Hyperscums of the cyanobacterium Microcystis aeruginosa in a hypertrophic lake (Hartbeespoort Dam, South Africa)

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Abstract
It is proposed that surface scums of densely packed planktonic cyanobacteria (blue-green algae) which exist for weeks to months, measure several decimeters in thickness and are covered by a crust of photo-oxidized cells, be called hyperscums. Hyperscums of Microcystis aeruginosa formed during prolonged periods of calm weather in wind-protected sites in a hypertrophic lake subject to low wind speeds (Hart beespoort Dam, South Africa). A hyperscum that extended over 1–2 hectares and persisted for 103 days during winter 1983 was studied. Chlorophyll a concentrations ranged from 100 to 300 mg l−2 Microcystis cell concentrations reached 1.76×109 cells ml−1 or 116 cm3l−1. The hyperscum environment was anoxic, aphotic, with a fluctuating temperature regime and low pH values. The densely packed Microcystis cells survived these conditions for more than 2 months. This was shown by comparing the potential photosynthetic capacity of Microcystis from the hyperscum with that of Microcystis from the main basin of the lake. However, after 3 months the hyperscum algae lost their photosynthetic capacity and decomposition processes prevailed. The hyperscum gradually shrank in size until a storm caused its complete disintegration.