When lakes freeze up

A phenomenon that allows for animals and plants to survive

The lakes in the Dolomites are irresistible even in winter, when their surfaces freeze and they are transformed into sheets of ice. And, if the layer of ice is thick enough to withstand our weight, then we can even walk on them (but be careful not to slip and, above all, before doing so, make sure that the ice is thick enough!) or speed across them on ice skates.

As cold as it may be, however, no lake freezes up completely. In fact, the sheets of ice that form on the surface shelter the water below from the cold, therefore it remains in its liquid form, allowing plants and aquatic animals to survive the winter.

This miracle of nature is due to the peculiar behaviour of water molecules that form the upper layer of the lake. When winter arrives, the water on the surface, which is most exposed to low external temperatures, cools down first. Usually, when in the water is colder, it also becomes denser and it consequently tends to slide towards the bottom. This in fact is what happens until temperatures do not fall below 4°C: as the water on the surface cools, it slides to the bottom and pushes up the warmer waters below, which in turn inevitably cool down and sink down, in a cycle that continues until all the lake reaches a uniform temperature.

How do animals survive in frozen lakes?

"The lakes in the Dolomites are irresistible even in winter, when their surfaces freeze and they are transformed into sheets of ice"

When the external temperature drops below 4°C though, the water begins to behave in an unusual way. Its molecules tend to move apart from each other, so the density of the water decreases rather than increases. At this point, the most superficial layer, although it is colder, no longer sinks, but stays afloat. And when the temperature reaches 0°C, it freezes.

Finally, since the ice sheets act as a thermal insulator, the lake water below them can be maintained in its liquid state and at a temperature that is mild enough to allow the lake animals to survive until the arrival of the spring thaw.

In the Trentino Dolomites, the magnificent lago di Tovel (Lake Tovel), located in the Parco Naturale Adamello Brenta (Adamello Brenta Nature Park), at 1178 m asl, can freeze completely during the coldest months, becoming a magical place where you can discover the secrets of Alpine lakes in the heart of winter.

Published on 14/11/2019