A fish-kill in Heart Lake, Ontario, associated with the collapse of a massive population of Ceratium hirundinella (Dinophyceae)

SUMMARY. A massive population of the common dinoflagellate Ceratium hirundinella developed in Heart Lake. Ontario, Canada during the summer of 1976 and its sudden collapse and subsequent decomposition depleted dissolved oxygen and resulted in a fish-kill in the lake. The lake was being artificially mixed at the time by supplying compressed air to the bottom waters and the limnological events contributing to the development of the Ceratium population and its collapse appear to be closely related to the artificial destratification process. Artificial destratification during 1976 precluded the development of blue-green algue. The process also led to an increase in the density of herbivorous zooplankters which controlled the development of smaller planktonic algae. Ceratium flourished in Heart Lake because there was little competition for nutrients from other algae and because Ceratium cells are too large to be grazed by the zooplankton. The maximum size of the Ceratium population (53 mm³ 1⁻¹) is apparently the highest biomass reported in the literature and its collapse may have been related to a depletion of inorganic nitrogen. There is apparently no previously published record of a Ceratium -induced fish-kill in a freshwater lake.