Lake classification. A lake is a body of water that does not have a direct connection with ...

How many closed depressions are there on land in which water accumulates. When water does not have time to evaporate, it forms lakes. That's what a lake is!

Definition of "lake"

A lake is an accumulation of water in a natural depression on land. It consists of a lake bowl or a bed filled with water to the brim. This body of water is not connected to the sea and ocean. Knowing what a lake is, it is easier to understand its origin. And it differs significantly. There is a lake: tectonic, glacial, river, seaside. There are also failure, mountain, crater and artificial.

Lake Features

What is a lake, what are its features? First, unlike rivers, lakes do not have currents and are not part of the oceans. Secondly, lakes have different mineralization of water. The deepest and freshest lake is Baikal. And the largest lake, and, in terms of salt composition, similar to ocean water, is the Caspian. Once it was the sea, as it was connected to the ocean.

There is also a division of lakes according to their position, according to the water balance, according to the chemical composition of the water and according to the nutritional value of the substances contained in the lake.

There are really a lot of features. There are lakes of different shapes, sizes, bottom topography. So, small lakes are called lagoons, and larger ones are called "seas". They receive water not only from rain, but also from underground rivers. Such close "cooperation" allows the rivers not to dry up. Often lakes even give life to new rivers.

- a reservoir formed on the surface of the land in a natural depression. Since the lake does not have a direct connection with the ocean, it is a reservoir of slow water exchange.

The total area of ​​lakes on the globe is about 2.7 million km 3, which is 1.8% of the land surface.

The main characteristics of the lake:

  • lake area - water surface area;
  • coastline length - water edge length;
  • lake length - the shortest distance between the two most distant points on a coastline, average width - ratio of area to length;
  • lake volume - the volume of the basin filled with water;
  • average depth - ratio of water mass volume to area;
  • maximum depth - found by direct measurements.

The largest lake in terms of water surface area on Earth is the Caspian (376 thousand km 2 at a water level of 28 m), and the deepest is Baikal (1620 m).

The characteristics of the largest lakes in the world are given in Table. one.

In each lake, three interconnected components are distinguished: the basin, the water mass, the vegetation and fauna of the reservoir.

Lakes of the world

By position lake basin lakes are divided into ground and underground. The latter are sometimes filled with juvenile water. The subglacial lake in Antarctica can also be classified as an underground lake.

Lake basins can be like endogenous, and exogenous origin, which most significantly affects their size, shape, water regime.

The largest lake basins. They can be located in tectonic depressions (Ilmen), in foothill and intermountain troughs, in grabens (Baikal, Nyasa, Tanganyika). Most large lake basins have a complex tectonic origin, both discontinuous and folded movements are involved in their formation (Issyk-Kul, Balkhash, Victoria, etc.). All tectonic lakes are large, and most of them have significant depths, steep rocky slopes. The bottoms of many deep lakes lie below the level of the World Ocean, and the mirror of the oxen is above the level. Certain regularities are observed in the location of tectonic lakes: they are concentrated along faults in the earth's crust, either in rift zones (Syrian-African, Baikal), or frame shields: the Great Bear Lake, the Great Slave Lake, the Great North American Lakes, along the Baltic Shield are located along the Canadian Shield - Onega, Ladoga, etc.

lake name

Maximum surface area, thousand km 2

Height above sea level, m

Maximum depth, m

Caspian Sea

North America

Victoria

North America

North America

Aral Sea

Tanganyika

Nyasa (Malawi)

Big Bear

North America

Great Slave

North America

North America

Winnipeg

North America

North America

Ladoga

maracaibo

South America

Bangweulu

Onega

Tonle Sap

Nicaragua

North America

Titicaca

South America

Athabasca

North America

North America

Issyk-Kul

Big Salty

North America

Australia

Volcanic lakes occupy craters and calderas of extinct volcanoes (Kronopkoye Lake in Kamchatka, lakes of Java, New Zealand).

Along with lake basins created by the internal processes of the Earth, there are very numerous lake baths formed due to exogenous processes.

Among them, the most common glacial lakes on the plains and in the mountains, located both in hollows plowed by the glacier, and in depressions between hills with uneven deposition of moraine. The destructive activity of ancient glaciers owes its origin to the lakes of Karelia and Finland, which are elongated in the direction of glacier movement from northwest to southeast along tectonic cracks. In fact, Ladoga, Onega and other lakes have a mixed glacial-tectonic origin. The glacial basins in the mountains include numerous, but small car lakes located in bowl-shaped depressions on the slopes of mountains below the snow line (in the Alps, in the Caucasus, Altai), and trough lakes - in trough-shaped glacial valleys in the mountains.

The uneven accumulation of glacial deposits on the plains is associated with lakes among the hilly and moraine relief: in the northwest of the East European Plain, especially on the Valdai Upland, in the Baltic States, Poland, Germany, Canada and in the north of the USA. These lakes are usually shallow, wide, with lobed shores, with islands (Seliger, Valdai, etc.). In the mountains, such lakes arose on the site of former tongues of glaciers (Como, Garda, Würm in the Alps). In areas of ancient glaciation, there are numerous lakes in the hollows of the flow of melted glacial waters, they are elongated, trough-shaped, usually small and shallow (for example, Dolgoye, Krugloye - near Moscow).

Karst lakes are formed in places where rocks are leached by underground and partly surface waters. They are deep, but small, often rounded in shape (in the Crimea, the Caucasus, in the Dinaric and other mountainous regions).

Suffusion lakes are formed in basins of subsiding origin at the site of intensive removal of fine earth and mineral particles by groundwater (south of Western Siberia).

Thermokarst lakes are formed when permafrost is burned or ice melts. Thanks to them, the Kolyma Lowland is one of the most lake regions in Russia. Many relict thermokarst lake basins are located in the northwest of the East European Plain in the former periglacial zone.

eolian lakes arise in blowout basins (Lake Teke in Kazakhstan).

Zaprudnye lakes are formed in the mountains, often after earthquakes, as a result of landslides and landslides blocking river valleys (Lake Sarez in the Murgab valley in the Pamirs).

In the valleys of lowland rivers, the most numerous are floodplain oxbow lakes of a characteristic horseshoe shape, formed as a result of meandering of rivers and subsequent straightening of channels; when rivers dry up in bochagas - reaches, river lakes are formed; in river deltas there are small ilmen lakes, in place of channels, often overgrown with reeds and reeds (ilmens of the Volga delta, lakes of the Kuban floodplains).

On the low coasts of the seas, coastal lakes are characteristic in place of estuaries and lagoons, if the latter are separated from the sea by sandy alluvial barriers: spits, bars.

A special type are organogenic lakes among swamps and coral buildings.

These are the main genetic types of lake basins, determined by natural processes. Their location on the continents is presented in Table. 2. But recently, more and more "man-made" lakes created by man have appeared - the so-called anthropogenic lakes: lakes - reservoirs on rivers, lakes - ponds in quarries, in salt mines, on the site of peat mining.

By genesis of water masses There are two types of lakes. Some have water of atmospheric origin: precipitation, river and groundwater. Such lakes insipid, although in dry climates they can eventually become salty.

Other lakes were part of the World Ocean - these are relic salty lakes (Caspian, Aral). But even in such lakes, the primary sea water can be greatly transformed and even completely displaced and replaced by atmospheric water (Ladoga and others).

Table 2. Distribution of the main genetic groups of lakes by continents and parts of the world

Genetic groups of lakes

Continents and parts of the world

Western Europe

Overseas Asia

North America

South America

Australia

Glacial

Glacial-tectonic

Tectonic

Volcanic

Karst

Residual

Lagoon

floodplain

depending from water balance, t.s. According to the conditions of inflow and runoff, lakes are divided into waste and non-drainage. Lakes that discharge part of their waters in the form of river runoff - sewage; a special case of them are flowing lakes. Many rivers can flow into the lake, but only one flows out (the Angara from Lake Baikal, the Neva from Lake Ladoga, etc.). Lakes that do not have a runoff into the oceans - drainless(Caspian, Aral, Big Salt). The water level in such lakes is subject to fluctuations of different duration, which is primarily due to long-term and seasonal climate changes. At the same time, the morphometric characteristics of lakes and the properties of water masses change. This is especially noticeable on lakes in arid regions, which are predicted to have long cycles of humidification and aridity of the climate.

Lake waters, like other natural waters, are characterized by different chemical composition and varying degrees of mineralization.

According to the composition of salts in the water, the lakes are divided into three types: carbonate, sulfate, chloride.

By degree of mineralization lakes are divided into insipid(less than 1% o), brackish(1-24.7% s), salty(24.7-47% o) and mineral(more than 47% c). Baikal can serve as an example of a fresh lake, the salinity of which is 0.1% c \ brackish - the Caspian Sea - 12-13% o, the Big Salt - 137-300% o, the Dead Sea - 260-270% o, in some years - up to 310% s.

In the distribution of lakes with varying degrees of mineralization on the earth's surface, geographical zoning is traced, due to the coefficient of moisture. In addition, those lakes into which rivers flow are distinguished by low salinity.

However, the degree of mineralization can be different within the same lake. So, for example, in the endorheic Lake Balkhash, located in the arid zone, in the western part, where the river flows into. Or, the water is fresh, and in the eastern part, which is connected to the western part only by a narrow (4 km) shallow strait, the water is brackish.

When the lakes are oversaturated from the brine, the salts begin to precipitate and crystallize. Such mineral lakes are called self-planting(for example, Elton, Baskunchak). Mineral lakes in which lamellar fine needles are deposited are known as mud.

plays an important role in the life of lakes thermal regime.

Fresh lakes of the hot thermal zone are characterized by the warmest water near the surface, with depth it gradually decreases. This distribution of temperature over depth is called direct thermal stratification. Lakes of the cold thermal zone have the coldest (about 0 ° C) and light water at the top for almost the entire year; with depth, the water temperature rises (up to 4 ° C), the water becomes denser, heavier. This distribution of temperature over depth is called reverse thermal stratification. Lakes of the temperate thermal zone have a variable stratification according to the seasons of the year: direct in summer, reverse in winter. In spring and autumn there come moments when the vertical temperature is the same (4 °C) at different depths. The phenomenon of temperature constancy over depth is called homothermy(spring and autumn).

The annual thermal cycle in lakes of the temperate zone is divided into four periods: spring heating (from 0 to 4 °C) is carried out due to convective mixing; summer heating (from 4 °C to maximum temperature) - by molecular heat conduction; autumn cooling (from maximum temperature to 4 °C) - by convective mixing; winter cooling (from 4 to 0 ° C) - again by molecular heat conduction.

In the winter period of freezing lakes, the same three phases are distinguished as in rivers: freezing, freezing, opening. The process of formation and melting of ice is similar to rivers. The lakes are usually covered with ice for 2-3 weeks longer than the rivers of the region. The thermal regime of freezing salt lakes resembles that of the seas and oceans.

Dynamic phenomena in lakes include currents, waves and seiches. Stock currents occur when a river flows into a lake and outflow of water from the lake into the river. In flowing lakes, they can be traced throughout the entire water area of ​​the lake, in stagnant lakes - in areas adjacent to the mouth or source of the river.

The height of the waves on the lake is less, but the steepness is greater compared to the seas and oceans.

The movement of water in lakes, along with dense convection, contributes to the mixing of water, the penetration of oxygen into the lower layers, and the uniform distribution of nutrients, which is important for a wide variety of lake inhabitants.

By nutritional properties of the water mass and the conditions for the development of life, lakes are divided into three biological types: oligotrophic, eutrophic, and dystrophic.

Oligotrophic- low-nutrient lakes. These are large deep transparent lakes with greenish-blue water, rich in oxygen, so organic residues are intensively mineralized. Due to the small amount of biogenic elements, they are poor in plankton. Life is not rich, but there are fish, crustaceans. These are many mountain lakes, Baikal, Geneva, etc.

Eutrophic lakes have a high content of nutrients, especially nitrogen and phosphorus compounds, are shallow (up to 1015 m), well heated, with brownish-green water. The oxygen content decreases with depth, which is why fish and other animals die in winter. The bottom is peaty or silty with an abundance of organic remains. In summer, there is a "bloom" of water due to the strong development of phytoplankton. The lakes are rich in flora and fauna. They are most common in forest-steppe and steppe zones.

Dystrophic lakes are poor in nutrients and oxygen, they are shallow. The water in them is acidic, slightly transparent, brown due to the abundance of humic acids. The bottom is peaty, there are few phytoplankton and higher aquatic vegetation, animals too. These lakes are common in heavily wetlands.

In the last decade, under the conditions of increased supply of phosphorus and nitrogen compounds from the fields, as well as the discharge of wastewater from some industrial enterprises, eutrophication of lakes has been observed. The first sign of this unfavorable phenomenon is a strong bloom of blue-green algae, then the amount of oxygen in the reservoir decreases, silts form, and hydrogen sulfide appears. All this will create unfavorable conditions for the life of fish, waterfowl, etc.

The evolution of lakes occurs in different ways in wet and dry climates: in the first case, they gradually turn into swamps, in the second - into salt marshes.

In a humid (humid) climate, the leading role in filling the lake and turning it into a swamp belongs to vegetation, partly to the remains of the animal population, which together form organic remains. Temporary streams and rivers bring mineral deposits. Small lakes with gently sloping shores are overgrown by pushing vegetative ecological zones from the periphery to the center. Eventually the lake becomes a grassy fens.

Deep lakes with steep banks overgrow in a different way: by growing from above alloys(quick) - a layer of living and dead plants. It is based on plants with long rhizomes (cinquefoil, watch, calla), and other herbaceous plants and even shrubs (alder, willow) settle on a grid of rhizomes. The raft first appears near the coast, protected from the wind, where there is no excitement, and gradually moves towards the lake, increasing in power. Part of the plants dies, falls to the bottom, forming peat. Gradually, only “windows” of water remain in the quagmire, and then they disappear, although the basin is not yet filled with sediments, and only with time does the raft merge with a layer of peat.

In dry climates, lakes eventually become salt marshes. This is facilitated by an insignificant amount of precipitation, intense evaporation, a decrease in the inflow of river waters, and the deposition of solid sediments brought by rivers and dust storms. As a result, the water mass of the lake decreases, the level decreases, the area decreases, the concentration of salts increases, and even a fresh lake can first turn into a salt lake (the Great Salt Lake in North America), and then into a salt marsh.

Lakes, especially large ones, have a softening effect on the climate of the surrounding areas: it is warmer in winter and cooler in summer. So, at coastal weather stations near Lake Baikal, the temperature in winter is 8-10 °C higher, and in summer by 6-8 °C lower than at stations outside the influence of the lake. The air humidity near the lake is higher due to increased evaporation.

Lake - a component of the hydrosphere, which is a naturally occurring body of water filled within the lake bowl (lake bed) with water and not having a direct connection with the sea (ocean). Lakes are the subject of study of the science of limnology.

From the point of view of planetology, a lake is an object that exists stably in time and space, filled with a substance in the liquid phase, the size of which is intermediate between the sea and the pond.

From the point of view of geography, the lake is a closed depression of the land, into which water flows and accumulates. Lakes are not part of the oceans.

Although the chemical composition of lakes remains constant for a relatively long time, unlike the river, the substance that fills it is renewed much less frequently, and the currents present in it are not the predominant factor determining its regime. Lakes regulate the flow of rivers, retaining hollow waters in their basins and releasing them in other periods. Chemical reactions take place in lake waters. Some elements pass from water to bottom sediments, others - vice versa. In a number of lakes, mostly without runoff, the concentration of salts increases due to the evaporation of water. The result is significant changes in the mineralization and salt composition of lakes. Due to the significant thermal inertia of the water mass, large lakes soften the climate of the surrounding areas, reducing the annual and seasonal fluctuations of meteorological elements.

The shape, size and topography of the bottom of lake basins change significantly with the accumulation of bottom sediments. Overgrowth of lakes creates new landforms, flat or even convex. Lakes and, especially, reservoirs often create groundwater backwater, causing waterlogging of nearby land areas. As a result of the continuous accumulation of organic and mineral particles in lakes, thick strata of bottom sediments are formed. These deposits are modified with the further development of water bodies and their transformation into swamps or dry land. Under certain conditions, they are transformed into rocks of organic origin.

Lake classification

By origin, the lakes are divided into:

  • Tectonic: formed by filling cracks in the earth's crust. A striking example of a tectonic lake is Lake Baikal.
  • Glacial: formed by a melting glacier. A typical glacial lake left from the last ice age is the Arbersee, located at the foot of the Great Arber (1456 m) - the highest mountain in the Bohemian Forest.
  • River(or old lady).
  • seaside(lagoons and estuaries). The most famous lagoon is the Venetian lagoon, located in the northern part of the Adriatic Sea.
  • Failed(karst, thermokarst). A feature of some failed lakes is their periodic disappearance and appearance, depending on the peculiar dynamics of groundwater. A typical representative is Lake Ertso in South Ossetia.
  • Zavalno-dammed: formed during the collapse of a part of a mountain (for example, Lake Ritsa in Abkhazia).
  • Mountain: located in mountain basins.
  • Crater: located in the craters of extinct volcanoes and explosion tubes. In Europe, similar lakes are located in the Eifel region (Germany). Near them, weak manifestations of volcanic activity in the form of hot springs are observed.
  • artificial(reservoirs, ponds). The creation of such lakes can be an end in itself, for example, to create reservoirs for various purposes. Often this creation is associated with more or less significant earthworks. But in some cases, such lakes appear as a side effect of such work, for example, in depleted quarries.

According to the position of the lake are divided into (in relation to the planet Earth):

  • Ground, whose waters take an active part in the water cycle in nature and groundwater, whose waters, if they take part in it, then only indirectly. Sometimes these lakes are filled with juvenile, that is, native water.
  • Underground. The subglacial lake in Antarctica can also be attributed to the number of underground lakes.

According to the water balance, the lakes are divided into:

  • sewage(have a drain, mainly in the form of a river).
  • Drainless(they do not have surface runoff or underground water drainage to neighboring watersheds. Water consumption occurs due to evaporation).

By type of mineralization

  • fresh;
  • ultra-fresh

mineral (salty).

  • brackish
  • salty

According to the chemical composition of water, mineral lakes are divided into

  • carbonate (soda)
  • sulfate (bitter-salty)
  • chloride (salty)

According to the nutritional value of the substances contained in the lake (trophicity), three types of lakes are distinguished:

  • Oligotrophic (with a small amount of nutrients) - lakes are usually characterized by large or medium depths, a significant mass of water below the temperature jump layer, high transparency, water color from blue to green, a gradual drop in O2 content to the bottom, near which water always contains significant amounts of O2 (at least 60% of its content on the surface)
  • Eutrophic (with a high content of nutrients) - well-heated lakes (the layer below the temperature jump is very small), transparency is low, the color of the water is from green to brown, the bottom is covered with organic silt. The water is rich in nutrient salts, the O2 content drops sharply towards the bottom, where it often disappears completely.
  • Dystrophic (poor in nutrients) - swampy lakes with low transparency and yellow or brown (from a high content of humic substances) water color. The mineralization of water is low, the content of O2 is low due to its consumption for the oxidation of organic substances.

In modern hydrology and hydroecology, intermediate levels of trophic classification are distinguished: mesotrophic (between oligotrophic and eutrophic) and hypertrophic.

According to their location on celestial bodies, lakes are divided into:

  • earthly;
  • extraterrestrial.

The largest lakes of the Earth

The total area of ​​the world's lakes is about 1.8% of the land (about 2.7 million km²).

lake name

Maximum surface area, thousand km²

Height above sea level, m

Maximum depth, m

part of the world

Caspian Sea
Upper

North America

Victoria
Huron

North America

Michigan

North America (USA)

Tanganyika
Baikal

Asia (Russia)

Malawi
Big Bear
Great Slave

North America (Canada)

Erie
Chad
Winnipeg

North America (Canada)

Balkhash

Asia (Kazakhstan)

Ontario

North America

Aral Sea
Ladoga

Europe (Russia)

LAKE
body of water surrounded by land. Lakes range in size from very large ones, such as the Caspian Sea and the Great Lakes in North America, to tiny bodies of water a few hundred square meters or even smaller. The water in them can be fresh, as in the lake. Upper, or salty, as in the Dead Sea. Lakes are found at any height, from the lowest absolute mark on Earth on the land surface -408 m (Dead Sea) and almost to the highest (in the Himalayas). Some lakes do not freeze all year round, while others, such as Lake. Vanda in Antarctica, ice-bound for most of the year. Many lakes exist permanently, while others (for example, Lake Eyre in Australia) are only occasionally filled with water. Despite their diversity, lakes of all types share a number of common physical, chemical and biological characteristics and are subject to many general laws. Therefore, the study of lakes in all their diversity and in all aspects is dealt with by one scientific discipline - lake science, or limnology (from the Greek lmn - lake, pond and logos - word, doctrine). Probably the best way to understand the nature of lakes is to consider them not only as landforms but also as aquatic ecosystems in which the interaction of all components leads to the establishment of observable conditions and where a change in one characteristic causes more or less significant changes in all other components of the ecosystem. In this sense, lakes are similar to oceans, but there are differences between them: lakes are smaller and more vulnerable to external influences, including natural climatic changes. Age is one of the significant differences between lakes and oceans. Only a few of the existing lakes, such as Tanganyika or Baikal, are several million years old. Most lakes are probably younger than 12,000 years old, and man-made lakes - artificial reservoirs - are only a few decades old.


EAST COAST OF LAKE TANGANIK, confined to the East African Rift Zone.


ORIGIN OF THE LAKE BELLS
The lakes fill basins that have different genesis. Since the formation of these basins is often dependent on local conditions, lakes are concentrated in certain areas, such as the Lake District in northwest England, the lake district in Austria, and the vast belt of lakes that covers the states of Minnesota, Wisconsin and Michigan. The formation of lake basins is influenced by tectonic activity, volcanism, landslides, glacial processes, karst and suffusion, fluvial processes, eolian processes, coastal processes, accumulation of organogenic deposits, damming of watercourses by humans or beavers, and meteorite fall. The oldest and deepest of the existing lakes arose under the influence of tectonic activity, but most of the lakes were formed due to glacial processes. Nevertheless, the role of other listed factors is also important.
Tectonic activity. Tectonic depressions arise as a result of movements of the earth's crust, and many lake basins of tectonic origin are large and ancient. They are usually very deep. Tectonic processes manifest themselves in different ways. For example, the Caspian Sea is confined to a trough at the bottom of the ancient Tethys Sea. In the Neogene, an uplift occurred, as a result of which the Caspian depression became isolated. Its waters gradually desalinated under the influence of atmospheric precipitation and river runoff. The basin of the lake Victoria in East Africa was formed by arching uplift of the surrounding land. The Great Salt Lake in Utah also arose due to the tectonic uplift of the area through which the flow from the lake was previously carried out. Tectonic activity often leads to the formation of faults (cracks in the earth's crust), which can turn into lake basins if a reverse fault occurs in the area or if a block enclosed between faults sinks. In the latter case, the lacustrine basin is said to be associated with a graben. Several lakes within the East African Rift System have this origin. Among them - lake. Tanganyika, formed ca. 17 million years old and very deep (1470 m). On the continuation of this system to the north are the Dead Sea and Lake Tiberias. Both are very ancient. The maximum depth of Lake Tiberias is currently only 46 m. ​​Lakes Tahoe on the border of the states of California and Nevada in the USA, Biwa (the source of freshwater pearls) in Japan and Baikal, which holds the world's largest mass of fresh water (23 thousand km3) are also associated with grabens. ), in Siberia.



Volcanic activity leads to the formation of a variety of lake basins - from small rounded craters with low sides (maars) to large deep calderas formed when magma erupts through a side crater located near the top of the volcano, which leads to the collapse of the volcanic cone. A good example of a caldera lake is Lake. Crater in Oregon, formed during the eruption of the Mazama volcano c. 6000 years ago. This picturesque lake of almost round shape has a depth of 608 m (seventh deepest in the world). In the middle of the lake is the island of Wizard, which arose as a result of a later eruption. Lakes of this type are found in Japan and the Philippines. In volcanic areas, lake basins can also form when hot lava flows from below a colder surface lava horizon, which contributes to the subsidence of the latter (this is how Yellowstone Lake was formed), or when rivers and streams are dammed by lava or mud lava flow during volcanic eruptions. This is how the basins of many lakes in Japan and New Zealand arose.



Landslides, podruzhivaya water flows, contribute to the formation of lakes. However, if the dam collapses or water overflows, these lakes soon disappear. For example, in 1841, the Indus River in the territory of modern Pakistan was dammed by a landslide resulting from an earthquake, and six months later the "dam" collapsed, and the lake, 64 km long and 300 m deep, was lowered in 24 hours. A lake of this type can only remain stable if excess water is drained through erosion-resistant hard rock. For example, Lake Sarez, formed in the Eastern Pamirs in 1911, still exists and has a depth of 500 m (the tenth deepest lake in the world). Glacial activity is the most effective factor in the creation of lake basins. Ice sheets several kilometers thick, which in geologically recent times covered much of North America and much of northern Europe, formed lake basins in various ways, and most of the lakes in these regions are of glacial origin. For example, many lakes are confined to plowing basins, which were formed during the movement of glaciers over a heterogeneous surface. At the same time, glaciers demolished loose sediments. Thousands of lakes that have filled such basins are found in northern Canada, Norway and Finland, where they occupy significant areas.



Karovye lakes are located on the slopes of the mountains in the upper reaches of the troughs. They are characterized by hollows, shaped like amphitheaters. Frost weathering processes also take part in the formation of the beds of such lakes. Fjord lakes have an elongated shape, steep banks and a U-shaped transverse profile. They occupy depressions at the bottom of river valleys, reworked and deepened by large glaciers. Good examples of lakes of this type are Loch Ness in Scotland and many lakes in Norway. Partly by glacial processes, a group of lakes has been formed, radiating from a single center in the Lake District in northwest England. The large lakes of northern Canada - Athabasca, Great Bear and Great Slave - have a similar origin. The depth of the latter reaches 640 m. Even the basins of the Great Lakes, which have a complex genesis, have been affected by glaciers. In addition, lakes are formed when river valleys are dammed by moraines. Finally, during the retreat of glaciers, huge blocks of dead ice were buried under the thickness of sediments carried by melted glacial waters beyond the glacier. Many of them melted only hundreds of years later, when the climate improved, and basins filled with water appeared in their place.
See also GLACIERS.


Karst and suffusion. Karst lakes are formed when soluble minerals and rocks such as limestone, gypsum and rock salt are carried away by water, either depressions on the surface or underground voids are formed, the roof of which then collapses. These lakes are not necessarily small: for example, Lake. Girot in the French Alps has a depth of 99 m with an area of ​​​​only 57 hectares.
fluvial processes. As a result of the activity of rivers, lakes are formed in several ways: water wells appear at the foot of waterfalls; depressions are developed in the rocky soil by flowing waters under the influence of the process of evorsion (when holes are drilled due to the friction of stones and other abrasive material on the bottom in whirlpools); river channels are blocked during the removal of river sediments by other rivers and their accumulation. For example, the Mississippi River formed a lake. St. Croy near St. Paul (Minnesota), having dammed the St. Croy River, but then itself was dammed downstream by sediments of the Chippewa River, and as a result, Lake was formed. Pippin. Finally, in valleys with well-developed floodplains, for example, in the valley of the Mississippi River in the states of Louisiana and Arkansas, as a result of the breakthrough of the necks of meanders and channel processes, oxbow lakes are cut off in the form of large meanders.
eolian processes. In basins of eolian origin, there are lakes dammed by eolian sands or enclosed among dunes. There are also deflationary lakes confined to blowout basins, which are common in arid or semi-arid regions of Texas, South Africa, and Australia. The origin of deflationary lakes, sometimes called playas, is not fully understood, but it is possible that they are sometimes formed by the combined action of wind blowing and excavation by animals that use them for watering.
coastal processes. During the movement of the alongshore sediment flow, sea bays can be separated by sand bars and turn into lakes. If such a bar remains stable, the resulting salt lake is then desalinated. Processes of accumulation of organogenic deposits. Lake Okeechobee in Florida is one of the better known lakes formed from such processes. Although its basin arose when a depression was raised at the bottom of the sea, originally Lake. Okeechobee was dammed by dense aquatic vegetation and accumulations of its remnants. Damping of streams by humans or beavers. Dams built by beavers can reach large sizes - more than 650 m long - but they are short-lived. Unintentional human activity has led to the creation of thousands of lakes at the site of quarries and mine workings, and, in addition, dams have been specially built. During the construction of large dams in Africa, huge reservoirs arose, including Nasser on the Nile River, Volta on the Volta River and Kariba on the Zambezi River. Some dams were built to generate electricity for aluminum smelting from large local deposits of bauxite.
impact of meteorites. Probably the most rare and unusual lake basins are depressions formed as a result of meteorite impacts. It has been reliably found out that one of the lakes of the Ungava Peninsula in the prov. Quebec (Canada) is confined to the Nouveau Quebec meteorite crater. This rounded lake is located among lakes of glacial origin, which have an irregular shape.
SOURCES OF LAKE WATER
To be called a lake, a basin formed by one of the methods described above, of course, must at least occasionally be filled with water, which can enter the lake in various ways. In many large lakes in humid regions, a significant part of the water can come directly from atmospheric precipitation falling on the surface of the lakes. For example, food Victoria in East Africa is about 75% atmospheric. The main source of water for smaller lakes or lakes in more arid areas is usually the surface runoff of rivers and streams. Lakes can be fed by groundwater that comes out in the underwater part of the lake basin. Many lakes, in particular of glacial origin, are associated with hollows worked out in the strata of loose aquifers and are located below the groundwater level. In this case, water enters the lake or flows out of it, seeping through the sides of the basin. There are also key lakes, at least partially fed by underwater springs. Sometimes, from sources, a huge amount of salts enters the lake, captured during the passage of a watercourse through easily soluble rocks (for example, in Lake Tiberias). The freshest waters are characteristic of lakes fed exclusively by atmospheric precipitation. However, the salinity of lakes also depends on how the water leaves the lake. The content of mineral salts in flowing lakes is usually close to their concentration in the feed stream. Lakes, in the basins of which water is filtered both into and out of the lake, are usually fresh. However, some lakes have an inflow of water, but no runoff, and water only evaporates from their surface, resulting in an increase in the concentration of soluble salts in water bodies. In such endorheic, or "closed" lakes (as opposed to "open"), highly specialized communities of plants and animals, such as some crustaceans or insects, often form. Another factor affecting the salinity of lakes is the amount of precipitation. Finally, the nature of the rocks among which the lakes are located is of great importance. Thus, the lakes in the area of ​​the Canadian Shield are mostly very fresh, since the rocks through which the water flows are completely insoluble. An essential aspect of the water balance of lakes is the rate of water exchange. This characteristic is determined either by the time of complete water change in the lake (in years), which is expressed through the ratio of the volume of the lake to the annual water flow from it, or through the inverse value, called the water exchange coefficient of the reservoir. The time for a complete water change can be very short - one week or less, which corresponds to a water exchange coefficient of 50 times a year - for reservoirs located on rivers above dams, but it can also be long - up to 500 years, with an annual water exchange coefficient of 0.002 (as in Lake Superior). Water bodies with a shorter cycle of complete water change (and, accordingly, with high water exchange coefficients) are cleared of pollutants faster and generally have lower concentrations.
SUBSTANCES DISSOLVED IN LAKE WATER
Water is an excellent solvent, and therefore lake waters contain a lot of dissolved substances. It is noteworthy, however, that the vast majority of these substances in most lakes are represented by a limited number of compounds, namely, positively charged ions (cations) of calcium, magnesium, sodium and potassium and negatively charged ions (anions) consisting of carbon and oxygen (bicarbonates), sulfur and oxygen (sulfates) and chlorine (chlorides) (both groups of ions are listed in descending order of their content). These seven ions account for 90 to 95% of the total amount of dissolved substances in the waters of most lakes, and their total concentration, usually measured in milligrams per liter (mg / l), characterizes the salinity (mineralization) of the water. Other substances, such as plant nutrients (nitrogen and phosphorus) and metals (iron and manganese), are present in much lower amounts, so that their concentrations are measured in micrograms per liter (µg/L). In drainless lakes, evaporation leads to a change in the composition of salts. Lakes are called chloride, sulfate or carbonate, depending on which anions have accumulated in them in the greatest amount under the influence of evaporation or precipitation.



STRATIFICATION OF LAKE WATER
In some lakes, especially in shallow waters or those exposed to strong winds, there is no noticeable water stratification at all. This means that the water masses are more or less constantly mixed by the action of the wind and are fairly homogeneous in all respects. However, for most deep lakes and those that are in the wind shadow, a distinct stratification of the water column according to physical properties is characteristic, as a result of which less dense waters are located above denser ones. Such stratification significantly affects the chemical composition and biology of lakes.



When solar energy interacts with water, the latter acquires a unique property: its density reaches its maximum value (1.0) at a temperature of approx. 4 ° C, gradually decreasing both with increasing and decreasing temperatures. In lakes, sunlight is used by plants for photosynthesis and by animals to see underwater. Light also influences the vertical migrations of some organisms, but the main effect of solar energy is water heating. The influx of energy from the Sun is significant. The arrival of solar energy during one summer day can reach 500 cal per 1 cm2 of the lake surface. Part of this energy is reflected from the mirror of the lake, part is scattered by the water surface into space, and part is absorbed by water and converted into thermal energy. This thermal energy is partially radiated back into the atmosphere or spent on evaporation. It is mainly the upper layer of water several meters thick that is heated, since the radiation is quickly absorbed as it penetrates deeper. Heating causes the water in this upper layer to expand, causing its density to decrease compared to that of the underlying cold layers. Heated water accumulates on top of cold and therefore denser waters. However, in early spring, especially in temperate regions, the temperature of the water as a whole remains low, so that the decrease in density due to such heating is insignificant, and the wind mixes the heated water throughout its thickness. Later, as the influx of solar energy increases, the water temperature in the lake as a whole rises, and the decrease in density per unit temperature increment becomes greater, as well as the volume of the heated near-surface water layer. Ultimately, the wind is no longer able to mix the entire water mass, and the influx of solar energy is concentrated in a few upper meters of water. As a result, the lake waters are divided into two horizons: the upper, less dense, warm - epilimnion, and the lower, denser, cold - hypolimnion. The intermediate layer, in which there is a rapid decrease in temperature with depth, is called the metalimnion, or thermocline. Such stratification is determined more by the density of water than by its temperature. Because in tropical regions, where water temperatures are generally higher, density changes are much greater (see graph), and the temperature difference between epilimnion and hypolimnion can be much smaller than in temperate areas. In any case, if the density of water in the epilimnion and hypolimnion differ by 0.001 to 0.003, a noticeable stable stratification is achieved. Such small differences allow lake waters to resist mixing even under the influence of strong winds. At the end of summer, when the days become shorter and the solar radiation decreases, the upper layer of water cools down, becomes denser and soon, together with the underlying waters, undergoes wind mixing, due to which the power of the epilimnion increases. This process continues until the water temperature throughout the entire depth of the lake, as a result of mixing, equals or becomes close to the temperature of the hypolimnion. In tropical regions, where temperatures are consistently above 0°C, this kind of circulation of lake waters can continue throughout the winter. However, where winter air temperatures drop below 0 ° C, lake waters continue to cool and mix until a temperature of 4 ° C is established. If further surface waters cool below this temperature, corresponding to the maximum density of water, they again become lighter and remain on the surface, creating a stratification in the lake, which not only depends on density, but is also inversely related to temperature. Ice binding of the water surface has a stabilizing effect, and such stratification persists throughout the winter, until complete mixing of lake waters occurs again in spring. Thus, periods of summer and winter stratification and spring and autumn mixing of lake waters are usually distinguished in the annual cycle of lakes. In most lakes, depending on the climatic features of the region, stratification is established once or twice a year, or is not established at all for a more or less noticeable period. However, the stratification of other lakes persists, usually due to the fact that the density of deep waters increases not due to temperature differences, but rather due to a higher concentration of dissolved chemical compounds. Such lakes, in contrast to periodically completely mixed ones, are called partially mixed lakes, since mixing does not occur in the lower layer. The same layer can exist in very deep lakes, such as Tanganyika, where the seasonal dynamics of air temperatures proceeds so quickly that the water in the lake does not have time to completely mix. The ability of lakes to store heat during the summer and release it in the winter can have a significant moderating effect on the local climate. This is especially true for large lakes such as the Great. For example, oz. Michigan annually absorbs and then gives off more than 50 kcal of heat per 1 cm2 of its surface.
HYDRODYNAMICS OF LAKES
The movement of water in lakes differs significantly from high-amplitude tidal and powerful ocean currents. Only in such large lakes as Superior and Michigan, there are constant currents, but even in them there are practically no tidal fluctuations (their amplitude in Lake Superior is only 3 cm). Nevertheless, under the influence of the temperature gradient, flowing streams and winds, water moves in the lakes. For example, at the end of summer, when heat is released from the surface of the lakes into the atmosphere at night, the water, cooling in this way, becomes heavier and sinks towards the hypolimnion, mixing with its upper layer. This is one of the main mechanisms of epilimnion growth in depth, which leads to complete mixing of water in autumn. When a river flows into a stratified lake, a runoff current occurs either in the surface layer or at medium depths. Surface currents are formed when the tributary waters have a lower density than the waters of the lake itself, as, for example, in summer when the Jordan River flows into Lake Tiberias. Medium-depth currents are formed if the watercourse rushes down to the layers corresponding to its own density. If water flows through the dam at the same time, this current can spread over long distances and carry waters with specific properties (for example, with a higher or lower silt content) through the entire reservoir. If the density of the watercourse is higher than the density of any layer of lake water, it will sink to the bottom and form a bottom current. In this case, even the formation of an underwater channel is possible, as, for example, at the confluence of the river. Rhone in Lake Geneva. Under the influence of wind, several types of movements of lake waters arise. One of them - the eddy wind current (or Langmuir circulation) - is clearly distinguished on the surface of lakes by alternating smooth and small rippled bands. When the wind blows, the water moves with the wind and forms cylindrical eddies whose axes are parallel to both the direction of the wind and the surface of the lake. In some vortices, the movement occurs clockwise, and in others - counterclockwise. As a result, longitudinal (windward-stretched) zones of convergence (oncoming and downward movement of water) are formed, alternating with longitudinal zones of divergence (ascending and divergent movement of water). Divergence zones are located at some distance from one another (for example, from 5 to 15 m). They are easily recognizable as smooth streaks as bubbles, dust and other floating objects collect along convergence zones where water sinks but is not fast enough to carry this material with it. Another type of water movement occurs when the wind constantly blows over the surface of the lake. Since the water moves with the wind, the water level at the far end of the lake rises somewhat, which leads to the formation of a compensatory current - either along the coast, if the lake is shallow, or, in deeper lakes, oppositely directed and passing at some depth from the surface. However, if the wind subsides, as a result of the surge of water to the far shore, a compensatory current forms on the surface of the lake, and the water moves first in one direction, then in the other, until these oscillations die out. Such surface movements of water with variable direction are called surface seiches. On large lakes, their height can exceed several meters. Seiches can cause great damage to low-lying coastal areas. Fortunately, these seiches fade fairly quickly and the lakes return to normal. If the lake is very deep or has a clear stratification, another type of water movement may occur, called inland seiches. When water moves with the wind, its level rises by approximately 1 mm per linear kilometer. If the wind is steady, then the balance of the water mass is disturbed. Both near the surge and surge shores of the lake, warm, less dense water masses are located above the cold and denser ones, but near the surge coast, the water layer is several millimeters larger. To balance the excess pressure created by this additional layer of water, the denser bottom waters move against the wind to the opposite shore of the lake, while the less dense surface waters move downwind. This leads to a distortion of the thermocline: it rises on the leeward side of the lake. However, since the density difference between surface and bottom waters is often only approx. 0.001 of the average density of water, the change in the ratio of these two types of water required to balance the shear exceeds the magnitude of the surge by about 1000 times. Therefore, the thermocline skew is very large compared to the magnitude of the surge: on such large lakes as Baikal, it can reach or exceed 150 m. . As a result, the surface and bottom waters continue to fluctuate, and the thermocline, like a pendulum, changes its inclination to one side or the other, until, finally, this movement dies out, and the lake comes to a state of internal equilibrium. The duration of such fluctuations is determined by the parameters of the lake basin, but it is much longer than the attenuation period of surface seiches, and, for example, on the lake. Baikal can reach 30 days. It is noteworthy that as a result of such oscillatory movements of bottom waters, only slight vertical mixing occurs, but the water is transported over long horizontal distances and may even come into contact with bottom sediments and change its chemical properties. In addition, such movements contribute to the transport of pollutants discharged into the upper part of the bottom water layer at one side of the lake for many kilometers to another place, where water is possibly abstracted for industrial or domestic needs. Under some conditions, inland seiches can even cause deep waters with very low dissolved oxygen content to reach the lake surface near the shore, where it causes fish to die. Such a phenomenon is periodically observed in Lake Tiberias with a characteristic 24-hour period of internal seiches, coinciding with the daily frequency of summer winds.
LIFE OF THE LAKES
The lakes are home to a wide variety of living organisms, from viruses and bacteria to freshwater seals and sharks. These organisms are not only influenced by the physical and chemical properties of their habitat, but also influence it themselves, especially in stratified lakes. There are three types of habitats in lakes: the zone of contact between the atmosphere and water, the zone of contact between bottom sediments and water, and the water column itself. In each zone there is a set of organisms adapted to the specific conditions of a given type of habitat.
The zone of contact between the atmosphere and water. Organisms living in this zone are collectively called "neuston" (from the Greek neusts - floating). Although these organisms are interesting in their own right, the group as a whole is quite small. Its most famous representatives are water strider bugs, swimming beetles and mosquito larvae that hang attached to the surface film of water.
Contact zone of bottom sediments and water. The totality of organisms living in this zone is called benthos (from the Greek. bnthos - depth). This group includes both plants and animals. Plants, commonly known as aquatic, or macrophytes, live in shallow waters where light is available and form a certain zoning. At the bottom, along the edge of the lake, semi-submerged macrophytes grow, including sedges and cattails. Farther from the shore and somewhat deeper, such macrophytes take root, such as, for example, water lilies with long stems topped with floating leaves, through which carbon dioxide from the atmosphere is absorbed. Even farther from the coast, at greater depths, macrophytes (for example, pondweeds) grow completely submerged in water. In North America, this group includes many species, including curly pondweed (Potamogeton scirpus), urt (Myriophyllum exalbescens), and others. Most (though not all) of these plants take root in the bottom soil, from where they extract nutrients. The size of the lake area occupied by such plants depends on a number of factors: on what proportion of the lake area is shallow, on the properties of bottom sediments, and on the characteristics of wave activity. While in some lakes with steep underwater slopes (for example, in the Upper) there are almost no macrophytes, in many lakes of smaller sizes or in large, but shallow ones (for example, in Lake Neusiedler See on the border of Austria and Hungary), the bottom can be completely covered with such plants. In tropical regions, floating aquatic plants are common, for example, eichhornia, or water hyacinth (Eichhornia), and pistia (Pistia), in temperate latitudes - tiny duckweed (Lemna). These plants, especially the larger ones, can grow strongly and form a dense continuous cover on lakes and reservoirs. The vast surface area of ​​shallow water plants serves as a habitat for a group of organisms attached to them called periphyton (from the Greek peri - around, around and phytn - plant), which includes bacteria, protozoa and algae. These organisms make the underwater parts of plants slippery to the touch. Shallow (littoral) areas also provide shelter for various animal organisms - gastropods and bivalves, leeches, insect larvae that live among plants and stones that are often found in the coastal zone. Deeper, outside the littoral zone, macrophytes do not grow. The sublittoral zone is located here, where the bottom gradually descends towards the deep part of the lake. The sublittoral zone is inhabited by bacteria, protozoa and true worms, as well as similar larvae of various insect species. With depth, habitat conditions become less favorable (especially in stratified lakes), and only a few adapted species are found there.
Water column. The organisms living here are divided into two groups: nekton and plankton, i.e. small organisms that float in water and are generally incapable of moving against the watercourse. Both terms have Greek roots: nektos - floating and plankton - wandering.
Nekton. According to their nutritional habits, lake fish are divided into several groups. Fish-eating or predatory fish, which are often non-commercial species, feed mainly on smaller fish and fry of other fish species. Planktivorous fish feed on plankton suspended in the water column and are themselves often eaten by predatory fish. Fish that feed on algae and herbivorous fish such as carp that feed on shallow water plants stand out. Benthivorous fish eat animals that live at the bottom of water bodies and organic particles that fall to the bottom of the lake.
Plankton. The term "plankton", originally introduced to refer to organisms (plants and animals) passively floating in the upper part of the ocean waters, is also used for organisms that live in lakes. There are phytoplankton (plant organisms) and zooplankton (animal organisms). All of them are microscopic and have a specific gravity close to that of fresh water, but if it were higher, the plankton would quickly sink to the bottom.



Blue-green algae: 1 - Oscillatoria, 2 - Microcystis aeruginosa, 3 - Anabaena, 4 - Coelosphaerium, 5 - Spirulina, 6 - Aphanizomenon flos-aquae. Green algae: 7 - Scenedesmus, 8 - Closterium, 9 - Spirogyra, 10 - Staurastrum, 11 - Chlorella, 12 - Micrasterias, 13 - Xanthidium, 14 - Cosmarium, 15 - Pediastrum.







Phytoplankton is represented by microscopic algae, consisting of individual cells or their colonies (sometimes immersed in mucus) or filamentous algae. In fresh water bodies, four functional groups of phytoplankton are distinguished, consisting of representatives of six or seven departments of the plant kingdom. The chloroplasts (specific intracellular formations) of green algae contain the green pigment chlorophyll, which is not masked by other pigments. In diatoms, chlorophyll is accompanied by other pigments that often give them a golden brown color. In blue-green algae, which many biologists consider bacteria (cyanobacteria), chlorophyll is dissolved in the protoplasm of the cell and masked by other pigments, which is why they have a bluish-green color. Pigmented flagellates, capable of actively moving, are a group of small organisms belonging to different departments of the plant kingdom. Although all types of algae are usually present at the same time, the prevalence of one or another of them is seasonal. For example, in temperate regions, diatoms are most abundant in spring, then at the end of spring they are replaced by green algae, blue-green in summer, and diatoms again in autumn. In the same climatic conditions, in lakes rich in nutrients, blue-green algae dominate for most of the year, which often happens in the tropics. Flagellates, like some blue-green algae, are often present under ice in winter. The reasons for successive changes in algae types throughout the year and the predominance of some of them over others are different. Numerous conflicting theories exist to explain these phenomena. In some lakes, up to 200 species of algae can be simultaneously detected at concentrations reaching hundreds of thousands of cells per 1 ml of water. The spring maximum concentration of diatoms is often called the spring bloom of water bodies, and the autumn maximum, respectively, the autumn bloom. An important property of diatoms is that they use silica (SiO2) to build a hard shell around the cell called a shell. Therefore, diatoms are heavier than other algae. In some blue-green algae, cell buoyancy is regulated by gas vacuoles. Algae play an important role in lakes, as they, together with larger plants, form the first link in the water food chain. In the process of photosynthesis, they, using sunlight captured by chlorophyll and other pigments, extract about 18-20 elements from lake water and use them in building a new cellular substance. At the same time, dissolved oxygen is released in the surface layer of water, where photosynthesis takes place. The energy accumulated in this way in primary production is then used for the life of other organisms living in the lake. Zooplankton are commonly referred to as microscopic animals or other microscopic organisms that do not carry out photosynthesis. Zooplankton includes some groups of bacteria, as well as protozoa, rotifers and tiny crustaceans. Although non-pathogenic (not disease-causing) bacteria are not animals, they are included in zooplankton. They abound in lake water, where their concentration can exceed 100 million in 1 ml. If not for these bacteria (many of which decompose organic matter into its constituent parts), the metabolism in lakes would slow down and eventually stop, since all available minerals would be bound into organic compounds in living or dead organisms. Instead, bacteria convert dead organic matter into free chemical elements and thus complete the cycle, again making these elements available for photosynthesis and growth. Protozoa are microscopic single-celled animals, sometimes called non-cellular, such as amoeba and paramecia (ciliary ciliates). They are often found in abundance in lake waters. Some of them attach themselves to larger organisms, others float freely in the water, feeding on bacteria or the smallest organic residues - detritus. A more complex structure than the simplest, have rotifers, so named for the corolla of hairs, or cilia, around the mouth opening. These cilia vibrate harmoniously in such a way that they give the impression of a spinning wheel. Rotifers are multicellular animals. They feed on small algae, bacteria and organic detritus, and occasionally other rotifers. In most cases, their reproduction is sexual, both females and males participate in it. However, in many cases, parthenogenetic reproduction occurs, in which only females participate. Females lay eggs that carry a diploid set of chromosomes, from which females also develop. Only in harsh environmental conditions do females lay eggs with a haploid set of chromosomes. Some of these eggs then develop (without fertilization) and hatch into males that produce haploid sperm. These males fertilize haploid eggs, and special, so-called. resting (latent) eggs that have increased resistance to harsh conditions, such as drying. When environmental conditions become favorable again, female individuals develop from resting eggs, reproducing parthenogenetically. The smallest crustaceans are one of the most visible constituents of zooplankton. These crustaceans are very small - 0.3-12 mm long. In most lakes, they are the main link between primary producers (algae) and subsequent links in the food chain (fish). They are so small that they feed only on microscopic algae, but they are large enough to be food for fish. Thus, the abundance of these crustaceans is controlled by two factors: food availability and predators. First of all, larger ones are eaten, i.e. more noticeable, crustaceans. In other words, predation is selective. There are two groups of lake crustaceans: copepods and cladocerans. Copepods resemble shrimp in appearance, as they have a clearly defined head, chest and abdomen, ending in a tail. Separate groups of copepods are distinguished mainly by the length of the antennules: in some they are very short, in others the length of the antennules exceeds the length of the body. Although some copepods feed on filamentous algae, many of them eat smaller animals. Reproduction is sexual, and approximately the same number of males and females are born. The eggs are carried in a single or double chambered oviduct located at the base of the tail. The eggs develop into larvae that look completely different from adult crustaceans. After six molts, they take on the appearance of adults. Copepods can be recognized by their characteristic spasmodic swimming style. The copepods include the Cyclopes, who, like the mythological namesake, have a single eye in the middle of the "forehead". The body of branched crustaceans is enclosed in a translucent bivalve chitinous carapace (shell). Most cladocerans are herbivores. They filter the water with swimming limbs equipped with feathery bristles, extracting from it the smallest particles of organic detritus, bacteria and especially algae, although some of the cladocerans are predators. Filtered food moves through a special groove to the mouth opening and enters the intestine, where digestion takes place. The eggs are carried and developed in a brood chamber on the back of the female. Juveniles leave her during molting. Basically, cladocerans reproduce parthenogenetically, laying diploid eggs, from which only females hatch. However, under harsh conditions, males hatch from these eggs and fertilize the resulting haploid eggs with haploid sperm, turning them into diploid "resting" ones. Such eggs are laid in pairs in intensely pigmented protective shells that are shed during molting and are able to survive unfavorable periods, and when conditions improve, females are hatched from them, breeding parthenogenetically. Sometimes, under the influence of wind, mass accumulations of such shells are formed along the edge of the coast. Other organisms are also found in zooplankton, such as mysids (Mysis) - small crustaceans that often live in the lower cold oxygen-rich water layers of deep lakes, and a transparent mosquito larva that usually lives at the bottom of lakes. Sometimes there are even freshwater jellyfish with a diameter of up to 38 mm.
CHEMICAL PROCESSES IN LAKES
Although the chemical composition of the lake is important for all organisms, as evidenced, for example, by specialized plant and animal species that live in salt lakes, it is plants that carry out photosynthesis that most strongly affect the chemistry of lake waters. Photosynthesis uses solar energy to convert carbon dioxide and water into hydrocarbons and oxygen. At the same time, in addition to carbon dioxide and water, 18-20 more chemical elements are involved in photosynthesis, and a decrease in the content of any of them below the optimal requirement significantly slows down the process of photosynthesis. This so-called. the hypothesis of the limiting role of nutrients, put forward in the middle of the 19th century. Justus Liebig, is still used in the characterization of aquatic ecosystems. In freshwater bodies, most nutrients are present in quantities in excess of the need for them, but two of them - nitrogen and phosphorus - are relatively rare. It is these elements, individually or together, that limit the process of photosynthesis, or primary production. Moreover, since some blue-green algae are able to bind atmospheric nitrogen, converting it into ammonium and using it in the process of photosynthesis, and phosphorus does not have such a source, the latter becomes the most important limiting element. As a result, many important characteristics of lakes, such as the total increase in primary production or the abundance of algae, are directly dependent on the phosphorus content in lakes. Therefore, lakes are classified according to this indicator. There are oligotrophic lakes (with a low content of nutrients), mesotrophic (with an average content) and eutrophic lakes (with a high content of nutrients). Epilimnion is almost always saturated with dissolved oxygen, which is formed here in the process of photosynthesis, as well as captured from the boundary layer of the atmosphere during the circulation of water. At the same time, all other elements necessary for photosynthesis and growth are extracted from the water by algae, and the chemistry of the waters of the epilimnion undergoes corresponding changes. At the same time, the epilimnion produces a lot of organic detritus, consisting of dead fragments of algae, which sinks into the hypolimnion. There, dissolved oxygen is used for respiration and decomposition, and many inorganic substances are returned to the water. Thus, in a stratified lake, the initially homogeneous water mass is subdivided into two distinctly different layers: the upper, warmer, with a deficit of available nutrients, and the lower, colder, with a higher concentration of nutrients. In temperate climates, this separation takes place both in winter and summer, although in winter it is less pronounced, since under the ice, due to less access to light, the level of primary water production is significantly reduced. In unstratified lakes, seasonal changes occur throughout the water column. In many lakes rich in nutrients, photosynthesis proceeds so intensively that dissolved oxygen is completely consumed directly at the surface of bottom sediments. In this case, even more significant changes in the chemical composition of water are observed. At the interface between bottom sediments and water, oxygen-containing insoluble iron compounds lose oxygen and become soluble, as a result of which a large amount of iron, manganese, phosphorus and nitrogen enters the water. This process is called internal eutrophication, because in some lakes, as a result of wind mixing or the influence of internal seiches, nutrients released from sediments enter the upper water layer, thus increasing the trophic level of the lake. In temperate regions, during the period of spring and autumn mixing of waters, the surface layer of sediments again absorbs oxygen, all differences in the chemical composition of water in depth disappear, and the water mass again becomes chemically homogeneous.
LAKE DEPOSITS
Lacustrine deposits, which play an important role in the chemistry of lakes, are mostly formed in the lakes themselves. Usually they consist of semi-decomposed remains of algae, zooplankton and larger organisms, and in lakes formed about 10 thousand years ago, they can reach a large thickness (about 20 m). The study of lacustrine sediment cores shows that the concentration of bacteria in them is very high, especially at the contact of bottom sediments and water. The same pattern can be traced in the concentration of various chemicals, such as phosphorus and ammonium. Since lacustrine sediments are usually cold and poor in oxygen, they provide excellent evidence of the state of the lake in the past, which is reflected either in the composition and quantity of specific algae pigments, or in the composition of identifiable remains of the most decay-resistant parts of organisms. Various methods have been developed to determine the age of individual layers of lacustrine sediments. Among them are methods based on the use of natural radioactive isotopes of lead 210Pb and carbon 14C; correlation of marker horizons in sediments, such as ash, with historical data on eruptions of nearby volcanoes. The study of sediments makes it possible to recreate a detailed picture of the changing conditions in a given lake. In addition, since lake sediments accumulate information about the natural conditions of the entire drainage basin, they also capture past climatic changes. For example, studying the composition of plant pollen in a lake sediment column allows us to determine which land plants were common at certain stages of geological history, and taking into account the modern environmental requirements of these plant species determines what temperatures and humidity were at that time.
PROBLEMS OF THE STATE OF LAKES
Lakes are ecosystems in which all components are interconnected. In the absence of external influences, the lakes reach a certain state of equilibrium with the environment, which eventually leads to a more or less stable position, when the organisms living in the lakes adapt to the existing conditions. However, lakes are rarely in equilibrium. On the contrary, they are often used as sources of water for irrigation, drinking water, for agricultural purposes, or for the discharge of such products of modern civilization as industrial wastewater, stormwater and agricultural runoff. Lakes are polluted by increasing levels of pesticides, herbicides and airborne organic compounds such as polychlorinated biphenyls, as well as acid rain from pollutant emissions from car engines and thermal power plants. Plant and animal species alien to them penetrate into them, brought in by fishermen on the bottoms of ships and by other random means. Eutrophication, or excessive enrichment of lakes with nutrients from anthropogenic sources, is on the menacing scale, causing significant environmental damage. In some cases, large lakes of economic importance are even under the threat of complete extinction. So, for example, the volume of water in the Aral Sea (a large salt lake) has now halved as a result of the analysis of the waters of the Amu Darya and Syr Darya flowing into it for irrigation. As a result, its salinity increased almost three times (from 9.6-10.3‰ to 27-30‰). Exposed areas of the seabed are blown by dust storms, which lead to the removal of salts and pesticides and their deposition within the nearby inhabited areas. Lake pollution is a very serious problem. For example, in order to reduce eutrophication of water bodies, many countries have adopted laws to limit the concentration of phosphorus in water that has passed through treatment plants and may enter lakes. A whole science of lake restoration has developed, based mainly on empirical relationships relating such indicators as the abundance of algae and water transparency to phosphorus concentrations in lake waters. In some regions, water abstraction from lakes is regulated. The use of pesticides is being carefully studied.
LARGEST LAKES IN THE WORLD
Area, thousand km2
Caspian Sea (Asia - Europe), salty 371.0* Upper (USA - Canada) 82.1 Victoria (Kenya, Tanzania, Uganda) 69.4 Huron (USA - Canada) 59.6 Michigan (USA) 57.8 Aral Sea sea ​​(Kazakhstan - Uzbekistan), salty 36.5 * Tanganyika (DRC, Burundi, Tanzania, Zambia) 32.9 Baikal (Russia) 31.5 Big Bear (Canada) 31.3 Nyasa (Malawi, Tanzania, Mozambique) 29, 0 Great Slave (Canada) 28.5 Erie (USA - Canada) 25.6 Winnipeg (Canada) 24.3 Balkhash (Kazakhstan), salty 22.0* Ontario (USA - Canada) 19.7 Ladoga (Russia) 17, 7 Chad (Niger, Chad, Cameroon, Nigeria), brackish 16.3* Maracaibo (Venezuela) 13.5 Onega (Russia) 9.7 Air (Australia), salty 9.3* Volta (Ghana) 8.5 Titicaca ( Peru - Bolivia) 8.3 Nicaragua (Nicaragua) 8.0 Athabasca (Canada) 8.0 Reindeer (Canada) 6.7 Rudolf (Kenya - Ethiopia), salty 6.5 Issyk-Kul (Kyrgyzstan), brackish 6.2 Kokunor (Qinghai) (China) Salted 5.7* Torrens (Australia) Salted 5.7* Venern (Sweden) 5.7 Albert (DRC - Uganda) 5.6 Nettilling (Canada) 5.4 Winn ipegosis (Canada) 5.39 Cariba (Zambia - Zimbabwe) 5.31 Nipigon (Canada) 4.9 Gairdner (Australia), salted 4.77* Urmia (Iran), salted 4.69 Manitoba (Canada) 4.66 Forest (US - Canada) 4.47 * The area is not constant.
LITERATURE
Bogoslovsky B.B. lake science. M., 1960 Muraveisky S.D. Rivers and lakes. M., 1960

Collier Encyclopedia. - Open society. 2000 .

Synonyms:

There are more than two million freshwater and salt lakes in Russia. The largest lakes in the European part of the country include Ladoga (17.87 thousand km²) and Onega (9.72 thousand km²) in the northwest, Lake Peipsi (3.55 thousand km²) on the Estonian border, as well as the Rybinsk reservoir ( 4.58 thousand km²) on the Volga north of Moscow.

Narrow lakes from 160 to 320 km in length are located behind the dams on the Don, Volga and Kama. In Siberia, similar artificial lakes are located on the upper Yenisei and its tributary, the Angara, where the Bratsk reservoir, 570 km long, is one of the largest in the world. But they are all insignificant compared to Lake Baikal, the largest reservoir of fresh water on the planet. With a length of 636 km and an average width of 50 km, the surface area of ​​Lake Baikal is 31.72 thousand km², and the maximum depth is 1642 m.

There are countless smaller lakes, located mainly in the poorly drained lowlands of the Russian and West Siberian Plains, especially in the more northern regions. Some of them reach significant sizes, in particular, Lake Beloe (1.29 thousand km²), Topozero (0.98 thousand km²), Vygozero (0.56 thousand km²) and Lake Ilmen (0.98 thousand km²) on the territory of the European north-west of the country, and Lake Chany (1.4-2 thousand km²) in south-west Siberia.

List of the largest lakes in Russia

We present to your attention the 10 largest lakes of the Russian Federation with a description, photo and geographical location on the map of the country.

Caspian Sea

The Caspian Sea is the world's largest inland water body (area: 371 thousand km²). It is called a sea, not a lake, because the ancient Romans who arrived in this region discovered that its water was salty and named it the sea after the tribes of the Caspian who lived near the shores of the lake. The Caspian Sea borders the following five countries: Russia, Kazakhstan, Turkmenistan, Azerbaijan and Iran. The main river feeding the lake is the Volga, which provides about 80% of the inflow of the Caspian Sea, and the remaining 20% ​​falls on other smaller rivers.

The Caspian Sea is rich in oil and natural gas deposits, but these are under development. Also, the extraction process is hindered by the problem of dividing the natural resources of the lake between the five countries bordering it. About 160 species and subspecies of fish from 60 genera live in the Caspian Sea and the deltas of the rivers flowing into it. About 62% of the species are endemic.

Baikal

Baikal is the deepest (1642 m), the oldest (25-35 million years) and the most voluminous (23.6 thousand km³) of all lakes in the world, it is a superstar reservoir in the field of hydrology, geology, ecology and history. Today, Lake Baikal contains about 20 percent of the fresh water on the Earth's surface, which is comparable in volume to the entire Amazon River basin. Baikal has 27 islands, including one over 70 km long (Olkhon Island).

More than 1,500 species of animals live off the shores of the lake, 80% of which are found nowhere else on the planet. The most famous representative of the Baikal fauna is the seal, which lives exclusively in fresh water. According to some reports, the population of seals is about 100,000 individuals. Also near the lake there are such large predators as wolves, which occupy the top positions of the Siberian food chain, feeding on deer, birds, rodents and smaller predators.

Ladoga lake

Lake Ladoga is the largest freshwater lake in Europe, located in the north-west of Russia, 40 km east of St. Petersburg. The area of ​​the lake is 17.87 thousand km², the volume is 838 km³, and the maximum depth at a point to the west of Valaam Island it reaches 230 m.

The depression of the lake appeared under the influence of glaciers. The northern shores are mostly high and rocky, and are also separated by deep, ice-covered bays. The southern shores have many sandy or rocky beaches, mostly low, slightly concave, overgrown with willow and alder. In some places there are ancient coastal embankments covered with pine trees. The largest tributaries are the Volkhov, Svir and Vuoksa rivers.

48 different species of fish were found in the lake, of which the most common are roach, carp, bream, pike perch, perch and smelt. Of the 48 species, 25 are of commercial importance and 11 are in the important food fish category.

Lake Ladoga also serves as a key stopping point for migratory birds of the North Atlantic Flyway, which typically mark the arrival of spring.

Lake Onega

Lake Onega is the second largest lake in Europe, located in the north-west of the European part of Russia, between Lake Ladoga and the White Sea. It covers an area of ​​9.72 thousand km², 248 km long and up to 83 km wide. The greatest depth is about 127 m.

The basin of the lake was formed by the movement of the earth's crust and glaciers. The high rocky shores in the north and northwest are composed of layered granite and covered with forest. There are deep bays in Petrozavodsk, Kondopoga and Pevenets. The southern shores are narrow, sandy, often swampy or flooded. Lake Onega has about 1650 islands, covering a total of about 260 km², usually in the northern and northwestern bays.

The lake is home to over 40 species of fish, including vendace (a small member of the salmon family), smelt, burbot bream, pike, perch, roach and salmon. Many types of fish have significant economic value.

Taimyr

Taimyr is the second (after Baikal) largest lake in the Asian part of Russia, located in the central regions of the Taimyr Peninsula. It is located south of the Byrranga mountains, in the zone.

The lake and tundra zone is a popular destination for birds such as geese, swans, ducks, buzzards, peregrine falcons and snowy owls. Lake Taimyr is home to a large number of fish, including grayling, muksun, char and whitefish. Although the area is relatively remote, depletion of stocks of certain commercial fish species is still observed.

Taimyr is famous for the largest population of reindeer in Eurasia. Also in this region there are such animals as argali, arctic fox, wolf and lemmings. In 1975, the area was re-introduced.

The lake and its environs have been included in the Taimyr Nature Reserve since 1983. Scientists have discovered plutonium in the sediments of a lake believed to have entered Taimyr via wind-blown radioactive particles from nuclear tests conducted on Novaya Zemlya during the Cold War.

Khanka

Lake Khanka has an area of ​​4 thousand km², of which approximately 97% is located in Russia. The maximum depth of the lake is 10.6 m, and the average volume is 18.3 km². The lake is fed by 23 rivers, 8 of which are in China, and the rest in the territory of the Russian Federation. The only outflow is the Sungacha River, which flows east to the Ussuri River, which forms the international border, and flows north where it joins the Amur River.

Khanka is famous for being home to the highest diversity of birds in the entire temperate zone of Eurasia. At least 327 species of nesting, wintering and migratory birds have been sighted in the lake area.

Chudsko-Pskovskoe Lake

Lake Peipus-Pskovskoye is the largest transboundary and fifth (after Ladoga, Onega, Swedish Venern and Finnish Saim) lake in Europe, located on the border between Estonia and Russia. It occupies 3.6% of the total area of ​​the Baltic Sea basin. A total of 30 islands are located on Lake Peipsi, and another 40 in the delta of the Velikaya River. Most of them rise only 1-2 m above the water level, and often suffer from floods.

About 54 species of coastal aquatic plants grow in the basin of Lake Peipus-Pskov, including reed, calamus, reeds and various herbs. 42 species of fish live in the waters of the lake, such as smelt, vendace, bream, perch, pike, roach and whitefish. Wetlands serve as important nesting and feeding grounds for migratory birds such as swans, geese and ducks that migrate from the White Sea to the Baltic Sea. The region is home to one of the largest swallow colonies in Estonia.

Ubsu-Nur

Ubsu-Nur is the largest lake in Mongolia in terms of surface area (3.35 thousand km²), as well as the largest salt lake in the country. The Ubsu-Nur basin is one of the most important biodiversity poles of Eurasia. Although most of the lake is in Mongolia, its northeastern shores are located in the Tyva Republic of the Russian Federation.

The lake is shallow, very salty, and is the remnant of a large sea that existed several thousand years ago. The basin covers an area of ​​about 70 thousand km² and is one of the best preserved natural steppe landscapes on the continent. It is here that the northernmost part of the desert and the southernmost part of the tundra meet.

Reed and freshwater river deltas serve as resting and nesting sites for numerous migratory birds. Over 220 species of birds can be found around the lake, including the black stork, osprey, white-tailed eagle, whooper, and black-headed gull. About 29 different species of fish live in the waters of the lake, one of which is suitable for human consumption. The mountainous region is home to Mongolian gerbils, wild sheep and the Siberian ibex.

vats

Although Lake Chany is not well known outside of Siberia, it is one of the largest lakes in the country. Chany is a shallow lake with salty and constantly fluctuating water, the level of which can vary from season to season and from year to year. The lands of the lake basin serve as pastures for cattle.

In terms of area, Beloye is the second (after Onega) natural lake in the Vologda region, and the third (after the Rybinsk reservoir). It is one of the ten largest natural lakes in Europe. The lake has a relatively round shape with a diameter of 46 km. Its area is 1.29 thousand km², and the basin area is about 14 thousand km².

The lake is famous for its fish stocks, the most famous delicacy is the Belozersky smelt. The forage base and high level of oxygen create favorable conditions for the life of many species. The following fish species are common in the waters of the lake: perch, pike, bream, ruff, sabrefish, roach, bleak, burbot, chub, rudd, whitefish, ide, tench, asp, dace and gudgeon).

Table of 10 largest lakes in Russia

lake name Area, km² Volume, km³
Dimensions, km Maximum depth, m
Average depth, m
Caspian Sea371000 78200 1200 by 4351025 208
Baikal31722 23615 636 by 79.51642 744,4
Ladoga lake17870 838 219 by 125230 46,9
Lake Onega9720 285 248 by 83127 30
Taimyr4560 12,8 - 26 2,8
Khanka4070 18,3 90 to 4510,6 4,5
Chudsko-Pskovskoe Lake3555 25 width 5015 7,1
Ubsu-Nur3350 35,7 85 to 8020 10,1
vats1400-2000 - 91 to 887 2,1
White Lake1290 5,2 46 to 3320 4