EFFECTS OF CO2 ON COMPETITION BETWEEN A CYANOBACTERIUM AND EUKARYOTICPHYTOPLANKTON

To distinguish whether there is a causal link between cyanobacterial d ominance 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, 5 00, and 5000 mu equiv. L-1) and CO2 (ca. 1.3, 13, and 130 mu M) genera ted three pH values (ca. 7, 8, and 9). At the end of about a 1-week in cubation, Aphanizomenon flos aquae was the only cyanobacterium present , while the chlorophytes Scenedesmus and Selenastrum along with uniden tified flagellates comprised the eukaryotic phytoplankton. The treatme nts had a dramatic effect on cyanobacterial biomass, which varied from >90% to 0% of the total phytoplankton biomass across treatments. Vari ation in percent cyanobacteria was better related to pH than to CO2. A t pH 8 and 9, percent cyanobacteria was relatively high at all CO2 con centrations. Only at pH 7 was percent cyanobacteria negatively related to CO2 concentration. These results demonstrate both direct and indir ect effects of CO2 on cyanobacterial dominance but suggest that, for A , flos aquae, the indirect impact of CO2 (pH alteration) is most impor tant. The impact of CO2 on this cyanobacterium, therefore, depends on the alkalinity of the system.