On the basis of investigations 1985-1995 the role of amphipods Monoporeia affinis (Lindström) in bottom communities of eastern Gulf of Finland is analyzed. The dissertation is the first detailed research of the macrozoobenthos of this badly investigated area of the Baltic Sea. The species composition, structure, ecological features and distribution of the bottom macrofauna of eastern Gulf of Finland are investigated with special reference to dominant species M. affinis. The data on seasonal and interannual variability of bottom communities are presented.

Growth and mortality of M. affinis are density dependent. Is established, that food is primary factor determining growth rate, duration of life cycle and the quantitative development M. affinis. The density-dependent mortality as well likely connected with predator activity of isopods Saduria entomon. Is shown, that the significant interannual fluctuations of abundance of amphipods populations are caused by an intraspecific competition for food resources. Production of populations M.affinis is calculated. The role M.affinis in consumption of organic matter is quantitatively appreciated. On the basis of 10 years observations a role of this species in formation of long-term changes of bottom communities is discussed.