It is shown that the level of total amylolytic activity (TAA) in the whole organism of potential preys (belonging to various taxonomic groups) of planktivorous and benthivorous fishes, as well as in the associated microbiota, depends largely on pH values in a medium. Maximal values of TAA in all species of invertebrates and in the micro flora isolated from their bodies are observed at pH 7.0; minimal ones are found at pH 10.0. The microbiota of Dreissena and of crustacean plankton may efficiently depolymerize polysaccharides within the zone of acid pH values.