Effects of CO2on competition between a cyanobacterium and eukaryotic phytoplankton

To distinguish whether there is a causal link between cyanobacterial dominance and low CO2and (or) the associated high pH, we ran duplicate competition experiments using a factorial design of CO2by alkalinity. In various treatments, three concentrations of alkalinity (ca. 50, 500, and 5000 µequiv. ·L-1) and CO2(ca. 1.3, 13, and 130 µM) 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 CO2concentrations. Only at pH 7 was percent cyanobacteria negatively related to CO2concentration. These results demonstrate both direct and indirect effects of CO2on cyanobacterial dominance but suggest that, for A. flos aquae, the indirect impact of CO2(pH alteration) is most important. The impact of CO2on this cyanobacterium, therefore, depends on the alkalinity of the system.