International Journal of Oceans and Oceanography Year : 2006, Volume : 1, Issue : 1 Print ISSN : 0973-2667. Biomachinery for maintaining water quality and natural water self-purification in marine and estuarine systems: elements of a qualitative theory Ostroumov  S.A. Faculty of Biology, Moscow State University, Vorob'evy Gory, Moscow, 119992 Russia. Abstract Basic elements are formulated for a qualitative theory of the polyfunctional role of the biota in maintaining self-purification and water quality in aquatic ecosystems. The elements of the theory covers the following: (1) sources of energy for the mechanisms of selfpurification; (2) the main functional blocks of the system of self-purification; (3) the list of the main processes that are involved; (4) analysis of the degree of participation of the main large taxons; (5) degree of reliability and the main mechanisms providing the reliability; (6) regulation of the processes; (7) the response of the system towards the external influences (man-made impacts); (8) the analogy between ecosystems and a bioreactor; and (9) conclusions relevant to the practice of biodiversity conservation. In support of the theory, results are given of the author's experiments which demonstrated the ability of some pollutants (surfactants, detergents, and some others) to inhibit the water filtration activity of marine filter-feeders (namely, the bivalve mollusks Mytilus galloprovincialis, Mytilus edulis, and Crassostrea gigas). key words pollutants (surfactants detergents), inhibiting the water filtration activity, marine filter-feeders, the bivalve mollusks Mytilus galloprovincialis, Mytilus edulis, Crassostrea gigas, functioning of ecosystems, ecosystem services, water quality, self-purification, S.A. Ostroumov