Lakes on a global scale constitute an enormous amount of water. They take up approx 1,8 % the surface of the earth, tj. about 2,5 million km² and are stored 1,17 x 105 km³ of water. The Caspian Sea covers the largest area (424 300 km², year 1977), however, it is relatively shallow (recently, its surface area has been rapidly diminishing as a result of less water inflow, thus the salinity of its waters increases). For this reason, the group of interconnected North American lakes with an area should be considered the largest freshwater lake 242 000 km2. If we take into account the volume of water accumulated, these are the largest lakes of Tanganyika (35 000 km2, 1435 m deep) and Baikal (31 500 km², 1741 m deep and approx 23 500 km3 of water).

The lakes were created by the forces of nature, mainly as a result of volcanic phenomena (tectonic) or glacier impacts (glacial processes). We distinguish between three main types: tectonic, moraine and gutter. Lakes, from a geological point of view, they are among the youngest creatures, and most of the glacial lakes are younger than 10 thousand years, the older tectonic lakes are from the Tertiary period.

In lakes and other stagnant waters, living conditions are shaped according to specific rules concerning mainly light and thermal conditions, oxygen content and its variation in the water column, food substances which form a suspension and are dissolved in water. The changes in these conditions depending on the correct one are also characteristic (caused by temperature) or incorrect (caused by the wind) water movement.

Main zones of light transmittance in stagnant waters.

Due to the light conditions in stagnant waters, three main zones are distinguished:

- littoral zone - shallow water zone, where light penetrates to the bottom and where favorable conditions are created for the existence of plants taking root;

– pelagic zone (limnetic) – free area, thoroughly overexposed water. The lower boundary of this zone is determined by the so-called. compensation level, at which the intensity of photosynthesis is equal to the intensity of respiration (oxygen consumption), which in practice corresponds to the depth, which it reaches 1 % total sunlight;

– profundal zone – the bottom and the area of ​​deep water below the limit defined by the compensation level (without light).

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