272 BIOLOGY OF THE SEAS OF THE tf.S.S.R. 



F. Aland Sea (transitional area) 



G. Southwest Finnish Quarken Sea 



(transitional area) 



rill. Outer part of Gulf: Bothnia Sea (Bot- 

 H. Gulf of Bothnia (marginal area) | tensee) 



(iV. Inner part of Gulf (Bottenwiek) 



The Belts are also known as the West Baltic Sea. The southern strait of 

 Oresund and the Darss Ridge, i.e. the eastern boundary of the German Belt, 

 form the western boundary of the Baltic Sea proper. The most westerly part of 

 the Baltic Sea proper, the Arcona or Riigen region (Arcona or Riigen Sea), lies 

 to the west of Bornholm Island ; to the east is the Bornholm region with its 

 deep Bornholm depression. These two areas are sometimes called the South 

 Baltic Sea or the South Swedish-Pomeranian Baltic Sea. Farther east and 

 north is the central area of the Baltic Sea, divided into eastern and western 

 parts by the island of Gotland. The transitional area between the Baltic Sea 

 proper and the Gulf of Bothnia is occupied by the Aland Sea west of the 

 Aland Islands and by the Quarken Sea, or the Southwest Finnish Quarken 

 Sea, to the east of them. 



Bottom topography 



The southern part of the Kattegat is nowhere more than 40 m deep. The three 

 narrow straits which connect the Kattegat with the Baltic Sea — the Great 

 and Little Belts and Oresund — are even shallower. Oresund, in the latitude 

 of Copenhagen, is only 7 m deep. The Little Belt is a little deeper, and in its 

 shallowest part is 10-5 m deep. In the Great Belt and its continuation in the 

 direction of the Baltic — the Langeland Belt — there is a continuous trench with 

 a depth of at least 30 m. However, farther to the east and before entering the 

 Baltic proper this system of straits becomes even shallower. Still farther to the 

 east depths begin to increase. Before the island of Bornholm is reached there 

 is the Arcona depression with depths down to 53 m. The next significant 

 hollow is situated east of Bornholm Island (the Bornholm depression) with 

 a maximum depth of 105 m (see Fig. 1 14). Farther north the floor rises again 

 slightly and then north of Gotland it goes down sharply ; the 80 m contour 

 line encloses a large area between the Gulf of Riga and Stockholm, which 

 stretches southwards with a tongue each side of Gotland (Gotland depres- 

 sion) and contains some exceptionally deep areas. Among them is the greatest 

 depth in the Baltic Sea — the Lansort depression, 459 m. deep. The Gotland 

 depressions extend even to the Gulf of Finland, becoming progressively shal- 

 lower as one moves east (40 m and less). 



The Gulf of Bothnia, on the contrary, is separated from the Gotland depres- 

 sion by shoals (30 to 50 m) off the Aland Islands (Aland Ridge). The Gulf of 

 Bothnia itself is also divided by a shallow ridge off the Quarken into two 

 deeper parts : the southern one, the Bothnian Sea (Bottensee) with a maxi- 

 mum depth of 294 m, and the northern one — Bottenwiek — with a maximum 

 depth of 140 m. Finally a closed hollow with depths down to 301 m lies to the 

 west of the Aland Islands. 



