In work results of research of intensive development of some kinds of seaweed in reservoirs and on a surface of soil in the conditions of Moldova are presented. So-called “fl owerings”, especially waters, usually negatively affects on functioning natural ecosystem as with ecological, and the economic point of view, and also negatively affects on health of people. It is especially strongly shown when toxic kinds of seaweed intensively develop. Losses from destruction of fi shes, cancers and others sea animal, together with house which use water from these reservoirs, are estimated in millions dollars. Authors consider, that a major factor which promotes intensive development are toxic seaweed pollution of reservoirs by sewage which is contain big quantities of various chemical elements.