COMMUNITY DEVELOPMENT IN FRESHWATER MICROCOSMS

Two cylindrical freshwater microcosms with a volume of 700 {ell} were maintained under controlled laboratory conditions for 190 days. The two microcosms were identical with regard to initial chemical composition and biological inocula, with the exceptions that in one microcosm (designated Tank 2) mosquitofish (Gambusia) and herbivorous catfish (Placostomas) were added. Three distinct communities developed in the tanks: (1) a phytoplankton-zooplankton assemblage and (2) two periphyton-zoobenthos communities associated with the sides and bottom of the tank, respectively. Community development and successional patterns were similar in both tanks. Major differences between the tanks involved timing of succession of the zooplankton and zoobenthos, attributable to predation by fish, principally Gambusia. A major drawback for these microcosms as use for experimental analogs such as lakes was a luxuriant periphyton growth which eventually overwhelmed the biomass of the system. The tanks displayed a degree of successional replicability, a large number of species, and a diversity of community development. Microcosms of this size could find use as experimental systems for higher level trophic manipulation and observation of life cycles not amenable to field studies