Unique habitats: Sea ice

Around november the northern Bothnian Bay starts to transform. From a warm, blue sea with a temperature of up to 20 degrees, to a solid, cold, white plain of ice.

Midwinter day on the Bothnian Bay sea ice (Photo by Linnea Bergdahl, County Administrative Board of Norrbotten)

The ice starts to form when the temperature of the water reaches below zero degrees. The more salt in the water, the lower the temperature needs to be. On lakes a clear, hard ice can be formed whereas ice in the salty oceans is more porous and soft. As the water of the Bothnian Bay is almost freshwater its sea ice is similar to the ice on a lake.

The new ice has a shiny, dark black colour. It first appears in sheltered, shallow bays and inlets along the coasts. The ice on the picture froze on an inlet in the town of Luleå, Sweden (Photo by Linnea Bergdahl, County Administrative Board of Norrbotten)

As the temperature continues to stay below zero, ice forms further and further off from the coasts. For the shipping traffic out at sea it is crucial to know where the sea ice currently is. The Swedish Meteorological and Hydrological Institute (SMHI) monitors the sea ice and publish daily maps of its extent. On today’s sea ice map you can see that the ice extends out to the archipelago islands on either side of the northern Bothnian Bay, but most of the bay still remains open. The maximum extent of the ice varies every year due to current weather situation. Last year the whole Bothnina Bay was frozen and in march the sea ice could be found all the way past the Quark area.

The se ice may seem lifeless and dark but truth is, it’s a basis for much of the life here. The scraping of the ice on the shores provides growth opportunities for plant species which wouldn’t be able to live there otherwise. For the ringed seal the ice provide both a home and safe place for new born pups. In february the female seal builds a “cave” on top of the ice in which she gives birth to one pup. The pup then grows up on the ice. Between april and may the adults change their fur and they are also then dependent upon the ice to rest on to stay warm.

Climate change pose a real threat to sea ice, being such a highly temperature-dependent habitat. In a recently publish assessment of threatened habitats in Finland the sea ice in the Baltic Sea was listed as one of the threatened habitats. The diminishing extent, duration and quality of the sea ice will in turn make it harder for the ringed seal, and change life conditions of the Bothnian Bay.

Send a thankful thought to the sea ice this winter, and remember to appreciate and enjoy it for what it brings!


Sunny day on the ice in late march (Photo by Linnea Bergdahl, County Administrative Board of Norrbotten)

Written by Linnea Bergdahl, County Administrative Board of Norrbotten

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