Two of 4 replicate 700-liter aquatic microcosms each were stocked with 2 mosquito fish (Gambusia affinis). The dominant zooplankter shifted from the large cladoceran Simocephalus vetulus to the smaller Alona guttata. The subsequent release of grazing pressure resulted in a rise in both phytoplankton and bacteria levels, which in turn were responsible for an increased rotifer biomass. Particulate organic carbon was higher and dissolved inorganic nitrogen was lower in the presence of Gambusia, reflecting a net shift of nutrients from inorganic to organic form, presumably because of smaller zooplankton respiratory losses. Ratios of particulate to dissolved organic carbon and of phytoplankton carbon to chlorophyll a were unnaturally high in the microcosms containing fish. An increase in total nitrogen was deduced for all 4 systems during the experiment; the increase could be explained by the presence of heterocystous Anabaena sp, and was independent of the presence of fish.

"Running head: Microcosm trophic structure." Research supported by the U.S. Energy Research and Development Administration and the Environmental Protection Agency D5-E681 through contract #77BCC. "LBL-5978." Issued Aug. 1977. Includes bibliographical references (pages 14-15).