The saprobic system is a tool to measure water quality, and specifically it deals with the capacity of a water body to self-regulate and degrade organic matter. The saprobic system derives from so-called saprobes — organisms that thrive through degradation of organic matter, which is called saprotrophic nutrition.
The saprobic system is based on a survey of indicator organisms. For example, the abundance of Lymnaea stagnalis water snails and other organisms is estimated, and using a formula, the listed saprobic and tolerance values of the organisms allow the water quality grade — the saprobic index — to be computed.
Saprobic water quality is expressed in four classes ranging from I to IV; and with three intermediate grades (I-II, II-III and III-IV). Water bodies of class I are the cleanest and of the highest quality. The inherent drawback of the saprobic systems as a water quality measure is that it only regards biodegradable organic material, and so ignores other factors like heavy metal pollution. Though the presence of certain organisms can rule out the presence of toxic substances, the incorporation of such organisms would deviate from the saprobic system's concept.