The Baltic Sea

The Baltic Sea and its parts — the Gulf of Riga and the Gulf of Finland — border the northern and western coasts of Estonia. The width of the national territorial waters is 12 nautical miles, i.e. ca 22.2 km. Most of the coastline of Estonia is highly indented. The total length of the coastline of mainland is 1242 km and of islands (in total ca 1500 sea islands) — ca 2551 km, including the coastline of Saaremaa (854 km), the largest island in Estonia.

The sea area located towards the open sea from Saaremaa and Hiiumaa islands hides numerous shoals and reefs, thus posing a threat to ships navigating in this area. Hiiu shoal (previously also known as Neckmansgrund) that is located 15 km northwest of Hiiumaa Island, near the waterway to the Gulf of Finland, is well known. Here the water above the limestone bottom is in some places only 1 m deep. There are also places (Mustpank, Uuskuiv) which are too shallow for big ships, situated 15–20 km to the west of Vilsandi and Saaremaa islands. The northernmost of them form an extension of the row of coastal scarps running along the northern coast of Muhu and Saaremaa (West-Estonian Clint). In some places the cliff terrace hidden under water is several tens of metres high. Westward of the territorial waters of Estonia, the sea becomes gradually deeper and in the central part of the sea area, in the Gotland Depth, the water layer is up to 249 m deep. Within the territorial waters of Estonia, the maximum depth of the Gulf of Finland slightly exceeds 100 m, while the maximum depth of the Gulf of Riga is 50–60 m.

Hydrology
The hydrological conditions of the Gulf of Riga and the Väinameri Sea, which both are shallow coastal seas with a relatively small water area and a poor connection to the open sea, are rather different from the hydrological conditions of the sea area to the west of Saaremaa, and of the western part of the Gulf of Finland. The Gulf of Riga and the Väinameri Sea have smaller waves, bigger fluctuations of water temperatures and of the water level, less salinity and transparency, and a thicker and more steady ice cover than the open sea. Compared to other seas, one of the hydrological peculiarities of the Baltic Sea is a distinct stratification of the water layer. West of the Estonian islands and at the mouth of the Gulf of Finland, at the depth of 60–70 m (at approximately 8‰ water salinity), there is a sharp transition between the upper water layer of low salinity (5–7‰) and the bottom water layer of relatively high salinity. As this significantly affects the vertical mixing of water, several hydrological, chemical and biological processes in these layers differ. In winter, the temperature of the water surface in the central part of the sea is 1°–2°C, while in bays and coastal sea areas covered with ice it is slightly below 0°C (water in the coastal sea areas freezes at –0.2° to –0.4°C). The maximum water temperatures can be measured in July–August when the water temperature in the sea areas west of islands and at the mouth of the Gulf of Finland is 16°–17°C, and the water temperatures in the coastal sea of the Väinameri Sea and of the Gulf of Finland, which both are separated from deep seas, are 18°–19°C.

Ice forms on the coastal sea every winter. The formation period of the first, thin ice cover, as well as the periods of occurrence of other ice-related phenomena, differ largely between years and depend on weather conditions. In some years, formation of ice on the bays of the western coast (the part of the sea with the most severe ice conditions in Estonia) starts already at the end of October or at the beginning of November, although sometimes, after very warm autumns, it may start even at the end of January, and at Vilsandi or Ruhnu, at the beginning of March. In very cold winters, the Väinameri Sea may freeze already in the first half of December (usually, a month later). Ice cover spreads slowly towards the open sea but reaches as far as 5–10 km from the coast only in cold winters. The whole sea area freezes over only in exceptionally severe winters (in 1939/1940, 1941/1942, 1946/1947). Usually, ice begins to break up in March and coastal sea areas are ice-free by the end of April. After mild winters, ice melts much earlier than after cold winters.

Occurrence of currents in the Baltic Sea depends on the direction and force of winds. Water movement along the Estonian coasts eastward is more frequent. Strong westerly winds raise the water level and easterly winds lower it. In exceptional years the water level has risen 2–2.5 m above, and sunk to 1.2 m below, the average water level. The Baltic Sea is virtually non-tidal. The tides here are less than 10 cm. The average wave height is 1–2 m, while during extremely stormy weather the wave height in the open sea may reach 10 m, in the Gulf of Finland 6 m, and in the Gulf of Riga 3–4 m.

Although the Baltic Sea is relatively poor in different species of flora and fauna, it is rich in specimens. The water salinity is suitable neither for fresh water species nor ocean species. The main commercial fish in the area are Baltic herring, sprat, cod and flounder.

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