The degree to which the decomposition of organic material is occurring in an aquatic environment.

The saprobity system is based on the observation that in the course of the self-purification process a body of water shows distinct zones of decreasing pollution (or improved water quality); these zones are termed polysaprobic (gross pollution), alpha-mesosaprobic, beta-mesosaprobic, and oligosaprobic; the latter may be divided into alpha- and beta- oligosaprobic.

Each zone is characterized by a particular content of oxygen, organic matter, products of septic decay, and products of mineralization. Biologically, each zone affords optimal conditions for certain species and communities of organisms, the so-called "indicator" organisms.

The particular saprobity zones may be characterized as follows:

• Polysaprobic zone, heavy pollution with sewage or other organic materials, mass development of bacteria that are involved in decomposition processes, a high rate of oxygen consumption, and a high production of ammonia and hydrogen sulfide.
• Alpha-mesosaprobic zone, vigorous oxidation processes, increased dissolved oxygen though oxygen consumption is still high, no hydrogen sulfide production, oxidation of ammonia starts.
• Beta-mesosaprobic zone, much dissolved oxygen, low oxygen consumption, mineralization of organic materials, and large amounts of the end-products of mineralization; for example, nitrates.
• Oligosaprobic zone, all mineralization processes have been completed, the dissolved oxygen content is high and oxygen consumption nearly zero; the beta-oligosaprobic level is characterized by rather moderate variety of species and low bioactivity, while the alpha-oligosaprobic level is characterized by a comparatively large variety of species and high bioactivity.