Gulf of Bothnia

The Gulf of Bothnia is the northernmost arm of the Baltic Sea.

It stretches between the west coast of Finland and the east coast of Sweden. This large body of water is divided into three distinct sections. The northern part is known as the Bothnian Bay. The southern part is called the Bothnian Sea. A narrow region called the Kvarken separates these two larger areas. In the south, the Åland islands sit between the Sea of Åland and the Archipelago Sea.

The physical structure of the gulf is shaped by complex geological processes. Geologically, the gulf began as an ancient depression of tectonic origin. This depression is partially filled with sedimentary rock from the Precambrian and Paleozoic eras. During the last 2.5 million years, glaciers repeatedly covered this area. While glacial erosion occurred, it had a limited effect on the overall topography. The area near the gulf is also part of the Sub-Cambrian peneplain.

A remarkable process called post-glacial rebound is currently changing the gulf. During the last ice age, massive continental ice sheets pressed the land down. Now, the land is rising as it recovers. This rise occurs at a rate of 80 centimeters every hundred years. As the land rises, the surface area and depth of the gulf decrease. Scientists estimate the land has another 100 meters to rise before it reaches equilibrium. In the Kvarken area, the land is rising by almost one centimeter every year. Within 2,000 years, the rising seafloor may split the gulf into a southern sea and a northern freshwater lake.

The water chemistry in the gulf changes significantly from south to north. This is due to a salinity gradient caused by many rivers flowing into the sea. In the south, the water is the standard brackish water found in the Baltic Sea. However, the salinity drops as you move toward the Bothnian Bay. Near the Kvarken, salinity is only 0.4%. In the northernmost part, it drops to 0.2%. This low salinity allows freshwater fish like pike, whitefish, and perch to thrive.

History shows that the gulf was once missing from many early maps. In 1427, the Danish cartographer Claudius Clavus did not indicate it. Even the Hartmann Schedel map from 1493 did not include it. The first map to show the gulf was made by Nicolaus Germanus in 1482. Although it showed the shape, it did not yet have a name. The name Bothnia is a Latinization of a Swedish term. The original Swedish name meant "gulf" or "bay."

The environment around the gulf is heavily forested, which drives much of the local economy. Trees are logged and transported to the coast for milling. The gulf also serves as a vital route for transporting oil and iron ore. For example, the city of Raahe uses it for ore transport to steel mills. In Sweden, the port of Luleå exports iron ore pellets and imports coal. Many other large ports serve the region, such as Kokkola and Tornio in Finland, and Umeå and Sundsvall in Sweden.

Because of its northern location, the gulf faces unique seasonal challenges. The water is nearly fresh, so it freezes over for five months every year.

The icing of the Baltic Sea typically begins and ends in the northern Gulf of Bothnia. Ships often require icebreaker assistance to navigate. In this region, the ice season can last as long as six months. This is much longer than the three-month ice season seen in the Gulf of Finland. While there is some fishing for Baltic herring, the area also faces pollution issues. Because the sea is enclosed, mercury and PCB levels have been relatively high. However, health authorities note that the fatty acids in the herring can offset some of these risks.