Sapropel consists of bottom sediments of freshwater lakes, containing more than 15% organic matter (OM) formed at the bottom of the freshwater bodies with reduced oxygen availability due to incomplete decomposition (oxidation) of dead and sank to the bottom biomass generated in the water (different algae, phyto-and zooplankton, aquatic plants, living organisms, pollen and spores of higher plants). Sapropel by its origin can be indigenous (sapropel accumulates only due to the lake biomass) and allochthonous (the sediments accumulate due to the humus matter getting from the catchment area and the adjacent lake peatlands).
The most valuable is autochthonous sapropel with a maximum content of organic matter (OM), because the original biomass and processes of its biodegradation and transformation of organic matter in sapropel in the freshwater bodies do not lead to the formation in OM of polynuclear aromatic structures (such as benz (o) pyrene) comparing with the formation of soil, peat and especially coal humic substances.
Sapropel is a complicated organic or organomineral complex of diverse chemical compounds preserved in the original biomass (algae residues, plant, etc.) and newly-formed ones in the process of biochemical transformation of organic substances (humic substances, vitamins, enzymes, free amino acids).
When the humus matter does not get into a lake from the catchment area, a depth of biochemical transformation of organic matter is indicated by the quantitative content and the chemical structure of humic acids. High amount of humic acids indicates the depth of the transformation of organic matter. If the process of profound transformation of OM is not passed, such sapropel retains most of the original components of biomass (carotenoids, flavonoids, zoo- and phytosterines, etc.) in their unmodified form or slightly modified form with a low content of humic acids (2-10%), and the color of sediments is green or light olive.
The deeper the transformation of sapropel is (only in the upper layer), the more dark color the sediments have (the accumulated humic acids of dark brown or black color cause the color change of the sediments to dark olive, brown or black).
Of course, with increasing depth of initial biomass biochemical decomposition, the sediments decrease in amount of organic matter because of its increased ash content after utilization of labile compounds (carbohydrates, pectin and other hydrolysable compounds). However, the total share of the major biologically active components of sapropel (lipids, humic acids, hydrolysable compounds) is maintained at about 70% of OM due to the reduction of easy-hydrolysable substances and increase of humic acids.
Due to the almost complete degradation (95-99%) of fragments of the original biomass, sapropel is characterized by homogeneous structure with high content of fine disperse particles (less than 0.05 mm) and high water-holding capacity (up to 25 g per 1 g of sapropel). The water captured in ultra- and micropores of sapropel particles has a structural memory of the distant times and is a unique peloid solution. The peloid solution, compared with the lake water, is enriched with biogenic amines, volatile fatty acids, free amino acids, vitamins and enzymes, simple phenols, microelements and other products of the organic matter decomposition.