The Gulf of Bothnia (; Finnish: Pohjanlahti; Swedish: Bottniska viken, also divided as Bothnian Bay + Bothnian Sea) is the northernmost arm of the Baltic Sea. It is situated between Finland's west coast (Ostrobothnia) and Sweden's east coast (Westrobothnia and North Bothnia). In the south of the gulf lie the Åland Islands, between the Sea of Åland and the Archipelago Sea.
The name botn was applied to the Gulf of Bothnia as Helsingjabotn in Old Norse, after Hälsingland, which at the time referred to the coastland west of the gulf. Later, botten was applied to the regions Västerbotten on the western side and Österbotten the eastern side ('East Bottom' and 'West Bottom'). The Finnish name of Österbotten, Pohjanmaa (maa, meaning 'land'), gives a hint as to the meaning in both languages: the meaning of pohja includes both 'bottom' and 'north'. Pohja is the base word for north, pohjoinen, with an adjectival suffix added.
Botn/botten is cognate with the English word bottom, and it might be part of a general north European distinction of lowlands, as opposed to highlands, such as the Netherlandic region, Samogitia (Lithuanian), and Sambia (Russia).[clarification needed]
Julius Pokorny gives the extended Proto-Indo-European root as *bhudh-m(e)n with a *bhudh-no- variant, from which the Latin fundus, as in fundament, is derived. The original meaning of English north, from Proto-Indo-European *ner- 'under', indicates an original sense of 'lowlands' for bottomlands. On the other hand, by north the classical authors usually meant 'outermost', as the northern lands were outermost to them.
In Saami, the cardinal directions were named according to the different parts of the typical tent used by this nomadic people. The door of the tent was traditionally pointed South, in the most sunny direction, and the bottom of the tent would be aligned with the North. Thus the origin of the word pohja in its use as 'north'. According to Lönnrot, north was viewed as the bottom direction because the lowest point of the sun's path is there.
From Simpnäsklubb (59°54'N) in Sweden, to Flötjan, Lagskær [sic], Fæstörne [sic], Kökarsörn, and Vænö-Kalkskær [sic] to the SW point of Hangöudde (Hangö Head, 59°49'N) in Finland, thus including the Aland islands and adjacent shoals and channels in the Gulf of Bothnia.
The gulf is 725 km (450 mi) long, 80-240 km (50-150 mi) wide and has an average depth of 60 m (200 ft, 33 fathoms). The maximum depth is 295 m (965 ft, 161 fathoms). The surface area is 117,000 km² (45,200 sq mi). The northernmost point is situated in Töre in the Bothnian Bay. its coordinates are 65° 54'07" N 22° 39'00 E.
The depth and surface area of the Gulf of Bothnia are constantly decreasing, as the land is rising after it had been pressed down by about 2,600 to 3,300 feet (800 to 1,000 meters) by the continental ice during last ice age. The rise is 80 cm every hundred years. It is estimated that the land has a further 300 to 400 feet (100 to 125 meters) to rise before equilibrium is reached. This recovery rate will progressively slow as isostatic equilibrium is approached.
Into the gulf flow a number of rivers from both sides; consequently, a salinity gradient exists from north to south. In the south the water is the normal brackish water of the Baltic Sea, but in the north, in the Bothnian Bay, the salinity is so low, from 0.4% near Kvarken to 0.2% in the northernmost part, that many freshwater fish such as the pike, whitefish and perch thrive in it.
Being nearly fresh, the gulf is frozen over five months every year. The icing of the Baltic Sea begins and ends in the northern Gulf of Bothnia. Traffic restrictions for icebreaker assistance are typically in force for all the gulf from late January to late April and for the northernmost ports from the middle of December to the middle of May.
Geologically the Gulf of Bothnia is an ancient depression of tectonic origin. The depression is partly filled with sedimentary rock deposited in the Precambrian and Paleozoic. Nearby plains adjoining the gulf are part of the Sub-Cambrian peneplain. While being repeatedly covered by glaciers during the last 2.5 million years glacial erosion has had a limited effect in changing the topography.
Some historians[who?] suggest that the adventurer Ottar was referring to the Gulf of Bothnia when he spoke of the Kven Sea in the 9th century. It is also possible that Claudius Clavus's usage of the term Mare Gotticus in the 15th century refers to the Gulf of Bothnia.
The land surrounding the Gulf of Bothnia is heavily forested. Trees are logged, then transported to the coast for milling. The gulf is also important for oil transport to the coastal cities and ore transport to steel mills, for instance in Raahe.
In terms of tonnage in international traffic, the largest ports on the Finnish side are Rauma, Kokkola and Tornio. The main ports of the Swedish side are in Luleå, Skellefteå, Umeå, Sundsvall, Gävle and Hargshamn. In Luleå, iron ore pellets are exported and coal is imported. Gävle is Sweden's third-largest container port. It also ships forest products and oil. In port operations in the Gulf of Bothnia, icebreaker assistance can be required for an ice season that averages as long as six months; whereas in the Gulf of Finland, the icebreaking season averages only three months.
There is some fishery, mainly Baltic herring, for domestic needs. A persistent problem has been pollution, because the sea is enclosed by a large drainage basin and is poorly connected to fresher waters from the Atlantic. Mercury and PCB levels have been relatively high, although the Finnish Food Safety Authority considers the herring edible. Although the levels exceed the limits, the fatty acids have health benefits that offset this risk.