The site of the “Scientific Russia” project has published a large amount of material about Lake Glubokoe.
The water column of the lakes is divided into epilimnion, metalimnion and hypolimnion (“limnos” - Greek for “lake”), that is, into the upper layer of the lake, middle and lower. The boundaries of the layers are determined depending on the temperature of the water. In summer, the epilimnion is usually warm, with the same temperature throughout the layer. Only in its lower part does it decrease slightly. The epilimnion temperature in the middle of summer reaches 20-25 degrees. In the lower part of the lake (in hypolimnion), the temperature is also even, but very cold, within 5-6 degrees. At times it can be slightly lower.
In the middle part of the lake (metalimnion), the water temperature drops abruptly. In the upper part of this layer, it can be within 20 ° C (as in the epilimnion), and after 3-5 m - already 7-10 ° C, or even lower. So, for example, in Lake Glubokoe (Moscow region), the temperature drops from 20 degrees to 10 ° C.
On the shore of this lake there is a biological station “Glubokoe Ozero” (“Lake Glubokoe”), which was founded in 1891. All this time, hydrobiological work has been carried out there. It belongs to the A.N. Severtsov Institute of Ecology and Evolution of RAS.
By the middle of the warm summer, the thickness of the epilimnion reaches 5-7 m, the metalimnion - 3-7 m or more, depending on the heating of the water. The hypolimnion at the end of summer starts from 7-10 m and extends to the maximum depth. Lake Glubokoe has a depth of 32 meters.
Such a clear stratification of the lake thickness is observed in the absence of mixing of waters due to currents or wind waves. Lake Glubokoe has a small area (about 60 hectares), surrounded by forests and to a small extent exposed to wind impact. In large bodies of water, the thermocline can be rapidly eroded due to mixing of waters.
Now let us consider what happens to the water during spring. Under the sun, the ice begins to melt. The rays, passing through the ice, heat the uppermost layer of water, which gives off heat to the nearby ice. So the ice melts not only from above, but also from below. In central Russia, water bodies are freed from ice in April or early May, depending on the weather.
The water continues to gradually warm up, soon it reaches a temperature of 4 ° C. It should be noted that water has an anomalous property: the highest density of water is observed not at 0 ° С, but at 4 ° С (to be more precise, at 3.8 ° С). At this temperature, the water is the heaviest (densest). The lower layers of the lake have a temperature of about 3-5 ° C or slightly lower.
As a result, heavy water, like an avalanche, rushes downward, displacing water from the lower layers. This is how the spring mixing of waters takes place. At the same time, the lower layers of the lake are enriched with oxygen, and the water column, including the upper layer of the lake, is enriched with minerals. At the bottom of the lake in winter, the destruction (decay) of settled organic matter (detritus) and the release of mineral salts into the environment occurs. Unwanted gases such as carbon dioxide, methane, hydrogen sulphide are also removed from the deep water layers with stirring. Sunlight and mineral salts help the development of microscopic algae, which enrich the epilimnion with oxygen. In early spring, mainly cold-loving algae develop.
The water gradually warms up during spring and summer, finally reaching 20-25 ° C. This happens in late July or early August. Epilimnion slowly warms up and gives off heat to the underlying layers. The less dense warm water lies on colder and denser water. Of course, during strong wind waves, these layers are mixed, and the stratification may not be so obvious.
In central Russia, the temperature of water bodies usually rises until the beginning of August, and then begins to decrease again. Finally, the water temperature drops to 4°С by the beginning - mid-October. The water becomes denser again and descends like an avalanche. The next mixing of waters takes place already in autumn. The temperature of the lake is leveling off, unwanted gases that have accumulated in the bottom layer over the summer are mixed with the bulk of the water or go into the atmosphere. It is mainly methane, hydrogen sulfide, which are toxic to living things. Enriched with oxygen, the water of the deep layers becomes suitable for life. This happens from year to year.
The upper layer of the lake is enriched with oxygen mainly due to the vital activity of phytoplankton. Photosynthesis is sometimes so intense that excess oxygen does not have time to escape into the atmosphere. In the absence of disturbance, if you put your hand in the water and make a sharp movement with your fingers, gas (oxygen) bursts out of the water, like from a bottle of soda.
Planktonic organisms living in the reservoir die off sooner or later. Many of them live from a few days to one month or a little more. These are microscopic algae, bacteria, zooplankton and others. Dying off, they slowly settle in the water column. The smaller the particles, the slower they settle. Animals, in particular crustaceans, fish, in the process of their vital activity, produce excrement, which also sinks to the bottom. Bacteria living in the water column and at the bottom mineralize the incoming organic matter with the release of mineral salts.
The destruction (decay) of organic matter is carried out with the consumption of oxygen. At the bottom, it has nowhere to come from, because enrichment occurs only in spring and autumn. This leads to the fact that the amount of oxygen gradually decreases, from the bottom to the surface. By the beginning of summer (after the spring mixing of waters) an oxygen-free zone can form at the bottom, which gradually spreads to the upper layers of the lake. Ultimately, by the end of August - beginning of September, the oxygen-free zone can reach 1/3 (sometimes even 1/2) of the lake's thickness. In Lake Glubokoe, the absence of oxygen was observed at a depth of 15-20 m. This is despite the fact that the maximum depth of this lake is 32 m.
During the summer, a large amount of organic matter enters the bottom of reservoirs (due to the death of aquatic organisms, foliage or different kinds of pollution), therefore, in winter, the entire thickness of the lake can lose oxygen. This leads to the death and death of a large number of fish and other aquatic organisms. They can only be saved by ice holes or by blowing air into the water column. However, it is necessary to blow through the water column of the lake very carefully so as not to stir up the silt at the bottom and not aggravate the situation.
The autumn and spring mixing of waters can be compared to the breathing of an organism. When you exhale, unwanted gases are removed, the gas composition of the lake is leveled, and when you inhale, the deep waters are enriched with oxygen.
What happens in the metalimnion? A sharp temperature drop is observed in this layer. In particular, in Lake Glubokoe, it can reach 10-12 degrees. In the epilimnion, to a depth of 7-8 m, the temperature is relatively even, and below - to a depth of 8-10 m, it is much colder. The warm water of the epilimnion lies, as it were, on the denser cold water.
Dead organisms, crustacean shells, which they shed during molting, excrement, etc. settle in the thickness of the lake. The sizes of these particles are small (within a few tens of microns), and they settle in the water column very slowly. The sedimentation of particles is prevented by a thermocline. The layer of the temperature jump differs from the overlying layers of water according to its increased density, which contributes to the retention of small particles in it, which are already settling at a low speed. In this regard, the presence of a large number of various particles is observed in the layer of the temperature jump, especially after the dying off of algae. They seem to lie in this layer, gradually decomposing under the influence of microorganisms. Because of this, the metalimnion is figuratively called the "second bottom". If you take a sample of water from this layer with a special device (bottle meter), you can detect the suspension with the naked eye. Under the microscope you can observe flakes of stuck together particles. The settling suspension is called detritus (from Latin - "worn out").
The presence of a large amount of organic substrate in the metalimnion creates favorable conditions for the life of bacteria, and accumulations of food particles attract zooplankton (crustaceans, rotifers, protozoa) to this layer, which, together with bacteria, takes part in the mineralization of organic matter and the consumption of oxygen. Its amount in this layer drops sharply. Intensive processes of destruction in the metalimnion lead to the appearance in this layer of the so-called metalimnial oxygen minimum. An interesting situation arises: in the epilimnion the amount of oxygen is large, in the hypolimnion it is also sufficient, but in the metalimnion it decreases to the minimum values (sometimes reaching the analytical zero).
In Lake Glubokoe, photosynthesis occurs mainly in the upper two meters, but the bulk of the algae is located at a depth of 5 m, above the temperature jump layer, where biogenic elements are present in sufficient quantities. In this layer, the biomass has been dominated by the cyanobacteria Coelosphaerium kuetzingianum, Microcystis aeruginosa, Oscillatoria agardhii, green Sphaerocystis polycocca, and dinophytic algae Peridinium cinctum. This is due to the fact that the decomposition of organic matter leads to the enrichment of this layer with mineral salts, which are always insufficient in water, especially in the second half of summer. Algae descend to a depth of 4-6 m to gain access to minerals. At these depths, there is little light, not enough for full-fledged photosynthesis, but algae gain access to mineral salts.
The work was carried out within the framework of the scientific school of the Moscow State University "The future of the planet and global environmental changes".
A.P. Sadchikov, professor at Moscow State University, vice-president of the Moscow Society of Nature Experts
Photos from the archive of the portal "Scientific Russia"
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