Sudden Mortality of a Massive Phytoplankton Bloom

Abstract

A dense bloom of the blue-green alga,Anabaena variabilisKuetzing suddenly died in a pond heavily stocked with channel catfish (Ictalurus punctatusL.). Low light intensity caused the formation of gas vacuoles inA. variabilis. The buoyant filaments accumulated at the surface in large numbers during a period of calm, warm weather. Light injury to cells in the surface scum apparently triggered the mass mortality of phytoplankton. Decomposition of dead algae quickly resulted in depletion of dissolved oxygen. Concentrations of CO₂and ammonia increased to high levels and the pH declined following the phytoplankton die-off. A period of 7 days was required for re-establishment of a phytoplankton community and little improvement in water quality occurred until high densities of healthy phytoplankters were present. Massive mortality of fish was prevented by pumping oxygenated water into the pond at two sites and by mechanically agitating surface water in a limited area.