Effects of CO2on competition between a cyanobacterium and eukaryotic phytoplankton

To distinguish whether there is a causal link between cyanobacterial dominance and low CO 2 and (or) the associated high pH, we ran duplicate competition experiments using a factorial design of CO 2 by alkalinity. In various treatments, three concentrations of alkalinity (ca. 50, 500, and 5000 µequiv. ·L -1 ) and CO 2 (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 CO 2 . At pH 8 and 9, percent cyanobacteria was relatively high at all CO 2 concentrations. Only at pH 7 was percent cyanobacteria negatively related to CO 2 concentration. These results demonstrate both direct and indirect effects of CO 2 on cyanobacterial dominance but suggest that, for A. flos aquae, the indirect impact of CO 2 (pH alteration) is most important. The impact of CO 2 on this cyanobacterium, therefore, depends on the alkalinity of the system.