Effect at the ecosystem level of elevated atmospheric C0₂ in an aquatic microcosm

We studied the responses of an aquatic microcosm in two different eutrophic conditions to elevated atmospheric CO₂ concentration. We used microcosms, consisting of Escherichia coli (bacteria), Tetrahymena thermophila (protozoa) and Euglena gracilis (algae), in salt solution with 50 and 500 mg l ^-1 of proteose peptone (eutrophic and hypereutrophic conditions, respectively) under ambient and elevated CO₂ (15504±100 μl l^-1) conditions. The density of E. gracilis increased significantly under elevated CO₂ in both eutrophic and hypereutrophic microcosms. In the eutrophic microcosm, the other elements were not affected by elevated CO₂. In the hypereutrophic microcosm, however, the concentrations of ammonium and phosphate decreased significantly under elevated CO₂. Furthermore, the density of T. thermophila was maintained in higher level than that in the microcosm with ambient CO₂ and the density of E. coli was decreased by CO₂ enrichment. Calculating the carbon biomasses of T. thermophila and E. coli from their densities, the changes in their biomasses by CO₂ enrichment were little as compared with large increase of E. gracilis carbon biomass converted from chlorophyll a. From the responses to elevated CO₂ in the subsystems of the hypereutrophic microcosm consisting of either one or two species, the increase of E. gracilis was a direct effect of elevated CO₂, whereas the changes in the density of E. coli and T. thermophila and the decreases in the concentration of ammonium and phosphate are considered to be indirect effects rather than direct effects of elevated CO₂. The indirect effects of elevated CO₂ were prominent in the hypereutrophic microcosm.