Baltic Sea: depths and relief, description, geographical location. Baltic Sea: salinity, depth, coordinates and interesting facts

The Baltic Sea is connected to the North Sea by the Øresund Straits (Zund), Greater and Lesser Belta, Kattegat and Skagerrak. It washes the shores of Russia, Estonia, Latvia, Lithuania, Poland, Germany, Denmark, Sweden, and Finland.

The maritime border of the Baltic Sea runs along the southern entrances of the Öresund, Greater and Lesser Belta straits. Area 386 thousand km². The average depth is 71 m. The shores of the Baltic Sea in the south and south-east. predominantly low-lying, sandy, lagoon-type; on the land side there are dunes covered with forest, on the sea side there are sandy and pebble beaches. In the north the shores are high, rocky, predominantly of the skerry type. The coastline is heavily indented and forms numerous bays and bays.

The largest bays: Bothnian (according to physical and geographical conditions, it is a sea), Finnish, Riga, Curonian, Gdansk Bay, Szczecin, etc.

Islands of the Baltic Sea of ​​continental origin. There are many small rocky islands - skerries, located along the northern shores and concentrated in the groups of the Vasiya and Åland islands. The largest islands are: Gotland, Bornholm, Sarema, Muhu, Hiuma, Öland, Rügen, etc. A large number of rivers flow into the Baltic Sea, the largest of which are the Neva, Western Dvina, Neman, Vistula, Odra, etc.

The Baltic Sea is a shallow shelf sea. The predominant depths are 40-100 m. The shallowest areas are the Kattegat Straits (average depth 28 m), Oresund, the Greater and Lesser Belts, the eastern parts of the Gulf of Finland and the Gulf of Bothnia and the Gulf of Riga. These areas of the seabed have a leveled accumulative topography and a well-developed cover of loose sediments. Most of the bottom of the Baltic Sea is characterized by highly dissected relief; there are relatively deep basins: Gotland (249 m), Bornholm (96 m), in the Södra-Kvarken Strait (244 m) and the deepest - Landsortsjupet south of Stockholm (459 m). There are numerous stone ridges; in the central part of the sea, ledges are traced - continuations of the Cambrian-Ordovician (from the northern coast of Estonia to the northern tip of the island of Öland) and Silurian cliffs, underwater valleys, glacial-accumulative landforms flooded by the sea.

The Baltic Sea occupies a depression of tectonic origin, which is a structural element of the Baltic shield and its slope. According to modern concepts, the main irregularities of the seabed are caused by block tectonics and structural denudation processes. The latter, in particular, owe their origin to the underwater cliff ledges. The northern part of the seabed is composed predominantly of Precambrian rocks, overlain by an intermittent cover of glacial and recent marine sediments.

In the central part of the sea, the bottom is composed of Silurian and Devonian rocks, which are hidden to the south under a thick layer of glacial and marine sediments.

The presence of underwater river valleys and the absence of marine sediments under the glacial deposits indicate that in pre-glacial times there was land in place of the Baltic Sea. During at least the last ice age, the Baltic Sea basin was completely occupied by ice. Only about 13 thousand years ago there was a connection with the ocean, and sea waters filled the depression; The Ioldian Sea was formed (based on the mollusk Joldia). The Ioldian Sea phase is slightly earlier (15 thousand years ago) preceded by the phase of the Baltic glacial lake, which was not yet connected to the sea. About 9-7.5 thousand years ago, as a result of tectonic uplift in Central Sweden, the connection between the Yoldian Sea and the ocean ceased, and the Baltic Sea again became a lake. This phase of the development of the Baltic Sea is known as Lake Ancylus (based on the mollusk Ancylus). A new subsidence of land in the area of ​​the modern Danish Straits, which occurred about 7-7.5 thousand years ago, and extensive transgression led to the resumption of communication with the ocean and the formation of the Littorina Sea. The level of the last sea was several meters higher than the modern one, and the salinity was greater. Deposits of the Littorina transgression are widely known on the modern coast of the Baltic Sea. The secular rise in the northern part of the Baltic Sea basin continues today, reaching 1 m per hundred years in the north of the Gulf of Bothnia and gradually decreasing to the south.

The climate of the Baltic Sea is maritime and temperate, strongly influenced by the Atlantic Ocean. It is characterized by relatively small annual temperature fluctuations, frequent precipitation fairly evenly distributed throughout the year, and fog in the cold and transitional seasons. During the year, westerly winds prevail, which are associated with cyclones coming from the Atlantic Ocean. Cyclonic activity reaches its greatest intensity in the autumn-winter months. At this time, cyclones are accompanied by strong winds, frequent storms and cause large rises in water levels near the coast. In the summer months, cyclones weaken and their frequency decreases. The invasion of anticyclones is accompanied by easterly winds.

The extension of the Baltic Sea by 12° along the meridian determines noticeable differences in the climatic conditions of its individual regions. Average air temperature in the southern part of the Baltic Sea: in January -1.1°C, in July 17.5°C; middle part: in January -2.3°C, July 16.5°C; Gulf of Finland: in January -5°C, in July 17°C; northern part of the Gulf of Bothnia: in January -10.3°C, in July 15.6°C. Cloudiness in summer is about 60%, in winter more than 80%. The average annual precipitation in the north is about 500 mm, in the south over 600 mm, and in some areas up to 1000 mm. The greatest number of days with fog falls on the southern and middle parts of the Baltic Sea, where it reaches an average of 59 days a year, the smallest - in the north. Gulf of Bothnia (up to 22 days a year).

The hydrological conditions of the Baltic Sea are determined mainly by its climate, excess fresh water and water exchange with the North Sea. Excess fresh water, equal to 472 km3 per year, is formed due to continental runoff. Amount of water entering precipitation (172.0 km³ per year), is equal to evaporation. Water exchange with the North Sea averages 1659 km3 per year (salt water 1187 km³ per year, fresh water - 472 km³ per year). Fresh water flows from the Baltic Sea into the North Sea through the runoff current, while salt water flows through the straits from the North Sea into the Baltic Sea through the deep current. Strong westerly winds usually cause an influx, and eastern winds cause a flow of water from the Baltic Sea through all sections of the Øresund, Greater and Lesser Belta straits.

The currents of the Baltic Sea form a counterclockwise circulation. Along the southern coast the current is directed to the east, along the eastern coast to the north, along the western coast to the south and at the northern coast to the west. The speed of these currents ranges from 5 to 20 m/sec. Under the influence of winds, currents can change direction and their speed near the coast can reach 80 cm/sec or more, and in the open part - 30 cm/sec.

The surface water temperature in August in the Gulf of Finland is 15°C, 17°C; in the Gulf of Bothnia 9°C, 13°C and in the central part of the sea 14°C, 18°C, and in the south it reaches 20°C. In February - March, the temperature in the open part of the sea is 1°C-3°C, in the Bothnian, Finnish, Riga and other bays and bays below 0°C. The salinity of surface water decreases rapidly with distance from the straits from 11‰ to 6-8‰ (1‰-0.1%) in the central part of the sea. In the Gulf of Bothnia it is 4-5‰ (in the north of the bay 2‰), in the Gulf of Finland 3-6‰ (at the top of the bay 2‰ and less). In the deep and bottom layers of water, the temperature is 5°C or more, salinity varies from 16‰ in the west to 12-13‰ in the central part and 10‰ in the north of the sea. In years of increased water inflow, salinity increases in the west to 20‰, in the central part of the sea to 14-15‰, and in years of decreased influx it drops in the middle parts of the sea to 11‰.

Ice usually appears in early November in the north of the Gulf of Bothnia and reaches its greatest extent in early March. At this time, a significant part of the Riga, Finnish and Bothnian gulfs is covered with motionless ice. The central part of the sea is usually ice-free.

The amount of ice in the Baltic Sea varies from year to year. In exceptionally severe winters, almost the entire sea is covered with ice; in mild winters, only the bays. The northern part of the Gulf of Bothnia is covered with ice 210 days a year, the middle part - 185 days; Gulf of Riga - 80-90 days, Danish Straits - 16-45 days.

The level of the Baltic Sea is subject to fluctuations under the influence of changes in wind direction and atmospheric pressure (progressive-standing long waves, seiches), influx of river waters and waters of the North Sea. The period of these changes varies from several hours to several days. Rapidly changing cyclones cause level fluctuations of up to 0.5 m or more off the coast of the open sea and up to 1.5-3 m at the tops of bays and bays. Particularly large rises of water, which are, as a rule, a consequence of the superposition of a wind surge on the crest of a long wave, occur in the Neva Bay. The greatest rise in water in Leningrad was noted in November 1824 (about 410 cm) and in September 1924 (369 cm).

Level fluctuations associated with tides are extremely small. The tides have irregular semidiurnal, irregular diurnal and diurnal patterns. Their size varies from 4 cm (Klaipeda) up to 10 cm (The Gulf of Finland).

The fauna of the Baltic Sea is poor in species, but rich in quantity. The Baltic Sea is home to a brackish-water race of Atlantic herring (herring), Baltic sprat, as well as cod, flounder, salmon, eel, smelt, vendace, whitefish, perch. Among mammals - the Baltic seal. Intensive fishing is carried out in the Baltic Sea.

Russian hydrographic and cartographic work began in the Gulf of Finland at the beginning of the 18th century. In 1738, F.I. Soimonov published an atlas of the Baltic Sea, compiled from Russian and foreign sources. In the middle of the 18th century. Many years of research in the Baltic Sea were carried out by A.I. Nagaev, who compiled a detailed navigation guide. First deep-sea hydrological exploration in the mid-1880s. were carried out by S. O. Makarov. Since 1920, hydrological work was carried out by the Hydrographic Directorate and the State Hydrological Institute, and after the Patriotic War of 1941-45, extensive comprehensive research was launched under the leadership of the Leningrad branch of the State Oceanographic Institute of the USSR.

Baltic Sea(also called the Eastern Sea) is considered an inland sea that extends deep into the continent.

The northern extreme point of the Baltic Sea is located near the Arctic Circle, the southern - near the German city of Wismar, the western - near the city of Flensburg, and the eastern - near St. Petersburg. This sea belongs to the ocean.

General information about the Baltic

The area of ​​the sea (not including islands) is 415 km. sq. It washes the shores of the following states:

• Estonia;
• Russia;
• Lithuania;
• Germany;
• Latvia;
• Poland
• Latvia;
• Denmark;
• Finland;
• * Sweden.

Large bays are considered to be: Bothnian, Finnish, Riga, Kursk (separated by an oblique). The largest islands: Öland, Wolin, Alandia, Gotland, Als, Saaremaa, Muhu, Men, Usedom, Fore and others. The largest rivers: Zapadnya Dvina, Neva, Vistula, Venta, Narva, Pregolya.

The Baltic Sea, through the Volga-Baltic basin, opens onto and is located on the continental shelf. In the area of ​​islands, shallows and banks, the depth varies within 12 meters. There are a couple of basins where the depth reaches 200 meters. The Landsort Basin is considered the deepest (470 meters), the depth of the basin reaches 250 meters, and in the Gulf of Bothnia - 254 meters.

In the southern region the seabed is flat, and in the north it is predominantly rocky. A huge part of the bottom is covered with sediments of glacial origin of various colors (green, brown, black).

A special feature of the Baltic Sea is that there is an excess of fresh water, which is formed due to river runoff and precipitation.

Its surface brackish waters constantly flow into. During storms, the exchange between these seas changes, as water is mixed from the bottom in the straits. The salinity of the sea is decreasing from the Danish Straits (20 ppm) to the east (in the Gulf of Bothnia it is 3 ppm, and in the Gulf of Finland – 2 ppm). Tides can be diurnal or semidiurnal (do not exceed 20 cm).

When compared with other seas, the disturbances in the Baltic Sea are completely insignificant. In the central parts of the sea, waves can reach 3-3.5 meters, less often - 4 meters. During large storms, waves 10-11 meters high were recorded. The clearest water with a bluish-green tint is observed in the Gulf of Bothnia; in coastal areas it is more cloudy and has a yellowish-green color. Due to the development of plankton, the lowest water transparency is observed in summer. The soils of the coastal zone are varied: in the southern regions there is sand, in the east there is silt and sand, and on the northern coast there is stone.

Climate of the Baltic Sea

The sea temperature is generally lower than other seas. In the mornings in the summer, thanks to the southern winds that drive the upper warm layers into the ocean, the temperature sometimes drops below 12 degrees. When northerly winds begin to blow, surface waters become significantly warmer. The highest temperature is in August - about 18 C. In January it varies from 0 to 3 C.

Due to its low salinity, harsh winters and shallow depth, the Baltic Sea often freezes, although not every winter.

Flora and fauna

The water in the Baltic Sea changes from sea salt to fresh water. Marine mollusks live only in the western region of the sea, where the water is saltier. Fish here include sprat, cod, and herring. The Gulf of Finland is home to smelt, vendace, salmon and others. Seals live in the area of ​​the Åland Islands.

Due to the presence of many islands, rocks, and reefs in the sea, sailing in the Baltic Sea is quite dangerous. This danger is somewhat reduced due to the presence of a large number of lighthouses here (most of them). The largest cruise ships leave the Danish Straits and enter the Atlantic Ocean. The Great Belt Bridge is considered the most difficult place. The largest ports: Tallinn, Baltiysk, Lubeck, Riga, Stockholm, Szczecin, Rostock, Kiel, Vyborg, Gdansk, St. Petersburg;

• Ptolemy called this sea the Venedian, which comes from the name of the Slavic peoples who lived in ancient times in the southern part of the coast - the Wends or Wends;
• The famous route from the Varangians to the Greeks ran through the Baltic Sea;
• "The Tale of Bygone Years" calls it Varangian Sea;
• The name "Baltic Sea" appears for the first time in the treatise of Adam of Bremen in 1080;
• This sea is rich in oil, manganese, iron and amber. The Nord Stream gas pipeline runs along its bottom;
• Every year on March 22, Baltic Sea Environmental Protection Day is celebrated. This decision was made by the Helsinki Commission in 1986.

Resorts

Among the Baltic Sea resorts, the most famous are: Zelenogorsk, Svetlogorsk, Zelenogradsk, Pionersky (Russia), Saulkrasti and

The Baltic Sea (from antiquity until the 18th century in Russia was known as the “Varangian Sea”) is an inland marginal sea that protrudes deeply into the mainland. The Baltic Sea is located in northern Europe and belongs to the Atlantic Ocean basin.

The northernmost point of the Baltic Sea is located near the Arctic Circle, the southernmost point is near the city of Wismar (Germany). The westernmost point is located in the area of ​​the city of Flensburg (Germany), the easternmost point is located in the St. Petersburg area. Due to the large elongation along the meridian and parallel, individual areas of the Baltic Sea are located in different physical-geographical and climatic zones. This, in turn, influences the oceanological processes occurring in the sea and its individual areas.
Sea area: 415 thousand kilometers. Depth: average - 52 meters, maximum - 459 meters.

The Baltic Sea has three large bays: Bothnian, Finnish, Riga. About 250 rivers flow into it, including the Neva, Vistula, Neman, Daugava, and Oder.

The connection between the Baltic Sea and the Atlantic Ocean is carried out through the North Sea, the Skagerrak, Kattegat and Danish straits (Great and Little Belt, Oresund (Sound) and Fehmarn Belt), however, this connection is difficult due to the shallowness of the straits (depth at the rapids is 7- 18 meters). Therefore, the Baltic waters are very slowly renewed due to the cleaner Atlantic waters. The period of complete water renewal in the Baltic Sea is about 30-50 years.

The Baltic Sea has low salt content. Its waters are a mixture of salt water from the ocean and fresh water coming from numerous rivers. The degree of sea salinity in different places has different indicators, which is due to the weak vertical movement of water layers. If in the southwestern part of the sea it is 8 ppm (i.e., every kilogram of water contains 8 g of salt), in the western part it is 11 ppm, then in the central water area it is 6 ppm, and in the Gulf of Finland, Riga and Bothnia it barely exceeds mark of 2-3 ppm (the average salinity of the World Ocean is 35 ppm).

The length of the Baltic coastline is 7 thousand kilometers. The coast is distributed between the countries as follows: Sweden owns 35% of the coast, Finland - 17%, Russia - about 7% (approximately 500 kilometers). The remaining part of the coast is shared by Lithuania, Latvia, Estonia, Poland, Germany, and Denmark. The sea coast and adjacent land areas are heavily populated and intensively used by humans. Transport complexes and large industrial enterprises are located on the coast. The Baltic Basin accounts for one tenth of global maritime trade.

The Baltic Sea is heavily polluted as a result of the active activities of people living on its shores. Environmental problems in the Baltic Sea are related to many aspects of society, such as energy production and consumption, industry, forestry, agriculture, fishing, tourism, transport, wastewater treatment.

The main environmental problems of the Baltic

Firstly, the excess supply of nitrogen and phosphorus into the water area as a result of washout from fertilized fields, with municipal wastewater from cities and waste from some enterprises. Since the water exchange of the Baltic is not very active, the concentration of nitrogen, phosphorus and other wastes in the water becomes very strong. Due to biogenic elements in the sea, organic substances are not completely processed, and due to a lack of oxygen, they begin to decompose, releasing hydrogen sulfide, which is harmful to marine life. At the bottom of the Gotland, Gdansk, and Bornholm depressions, dead hydrogen sulfide zones already exist.

The second significant problem of the Baltic is water pollution with oil. Thousands of tons of oil enter the water area annually through various discharges. The film of oil that covers the surface of the water mirror does not allow oxygen to penetrate deeper. Toxic substances that are harmful to living organisms also accumulate on the surface of the water. Accidental oil spills in most cases occur in coastal and shelf zones, the most productive and at the same time vulnerable areas of the sea.

The third problem in the Baltic Sea is the accumulation of heavy metals. Mercury, lead, copper, zinc, cobalt, and nickel enter mainly into the Baltic waters with precipitation, the rest ends up through direct discharge into the water area or with river runoff of household and industrial waste. The amount of copper entering the water area annually is about 4 thousand tons, lead - 3 thousand tons, cadmium - about 50 tons, and mercury - 33 tons, per 21 thousand cubic kilometers of water volume of the water area.

Due to its geographical location, the Baltic Sea has always been at the crossroads of historical events. There is more than one ship graveyard at the bottom of the Baltic. Many sunken ships contain dangerous cargo. Containers containing cargo deteriorate over time.

For decades in the Baltic, the practice of sinking and burying obsolete bombs, shells, and chemical munitions was practiced. After the end of World War II, by a joint decision of the countries of the anti-Hitler coalition (USSR, Great Britain and the USA) and in accordance with the decision of the Potsdam Conference of 1951, over 300 thousand tons were sunk in various areas of the Baltic, as well as in the straits connecting the Baltic Sea with the North Sea German chemical weapons and ammunition.

For more than half a century, ammunition has been lying on the bottom of the Baltic, creating a potentially deadly threat. Metal in seawater is corroded by rust, and toxic substances can enter the water at any time.

The material was prepared based on information from open sources

Strongly embedded in the mainland. It is not as harsh as the climate of the Arctic seas, although the Baltic Sea is located in the northwestern part of Russia. This sea is almost completely limited by land. Only from the southwest is this sea connected to the waters by various straits. The Baltic Sea belongs to the type of inland seas.

The shores washed by this sea have different origins. Quite complicated and... The Baltic Sea has a rather shallow depth, due to the fact that it is located within the boundaries of a continental shallow.

The greatest depth of the Baltic Sea is recorded in the Landsort Basin. The Danish Straits are characterized by shallow depths. The depth of the Great Belt is 10 - 25 m, the Little Belt - 10 - 35 m. The waters of the Sound have a depth of 7 to 15 m. The shallow depths of the straits interfere with the smooth exchange of waters between the Baltic Sea and. The Baltic Sea covers an area of ​​419 thousand km2. The volume of water is 321.5 km 3 . The average water depth is about 51 m. The maximum sea depth is 470 m.

The climate of the Baltic Sea is influenced by its location in the temperate latitude zone, the proximity of the Atlantic Ocean and the location of most of the sea inland. All these factors contribute to the fact that the climate of the Baltic Sea is in many ways close to the maritime climate of temperate latitudes, and there are also some features of a continental climate. Due to the fairly significant extent of the sea, there are some distinctive climate features in different parts of the sea.

In the Baltic, it is largely due to the influence of the Icelandic, Siberian and. Seasonal patterns vary depending on whose influence is dominant. In autumn and winter, the Baltic Sea is influenced by the Icelandic low and the Siberian high. As a result of this, the sea is at the mercy of the sea, which spreads in the fall from west to east, and in winter to the northeast. This period is characterized by cloudy weather with strong southwestern and western winds.

In January and February, when the lowest is observed, the average monthly temperature in the central part of the sea is – 3°C, and in the north and east – 5 – 8°C. As the Polar Maximum intensifies, cold temperatures reach the Baltic Sea. As a result, it drops to – 30 – 35°C. But such cold snaps happen quite rarely and, as a rule, they are short-lived.

In the spring-summer period, the Siberian High loses its strength, and the dominant influence on the Baltic Sea is exerted by the Azores and, to a lesser extent, the Polar High. At this time, it is observed at sea. Cyclones coming to the Baltic from the Atlantic Ocean are not as significant as in winter. All this causes the unstable direction of the winds, which have low speeds. In the spring season, northern winds have a great influence on the weather; they bring cold air.

In summer, winds from the west and north-west prevail. These winds are predominantly light or . Due to their influence, cool and humid weather is observed in summer. The average July temperature reaches + 14 – 15°C in the Gulf of Bothnia and +16 – 18°C ​​in other areas of the sea. Very rarely, warm air masses arrive in the Baltic Sea, causing hot weather.

The temperature of the Baltic Sea waters depends on the specific location. In winter, the water temperature off the coast is lower than in the open sea. In the western part the sea is warmer than in the eastern part, which is due to the cooling influence of the land. In summer, the coldest waters are off the western shores in the central and southern zones of the sea. This distribution of temperatures is due to the fact that the western ones move heated upper waters from the western shores. Their place is taken by cold deep waters.

Shore of the Baltic Sea

About 250 large and small rivers carry their waters into the Baltic Sea. During the year they give to the sea about 433 km 3, which is 2.1% of the total volume of the sea. The most abundant are: the Neva, which flows 83.5 km 3 per year, the Vistula (30.4 km 3 per year), the Neman (20.8 km 3 per year) and the Daugava (19.7 km 3 per year). The share varies in different areas of the Baltic Sea. For example, in the Gulf of Bothnia, rivers contribute 188 km 3 per year, while the volume of continental waters is 109.8 km 3 / year. The Gulf of Riga receives 36.7 km 3 /year and in the central part of the Baltic it is 111.6 km 3 /year. Thus, the eastern regions of the sea receive more than half of all continental waters.

Throughout the year, rivers bring unequal amounts of water to the sea. If the full flow of rivers is regulated by a lake, as, for example, near the Neva River, then the greater flow occurs in the spring-summer period. If the full flow of rivers is not regulated by lakes, as, for example, near the Daugava River, then the maximum flow is observed in the spring and a slight increase in the fall.

They are practically not observed. Currents affecting surface waters are caused by winds and river flow. In winter, the waters of the Baltic Sea are covered with ice. But during the same winter, the ice can melt several times and freeze the waters again. This sea is never completely covered with ice.

Fishing is widely developed in the Baltic Sea. Herring, sprat, cod, whitefish, lamprey, salmon and other types of fish are caught here. A large amount of algae is also found in these waters. There are many marine farms in the Baltic Sea where the most sought-after fish species are grown. There are a large number of placers on the Baltic Sea coast. Amber mining work is being carried out in the area. There is oil in the depths of the Baltic Sea.

Shipping is widely developed in the waters of the Baltic Sea. Sea transport of various goods is constantly carried out here. Thanks to the Baltic Sea, it maintains close economic and trade relations with Western European countries. There are a large number of ports on the Baltic Sea coast.

The Baltai Sea (Fig. 222) is a semi-enclosed body of water, deeply covered by the mainland, communicating with the open sea by a system of narrow and shallow straits and stretching from south to north for almost 1,200 km.

Figure 222.

In terms of area, this body of water is close to the Black and Caspian Seas - its surface is 386 thousand square meters. km, and in terms of volume it is much inferior to them, having 33,700 cubic meters. km.

In the southern part of Kattegat, the depth does not exceed 40 m. The three narrow straits connecting Kattegat with the Baltic Sea - the Small and Great Belts and Öresun - have even shallower depths. The last of them, at the latitude of Copenhagen, has a depth of only 7 m, the depth of the Little Belt is 10 m. The western border of the Baltic Sea proper is the southern Strait of Oresun and the so-called Darssky ridge, i.e. the southern border of the German Belt.

To the east of the Darssky ridge, the depths increase, and between it and Fr. Bornholm is the Arkona depression with depths of up to 40 m, and to the east of the island. Bornholm - Bornholm Depression with a depth of up to 105 m. Further north, the bottom rises slightly again, and then forms the Gotland Depression up to 240 m and the deepest Landsort Depression northwest of the island of Gotland with a depth of 459 m. The average sea depth is 86 m.

The Gulf of Finland is not fenced off from the sea by a barrier, and the Gulf of Bothnia forms two deep depressions, separated from the rest of the sea by an elevation with depths of 30–50 m, while the depressions themselves have a depth of 294 and 135 m. A small but deep depression (up to 300 m) is also present west of the Åland Islands.

The Baltic Sea is the most desalinated of all our seas. Only in the very southwestern part of the sea the surface salinity is 8‰, with movement to the east and north the salinity decreases - in the middle parts of the sea it is 6-7‰, and in the inner parts of the Gulf of Bothnia and the Gulf of Finland it drops to 3 and 2‰ (Fig. 223 ).

Figure 223.

With depth, the salinity of the Baltic Sea water increases significantly (Table 58).

Moreover, in the southwestern part of the sea, adjacent to the straits, salinity can undergo very strong fluctuations, sometimes over short periods of time. Thus, in Öresun at the same point, fluctuations in salinity were observed from 7.2 to 22.4‰ on the surface and from 11.7 to 22.5‰ at a depth of 17 m. In Kiel Bay, fluctuations in water salinity are even greater - from 3. 9 to 26.3‰ on the surface and from 10.3 to 28.8‰ at a depth of 14 m.

The significant difference in the salinity of the surface and deep water of the Baltic Sea is explained primarily by the presence of two currents: the surface current (drain) carries fresher waters into the straits, while more salty waters are carried into the sea by the bottom current. The influx of saltier waters through the straits occurs not so much through this constant bottom current, but as a result of non-periodic strong backwaters of sea water from the Skagerrak and Kattegat under the influence of various meteorological (surge winds) and geophysical factors. The wave of salty heavy water that penetrates in this way moves along the bottom deep into the Baltic Sea and “fills” its depressions. The isolation of deep depressions and strong differences in the salinity of the surface and deep layers of water in the Baltic Sea create conditions for weak vertical mixing and “stagnation” of deep waters. This affects the temperature distribution and gas regime (Table 58).

Surface waters have a different temperature regime than deep waters. In summer, the surface temperature is very similar to the temperature of the North Sea (Fig. 224) (in August 12–16 °), but in winter the difference is quite sharp - in the North Sea 4–6 °, in the Baltic 1–2 °, and in the Bothnian and In the Gulf of Finland, the temperature drops below zero and the water is covered with ice (in the internal parts for 4–6 months a year). At depths of over 50 m and to the bottom, the water temperature is low and uniform throughout the year (3–4° in the southern part of the sea and close to 0° in the northern).

Figure 224.

Poor aeration of deep waters also affects the decrease in oxygen with depth and the increase in dissolved carbon dioxide (Table 58), which is very important for the distribution of bottom fauna. All of the above aspects of the hydrological regime are needed to understand the composition and distribution of life in the Baltic Sea, but the geological past is no less important for this.