A body of water with low organic pollution. In such a reservoir, oxygen is present in significant quantities, hydrogen sulphide is present in insignificant quantities, there are no undecomposed proteins and, in addition to nitrates and nitrites, there are slightly acidic nitrogen compounds. The corresponding complex of organisms is characteristic. A moderately polluted reservoir, a reservoir with an average content of organic matter, in which protein degradation processes take place with a fairly high number of saprophytic bacteria (from tens to several thousand specimens in 1 ml of water). There are no non-decomposing proteins in the mesosaprobe reservoir, hydrogen sulphide (H2S) and carbon dioxide (CO2) are low, oxygen is present in significant quantities, but the water contains slightly oxidised nitrogen compounds - ammonia (NH3), amino and amido acids.

Saprobionts are divided into three groups: organisms of wastewater proper - polysaprobionts (p-saprobionts), organisms of highly polluted waters - mesosaprobionts (two subgroups - α-mesosaprobionts and β-mesosaprobionts), organisms of slightly polluted waters - oligosaprobionts (o-saprobionts). In order to assess the degree of organic pollution, four pollution zones have been defined: poly-, α-meso-, β-meso- and oligosaprobic. The mesosaprobic zone is close to the polysaprobic zone in the nature of the biochemical processes, but here free oxygen is present, hydrogen sulphide and methane are absent, but it differs from the rest by the predominance of oxidative over reducing processes. Due to the intensive photosynthesis of many plants in summer, the waters are oversaturated with oxygen.