The northern Baltic Sea is a realm of ice in winter. The sea’s organisms and coastal inhabitants have adapted to the ice. Ice winters naturally vary, but due to climate change, the ice is diminishing. What is the nature of the Baltic Sea ice, and what does it mean for marine life?
Seasonal sea ice is found in only a few places outside the polar seas. Such areas include the White Sea, the Caspian Sea, and Hudson Bay in Canada. However, the Baltic Sea is the only brackish water basin where a significant portion of the sea surface freezes every winter.
In autumn, as the air cools, the water chills and eventually freezes. Ice that forms directly from water is called columnar ice and it is very strong and clear. Another type of ice is slush ice, which forms when slush on top of the ice freezes. Slush ice is pale and formed from small crystals, containing many air bubbles. It is much weaker than columnar ice.
If you saw a piece of ice, you can see the layers of columnar ice and slush ice. From these layers, you can deduce the conditions under which the ice formed.
On the coasts and in the archipelago, there is mostly fast ice, which is attached to islands, skerries, and shallow areas. Fast ice forms in the early winter and remains in place until the ice melts.
In the open sea, sea ice is practically drift ice. Drift ice moves with the winds and currents, breaking into ice floes of various sizes. These floes can pile up into large ridges, known as pack ice.
While the Baltic Sea has sea ice every winter, its extent and duration varies. On average, ice covers the Finnish coast for 5-7 months of the year. In milder winters, ice is mainly found in the Bothnian Bay, whereas in harsher winters, almost the entire Baltic Sea is frozen.
As climate change progresses, mild winters are becoming more common. The ice cover of the Baltic Sea is currently decreasing by an average of 3,400 square kilometres per decade. This is equivalent to one-tenth of Finland’s land area. The impact of climate change is particularly evident in the reduction of the Baltic Sea ice cover since the early 1990s.
The reduction of ice cover puts some species in distress and can affect the entire marine food web. A mild ice season can also be challenging for shipping. In mild winters, ice drifts and packs easily. The sea becomes difficult to navigate, and broken channels quickly close up.
The species of the northern Baltic Sea have adapted to the winter ice cover. Many species are actually dependent on ice. Ice dampens the waves, preventing them from tossing around, for example, fish larvae. Ice is crucial for the reproduction of whitefish. The Baltic ringed seal also needs ice, as it gives birth to its pups in snow caves on the ice.
A unique aspect is the microscopic life within the ice. Sea ice contains small cavities, less than a millimetre in size, filled with salty water. These salt pockets form because ice crystals are made of pure water, meaning they are salt-free. The leftover salt gets trapped inside the ice.
The cavities and channels filled with saltwater create a stable habitat for certain planktonic algae and small organisms. These organisms survive better within the ice than in open water and are ready to reproduce quickly in the spring. The first algae bloom actually occurs within and on the underside of the ice. Sometimes, the ice may even appear to bloom! Through this process, ice plays a significant role in the marine food web.
