Effect at the ecosystem level of elevated atmospheric CO2 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 (1550 +/- 100 mul 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.