Effect at the ecosystem level of elevated atmospheric CO2in an aquatic microcosm

Abstract

We studied the responses of an aquatic microcosm in two different eutrophic conditions to elevated atmospheric CO2concentration. We used microcosms, consisting of Escherichia coli(bacteria), Tetrahymena thermophila(protozoa) and Euglena gracilis(algae), in salt solution with 50 and 500 mg l−1of proteose peptone (eutrophic and hypereutrophic conditions, respectively) under ambient and elevated CO2(1550±100 μl l−1) conditions. The density of E. gracilisincreased significantly under elevated CO2in 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. thermophilawas maintained in higher level than that in the microcosm with ambient CO2and the density of E. coliwas decreased by CO2enrichment. Calculating the carbon biomasses of T. thermophilaand E. colifrom their densities, the changes in their biomasses by CO2enrichment were little as compared with large increase of E. graciliscarbon biomass converted from chlorophyll a. From the responses to elevated CO2in the subsystems of the hypereutrophic microcosm consisting of either one or two species, the increase of E. graciliswas a direct effect of elevated CO2, whereas the changes in the density of E. coliand T. thermophilaand 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 CO2were prominent in the hypereutrophic microcosm.