Beneath the Surface: How do fish survive under ice?

It may surprise you to know the answer lies in aquatic plants; the same ones that may be a nuisance to some of our lake activities.

Did you know that only the surface layer of Lake Windermere freezes in the winter?

Imagine a glass of water with ice cubes in it. Does the ice sink to the bottom or float to the top? Ice floats because water in its solid state is actually lighter and less dense than water in its liquid form. Picture an ice molecule as a singular crystal: when they are packed together, larger spaces will remain between molecules, making it lighter.

The deeper you go into Lake Windermere, the warmer the water will be, though you’ll still want to avoid a plunge. The water at the bottom of our lake is likely around 4 C — this is the temperature at which water is heaviest, or most dense.

Fish have a special adaptation that allows them to survive cold waters. They are cold-blooded, meaning their body temperatures fluctuate with the temperature of the water. In fact, fish and other aquatic animals can live quite comfortably under the ice as long as oxygen is available. But as organisms take in and remove oxygen from the water, it must be replenished and recharged.

Can you guess how oxygen gets in the water? Remember that the lake surface is sealed from the atmosphere with a layer of ice.

It may surprise you to know the answer lies in aquatic plants; the same ones that may be a nuisance to some of our lake activities.

Aquatic vegetation can actually prevent fish kills in lakes that would otherwise lack oxygen in the winter months.

As long as the snowfall is not too heavy, light can penetrate the ice surface, allowing the plants and algae to carry out photosynthesis and create oxygen.

In the middle of winter, underwater plants will photosynthesize at only 10 to 20 per cent of their normal summer rate. Fortunately for the fish, it’s enough to keep the water oxygenated and habitable. (Sources: C.J. Andrews. 1996. ‘How Do Plants Survive Ice’ Annals of Botany, 78: 529-536; Aquarius Systems February 2013 e-Newsletter.)

Beneath The Surface is based on the principle that there is often more to know than what is visible from the “surface” of an issue. If there is something that concerns you about the lake and you want to get to the “bottom of it,” call Lake Windermere Ambassadors program co-ordinator Megan Peloso at 250-341-6898 or email and inspire the next column!

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