Contemporary Problems of Ecology, 2008, Vol. 1, No. 1, p. 147-152. http://scipeople.com/publication/99105/; Water self-purification is an important example of ecosystem services. This function of aquatic ecosystems is necessary for sustainable development, and for sustainable use of aquatic resources (water resources), as well for environmental safety. The analysis made by the author in a published paper (Ostroumov, 2008) showed that aquatic ecosystem (both marine and freshwater one) features a multi-component molecular-ecological mechanism for upgrading water quality. In other words, it is a multi-component, multi-process biomachinery for water quality formation and self-purification. The biomachinery includes the following items: (1) sources of energy for self-purification mechanisms, (2) the major taxa of living organisms as components of the biomachinery; (3) the biomachinery contains the functional blocks that perform functions of filters, mills, and pumps. The experiments that the author carried out demonstrated how several types of chemical pollutants may produce damage to this delicate and very useful biomachinery which makes water clean. The experiments demonstrated that some chemical pollutants decreased important and useful functions of aquatic invertebrate animals which contribute to purification of water. We have found that the chemical pollutant as diverse as synthetic surfactants, detergents, salts of Cd, Cu, Pb, Hg, Co, Ti, V (Na3VO4 •12 H2O), and oil hydrocarbons inhibited water filtration by bivalve molluscs, the marine mussels (the Latin name of the mussels: Mytilus galloprovincialis). One of the practical conclusions is that we now see another additional aspect of important hazard from the low, sublethal concentrations of the chemicals that pollute the aquatic environment. Not only death of aquatic organisms, but also even a decrease of their functional activity in polluted aquatic ecosystems poses some danger and risk to the water system. The potential hazard of those low levels of the chemical pollutants is that the potential of the ecosystem to purify water will be decreased. As a final result, in future we may have water of worse quality. This may be of interest to the scientists and specialists in the following fields: aquatic ecology, water science, environmental toxicology and chemistry, ecotoxicology, and water resource management. The reference to the paper where this analysis was published: Ostroumov S. A. Basics of the molecular-ecological mechanism of water quality formation and water self-purification. - Contemporary Problems of Ecology, 2008 (Feb), Vol. 1, No. 1, p. 147-152. [ISSN 1995-4255 (Print) 1995-4263 (Online); DOI 10.1134/S1995425508010177; http://www.springerlink.com/content/e380263154u73045/; http://www.researchgate.net/file.FileLoader.html?key=e533be77c87735c6dcc5cfdb9db96cec;