Sapropels are formed in anaerobic conditions as the result of physical and chemical, as well as biological transformations of the lakes’ aquatic organisms provided different levels of the mineral and organic components involvement from terrigenous flow. The lakes flowage factor plays a great role in the benthic sediments formation. The average annual increase of sediments is 1 to 6.6 mm.
The sapropel sediments in modern lakes are typically not older than 12 thousand years. The freshwater pools sediments with organic substance content not less than 15% are considered sapropel, or else, if the organic substance content is lower, they are considered limnetic sediments. The composition and properties of sapropels from different deposits vary widely due to the parent lake productivity, surface flow peculiarities and climatic conditions.
Sapropel is a jelly-like homogeneous mass, its texture in upper layers is close to cream-like, and in lower layers it becomes denser. The sediments are odorless except for separate types which smell of hydrogen sulfide. Sapropel color depends on organic substance and mineral additions.
Sapropel consists of silt solution, skeleton and colloidal complex. Silt solution includes water and substances dissolved in it – mineral salts, low molecular weight organic substances, vitamins and enzymes. The sapropel skeleton consists of non-decomposed flora and fauna remnants, and the colloidal complex includes compound organic substances causing the jelly-like texture of sapropel.
Sapropel properties are determined by three principal components: water, cindery share and organic substances of a very complicated and versatile composition. Natural humidity of sapropel sediments is 84–96% (in average – 88.4%). The humidity differences can be explained by the versatility of the sapropels chemical composition and different ratio between the cindery and the organic share. The bigger is organic share, the higher is sapropel humidity. Principal share of the water retained by sapropel (up to 80%) is loosely bound water of macropores, retained in the material mechanically, 12–15% is the water immobilized inside of soft colloids, 8–15% is physically bound water, including 3–5% of tightly bound one. Loose water is a medium for microbiological and related processes in the sapropels due to which a range of substances accumulates therein. Well developed specific surface of sapropels spurs the development of chemical interaction processes between water and solid phase, thus water becomes saturated with many soluble organic and mineral components. Thus the chemical composition of lake sediments water phase has higher overall mineralization compared to the relevant lake water, increased content of specific macro- and microelements.
Organic matter in sapropels is represented by bitumoids, carbohydrate complex (hemicellulose and cellulose), humic substances (humic acids, fulvic acids), nonhydrolyzed residue. Organic matter content in sapropels is 15–95% of the dry mass. The variegated nature of sapropel formers caused the formation of sediments with different share of organic matter. Humic acids are a principal group of biologically active substances in sapropels, their content is sapropel sediments widely varies, from 4–9 to 50–60% of organic matter. Moreover, the organic matter includes carotenes, chlorophyll, xanthophylls, stearines, organic acids, alcohols, hormone-like substances, enzymes, vitamins of B group (В1, В2, В6, В12), С, Е, Р and other compounds. The nitrogen amount in sapropels of different types is 2.7–6.0% of the organic matter. 25-50% of nitrogen is included in aminoacids. There were 17 aminoacids found in sapropels (lysine, arginine, methyonine, leucine etc.). Hemicelluloses content is 5–8% of the organic matter.
Mineral share of the sapropels contains a lot of microelements, such as Co, Mn, Cu, B, Br, Mo, V, Cr, Be, Ni, Ag, Sn, Pb, As, Ba, Sr, Ti, and many other enzymes, e.g., catalases, peroxidases, reductases, proteases.
Thus, sapropel is an organic and mineral complex of substances, which contains wide range of chemical compounds and elements, including biogenic stimulants, vitamins and minerals.