solid, fish routinely survive the deepest winter. This feat is made possible by a combination of unique physical properties of water and remarkable biological adaptations. The ability of a fish to endure this extreme seasonal change depends on managing its energy needs and navigating the unique, stratified environment beneath the ice.
How Fish Survive Extreme Cold
Fish are cold-blooded creatures, or ectotherms, meaning their internal body temperature mirrors that of the surrounding water. As the water temperature drops toward the freezing point, their survival relies on a sophisticated physiological slowdown. Fish enter a semi-dormant state known as torpor, which is similar to hibernation but allows for occasional, slow movement. This state is characterized by a dramatic reduction in bodily functions, including heart rate and respiration, which conserves energy.
The fish’s metabolism decreases significantly, sometimes to less than ten percent of its summer rate. This metabolic depression minimizes the need for food and lessens the demand for dissolved oxygen in the water. Most species move to the deepest parts of the lake where temperatures are more stable, forming schools and remaining almost motionless throughout the coldest months. Certain species, like the brown bullhead, can even bury themselves into the soft bottom sediment to wait out the winter.
Conditions Beneath the Frozen Surface
The survival of aquatic life hinges on a unique physical characteristic of water: its density anomaly. Unlike most liquids, freshwater reaches its maximum density not at its freezing point (0°C), but at approximately 4°C. As the surface water cools in the autumn, the denser 4°C water sinks to the bottom, displacing lighter water in a process known as fall turnover. Once the surface temperature drops below 4°C, it becomes less dense and remains at the top until it freezes at 0°C.
This process results in thermal stratification, creating a stable, liquid refuge for fish at the lake bottom. The ice layer, being less dense than liquid water, floats and acts as an insulator, protecting the water below from colder air temperatures. Consequently, the water column remains layered, with the coldest water near the surface and a pocket of 4°C water settled at the deepest point. This stable, temperate zone is where fish congregate to conserve energy during the winter.
The Danger of Oxygen Depletion
Despite the adaptations and the thermal refuge, the sealed environment beneath the ice creates a major life-threatening issue: oxygen depletion, commonly resulting in a phenomenon called “winterkill.” The ice layer forms a barrier that prevents the transfer of atmospheric oxygen into the water. Simultaneously, the aquatic environment continues to consume the dissolved oxygen present before the freeze.
Oxygen is consumed by the respiration of all living organisms, including the fish in torpor. More significantly, bacteria on the lake bottom continuously consume oxygen as they decompose organic matter, such as dead algae and plants. This consumption is accelerated if snow covers the ice, blocking sunlight and stopping the photosynthesis that plants use to produce oxygen.
As winter progresses, dissolved oxygen (DO) levels drop, particularly in shallow lakes with significant organic sediment. If the DO concentration falls below a critical threshold, often 2 to 3 milligrams per liter for game fish, the fish begin to suffocate. In severe cases, anaerobic decomposition produces toxic gases like hydrogen sulfide (H₂S), which is highly poisonous and can accumulate in the deepest, oxygen-depleted layer, causing mortality.