The roles of three size groups of metazoans (meio-, macro-, and megabenthos) in bottom communities are assessed by the example of the Neva Bay. During the study period (in 2010 and 2011), meiobenthic organisms were dominant in bottom communities of the bay in abundance, while megabenthic organisms (large mollusks of the families Unionidae and Viviparidae) were dominant in biomass. Macrobenthic organisms were dominant in functional parameters (production, metabolic cost, and energy flow) and accounted for about 3/4 of the total energy flow through the bottom community of the bay. The proportional abundance levels of different size groups were inconstant; larger animals (macro- and megabenthos) were distinguished by greater long-term variation in abundance and biomass when compared to meiobenthos. It is shown that megabenthos (>1 cm) should be distinguished as a separate size category in studies of fresh-water bottom communities, as is done in marine biology.