Blue-green algal scums: An explanation for their occurrence during freshwater blooms1

Ahs tract 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 cyanobacteria can dominate surface waters. The mechanism is based on a preference for CO, over HCO,- and COs2- 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, are depleted, resulting in an increase in ambient pH. In response to CO, depletion, cyanobacteria form surface scums, enabling them to use CO, at the air-water interface. This also promotes optimal photosynthetic growth and N, fixation while simultaneously shading underlying noncyanobacterial phytoplankton. One of the most obvious signs of advanced freshwater eutrophication is a blue-green algal (cyanobacterial) scum. During calm periods, scums often lead to rapid deterioration, exemplified by deoxygenation of underlying waters, subsequent fish kills, toxicity, foul odors, and lowered esthetic values of affected waters. Increased cell buoyancy, due to intracellular gas vacuole formation, leads to surface scums of the dominant bloom genera Anabaena, Aphanixomenon, and