Microcystis Aeruginosa and Underwater Light Attenuation in a Hypertrophic Lake (Hartbeespoort Dam, South Africa)

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Abstract
(1) Underwater light attenuation, primary productivity and phytoplankton dynamics were studied in the hypertrophic Hartbeespoort Dam. Microcystis aeruginosa dominated the phytoplankton population during most of the year. Nitrogen and phosphorus availabilities were always in excess of algal requirements. (2) Green light (550 nm) penetrated furthest and blue light (443 nm) the least. The euphotic zone (Zeu) depth (1% PAR) ranged between 0.6 m and 6.4 m (x̄ = 3.7 m) and surface chlorophyll a concentrations ranged between 3.4 mg m-3 and 1000 mg m-3 in 1982-83. The bulk of the chlorophyll was usually contained within the upper 4 m of the water column. The chlorophyll a concentration was a major factor determining underwater light attenuation. (3) changes in Microcystis aeruginosa colony size moderated the effect of chlorophyll a concentration on attenuation. The euphotic zone depth increased when colony size increased and decreased when colony size decreased. (4) Primary productivity, measured by uptake of 14C-CO2, ranged between 46.3 mg C m-2 h-1 and 2290 mg C m-2 h-1 in 1982-83. Production was usually restricted to the upper 4 m of the water column (maximum depth 22-32 m) due to the vertical stratification of the phytoplankton and algal self-shading. Primary production was significantly correlated with chlorophyll a concentration. (5) It is proposed that two major reasons for the dominance of M. aeruginosa of the phytoplankton population in this hypertrophic lake which is subjected to low wind speeds are the ability of the alga to maintain itself in the upper part of the water column and the increase in colony size as the population accumulates at the surface with any decrease in mixing and turbulence.