The factors which influence algal growth in the P.K. le Roux impoundment, Orange River, South Africa

Abstract

Introduction: In South Africa, much of the work on algal limitation in man-made lakes has centered on nutrients rather than the possible effects of physical factors such as turbidity.In order to determine which is the most important factor in algal limitation in the turbid Orange River system, a comprehensive study of spatial and seasonal changes of the following factors was undertaken in the P.K. Ie Roux impoundment: trace elements , silicon, nitrate, phosphate, total phosphate, phytoplankton standing stock, total suspended solids, secthi disc transparency, light attenuation and the spectral quality of the light. None of the chemical factors appeared to be in short supply but the shallowness of the euphotic zone (0.8 m) had two effects. Firstly. the quantity of light in the upper 1.5 metres was extremely small due to the rapid attenuation of the light by silt. Secondly. the quality of the light was poor due to the selective screening of the shorter wavelengths of light. Once light limitation had been implicated as an important factor in the control of phytoplankton stocks a series of column experiments were used to demonstrate that increased circulation rate. and therefore a higher light dosage. would elevate the algal standing stocks without the addition of nutrients. It was demonstrated that light. as a result of the negative effects of turbidity was the principal cause of low algal standing stocks within the impoundment. and not nutrients as has previously been suggested by other workers.