When buoyant cyanobacteria form a surface bloom, they may obtain inorganic carbon from the water and the atmosphere. In this study, artificial blooms of cyanobacteria were exposed to different concentrations of CO(2 )in the atmosphere and HCO3- in the water. The conditions and photosynthetic activity within the bloom were measured with microelectrodes sensing O-2, pH, and light. Net rates of photosynthesis increased with the concentration of CO2 in the atmosphere (air = 350 ppm). However, even under 3,500 ppm CO2, reducing the concentration of inorganic carbon in the water reduced rates of photosynthesis. Thus, both air and water acted as sources of inorganic carbon. Bloom formation may give cyanobacteria an advantage in competition for light and inorganic carbon with other groups of phytoplankton; by placing a dense biomass close to the water surface they are able to intercept a large amount of the flux of light and inorganic carbon. The high surface pH produced in the bloom will promote influx of CO2 from the atmosphere by "chemical enhancement." The obvious advantage of being close to the water surface may be offset by the risk of photoinhibition at high photon irradiance. The dense packing of colonies in the bloom furthermore caused a high local demand for inorganic carbon and consequently extreme carbon depletion. This combination of high photon irradiance and carbon limitation proved to be especially detrimental in blooms. When the availability of inorganic carbon was restricted, nonphotochemical quenching of fluorescence increased, showing some removal of excess excitation energy, but not to the extent that photodamage was prevented. Depending on the outcome of several interacting processes, surface bloom formation may be advantageous or deleterious for the cyanobacteria involved. This also depends on the environmental conditions to which cells were acclimated prior to bloom formation. Cyanobacteria have a better chance of withstanding the conditions in surface blooms if the cells were already acclimated to high average irradiance prior to floating to the surface.