Effects of CO2 on competition between a cyanobacterium and eukaryotic phytoplankton

To distinguish whether there is a causal link between cyanobacterial dominance and low CO2 and (or) the associated high pH, we ran duplicate competition experiments using a factorial design of CO2 by alkalinity. In various treatments, three concentrations of alkalinity (ca. 50, 500, and 5000 mu equiv. L-1) and CO2 (ca. 1.3, 13, and 130 mu M) generated three pH values (ca. 7, 8, and 9). At the end of about a 1-week incubation, Aphanizomenon flos aquae was the only cyanobacterium present, while the chlorophytes Scenedesmus and Selenastrum along with unidentified flagellates comprised the eukaryotic phytoplankton. The treatments had a dramatic effect on cyanobacterial biomass, which varied from >90% to 0% of the total phytoplankton biomass across treatments. Variation in percent cyanobacteria was better related to pH than to CO2. At pH 8 and 9, percent cyanobacteria was relatively high at all CO2 concentrations. Only at pH 7 was percent cyanobacteria negatively related to CO2 concentration. These results demonstrate both direct and indirect effects of CO2 on cyanobacterial dominance but suggest that, for A, flos aquae, the indirect impact of CO2 (pH alteration) is most important. The impact of CO2 on this cyanobacterium, therefore, depends on the alkalinity of the system.