Factors controlling primary production in a hypertrophic lake (Hartbeespoort Dam, South Africa)

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Abstract
The physical factors controlling algal primary production were demonstrated from data collected for a hypertrophic lake. Amax ranged between 12.4 and 5916 mg C m −3 h −1 . Areal rates (Σ A ) varied between 46.9 and 3381 mg C m −2 h −1 . The factors permitting and controlling production were subjectively separated into two categories. In category 1, nutrients (N + P), which were in overabundance, permitted large standing crops of Microcystis aeruginosa to develop (>1000 μg chl a 1 −1 ). Wind patterns determined the dramatic spatial and temporal changes in algal standing crop which could drop to 2.7 μg chl a 1 −1 . In category 2 were the factors which affected the rate processes. The buoyancy mechanism of Microcystis usually kept the alga in the euphotic zone. A power relationship ( r = 0.92, n = 54) between Σ A and Amax /ε min showed that with increasing phytoplankton vertical stratification, Amax was increasingly important in the integral. The saturation parameter IK and photosynthetic capacity were temperature dependent. Variations of Σ A were significantly related to changes in water column stability (g cm cm −2 ) because both axes of the photosynthesis depth-profile were affected by stability changes.

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