Nuisance blue‐green algal (cyanobacterial) scums are a sign of advanced eutrophication in aquatic systems. Previous workers have concluded that scums arc a sign of cyanobacterial senescence, or uncontrollable buoyancy. Our studies, however, indicate that scum formation may represent a mechanism by which the cyanobactcria can dominate surface waters. The mechanism is based on a preference for CO2 over HCO3− and CO32− as a photosynthetic carbon source, particularly in poorly buffered (low total inorganic carbon) waters. On calm days, when surface waters are thermally stratified, supplies of free CO2 are depleted, resulting in an increase in ambient pH. In response to CO2 depletion, cyanobacteria form surface scums, enabling them to use CO2 at the air‐water interface. This also promotes optimal photosynthetic growth and N2 fixation while simultaneously shading underlying noncyanobacterial phytoplankton.

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