The Gulf of Finland is the easternmost part of the Baltic Sea. To the north, it is bordered by the shores of Finland, to the east by Russia, while its south coast belongs to Estonia. In the west, the boundary between the gulf and the open waters of the Baltic Sea is not clearly marked. The gulf is tubular in shape, with a length of 428 km and a width of 120 km. At the narrowest point the gulf is only 45 km wide. Because of the shallow depth and mixing of waters, it is defined as an estuary. The maximum depth is 123 m and the average is 38 m. The western part is deeper, whereas the eastern end is much shallower. The gulf has an area of 29,498 km2 and a volume of 1100 km3. In the northern part of the gulf the bottom consists of alternate areas of crystalline rocks and clays and areas covered with gravel, pebbles and boulders. Only a few depressions in the bottom are filled with clays. In the south, there are larger areas covered with postglacial clays. At the bottom of the gulf there are ferromanganese concretions. The Gulf of Finland has a continental climate, with relatively harsh winters compared with other regions of the Baltic Sea. The water temperature ranges from 0 °C in winter to 17 °C in summer. In winter the gulf freezes over and the ice cover can persist from 30 to 130 days. A significant nutrient load enters the gulf from rivers, the Baltic Proper and the atmosphere, making it one of the most eutrophic regions of the Baltic. Temporary water stratification leads to shortages of oxygen in the deeper areas, or even anoxia. Hypoxia occurs after algal blooms, when dead organic matter sinks to the bottom of the sea (Fig. 6.1).
Even though oxygen conditions are the worst in the western part of the gulf, a significant improvement has taken place since the 1990s. The gulf is the largest recipient of freshwater in the Baltic Sea. About 75% of this flows in with the Neva, which is why the impact of this river on the physical, chemical and biological processes of the gulf is so pronounced. At the mouth of the Neva the surface water salinity is about 0‰, but within 100 m of the shore, the salinity is already around 2.5–3‰. At the western end of the gulf the salinity is higher at about 7‰. The salinity of near-bottom waters is about 5‰ in the east and 10‰ in the west of the gulf. There is a periodic halocline, more pronounced in the west. Oxygen shortages create difficult conditions for benthic invertebrates, but a rich macrofauna inhabits the gulf’s open waters. The coastal zone is dominated by the crustaceans Monoporeia affinis and Corophium volutator. The gulf is inhabited by about 50 species of marine and fresh water fish, the most numerous being herring Clupea harengus and sprat Sprattus sprattus.