COMPARISON OF MICROBIAL TROPHIC INTERACTIONS IN AQUATIC MICROCOSMS DESIGNED FOR THE TESTING OF INTRODUCED MICROORGANISMS

Two aquatic microcosms of different complexity were calibrated with a eutrophic lake. The simple microcosm consisted of an intact sediment c ore with overlying water. The complex microcosm was compartmentalized (phytoplankton tank, grazer tank, and sediment cores) to allow manipul ation of individual components of the microbial food web. The comparis on showed that whereas average phytoplankton biomass and productivity were approximately 30% lower in microcosms, biomass and activity of th e heterotrophs (bacteria and nanoflagellates) were equal to or higher (up to 40% higher) than the corresponding field values. Important comp onents of the microbial loop were represented in both microcosms. In t he field and simple mi crocosm, approximately 30% of the phytoplankton production was cycled through the microbial loop, whereas close to 50 % was fluxed through the loop in the complex model. Similarly, bacteri al production amounted to almost the same fraction of the primary prod uction in the simple microcosm and the field (16 and 18%, respectively ) but 29% in the complex model. The percentage of the primary producti on that ended up as nanoflagellate production, on the other hand, was almost similar in all systems (0.4%). These data indicate that while t he simple microcosm was the better surrogate of the field, variability between replicate microcosms of the complex model was lower (10-25% a nd 25-41% for the complex and simple models, respectively). The choice of model, therefore, depends on the questions asked. The results sugg est that the trophic-level analysis is useful in calibration studies o f microcosms intended for testing of microorganisms.