The aim of this thesis is to incorporate the smallest organisms, in particular the heterotrophic bacteria and their predators, more fully into current descriptions of aquatic community structure and dynamics. A strong, positive empirical relationship was found between bacterial abundance and chlorophyll concentration in freshwater and marine systems. Common members of the photosynthetic phytoplankton (all chrysophyceans) were shown to be major, even dominant, grazers of these bacteria. This phagotrophic capability is quantitatively important to the bacterioplankton, to the mixotrophs themselves, and in some cases, to the structure of the limnological community as a whole. Grazing by mixo- and heterotrophic protozoans is concentrated on the largest bacterial cells that also have the greatest growth rates. Relatively lower removal rates of the tiniest cells, with low growth rates, is proposed to explain their numerical dominance in lakes.

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