Blue‐green algal scums: An explanation for their occurrence during freshwater blooms 1

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 CO 2 over HCO 3 − and CO 3 2− 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 CO 2 are depleted, resulting in an increase in ambient pH. In response to CO 2 depletion, cyanobacteria form surface scums, enabling them to use CO 2 at the air‐water interface. This also promotes optimal photosynthetic growth and N 2 fixation while simultaneously shading underlying noncyanobacterial phytoplankton.