Baltic Sea

Sea ice

As a long-term average the Baltic Sea is ice covered for about 45% of its surface area at maximum annually. The ice-covered area during such a typical winter includes the Gulf of Bothnia, the Gulf of Finland, Gulf of Riga and Väinameri in the Estonian archipelago. The remainder of the Baltic itself does not freeze during a normal winter, with the exception of sheltered bays and shallow lagoons such as the Curonian Lagoon. The ice reaches its maximum extent in February or March; typical ice thickness in the northernmost areas in the Bothnian Bay, the northern basin of the Gulf of Bothnia, is about 70 cm for landfast sea ice. The thickness decreases further south.

Freezing begins in the northern coast of Gulf of Bothnia typically in middle of November, reaching the open waters of Bothnian Bay in early January. The Bothnian Sea, the basin south of it, freezes on average in late February. The Gulf of Finland and the Gulf of Riga freeze typically in late January.

The ice extent depends on whether the winter is mild, moderate or severe. Severe winters can lead to ice formation around Denmark and southern Sweden. According to William Derham during severe winters in 1703 and 1708 only Ostsee Danish straits were frozen; only restricted parts of the Bay of Bothnia and Gulf of Finland are ice covered, in addition to coastal fringes in more southerly locations such as the Gulf of Riga. In recent years a typical winter produces only ice in the northern and eastern extremities of the Sea. In 2007 there was almost no ice formation except for a short period in March. 

In spring, the Gulf of Finland and of Bothnia normally thaw during late April, with some ice ridges persisting until May in the eastern Gulf of Finland. In the northernmost reaches of the Bothnian Bay ice usually stays until late May; by early June it is practically always gone.

During winter, fast ice, which is attached to the shoreline, develops first, rendering the ports unusable without the services of icebreakers. Level ice, ice sludge, pancake ice or rafter ice form in the more open regions. The gleaming expanse of ice is similar to the Arctic, with wind-driven pack ice and ridges up to 15 m, and was noted by the ancients. Offshore of the landfast ice the ice remains very dynamic all year, because of its thickness it is relatively easily moved around by winds and therefore makes up large ridges and piles up against the landfast ice and shores.

The ice cover is the main habitat only for a few larger species. The largest of them are the seals that both feed and breed on the ice, although the sea ice also harbors several species of algae that live in the bottom and inside brine pockets in the ice.