From Resumé: The effect of physico-chemical factors on the biology of the upper reaches of Swartvlei has been investigated during 1971-1972. Physico-chemical data have shown that Swartvlei was characterized by an extremely labile ectogenic meromixis. This instability was due to three factors : (1) the inflow of freshwater, (2) the inflow of sea water, and (3) wind stress. The magnitude of the effects of these factors upon the physics, chemistry and biology of the upper reaches was dependent upon whether or not the estuary mouth was open or closed. The phytoplankton of the pelagic zone of Swartvlei was dominated by nannoplankton. Three major categories were recorded: dinoflagellates, flagellates and diatoms. The major factor regulating their productivity in the upper reaches was light. As a result of humate staining and suspended detrital matter light conditions in Swartvlei were comparable to those in extremely eutrophic northern hemisphere lakes. A maximum integral primary productivity of 39.66 mg C m⁻² h⁻¹ was recorded in November 1972. The aerobic heterotrophic bacterial population in Swartvlei was usually less than 300 col. ml⁻¹ (plate counts). The activity of the total microbial population was measured with ¹⁴C techniques. Uptake of acetate was dominated by flagellates and one species of dinoflagellate when they were present. Glucose uptake was due to bacteria as was acetate uptake in the absence of heterotrophic phytoplankton. Glucose uptake was usually greatest in the anaerobic zone. This, and the presence of H₂S, suggested that a large active photosynthetic and chemosynthetic bacterial population may have been present in the monimolimnion. The possible importance of these bacterial processes in the total productivity of the pelagic zone of Swartvlei was discussed. Daytime zooplankton population size was statistically correlated with the size of the flagellate population. The zooplankton was dominated by Acartia and Halicyclops. These two animals were found in the anaerobic bottom of Swartvlei after October 1971. This corresponded to the disappearance of the flagellate population from the water column. The dominance of these animals in the zooplankton population may have been related to their ability to live in anaerobic water where the bacterial population appeared to be considerably more abundant than in the aerobic zone. Further implications of the results are discussed in reference to phytoplankton cell size and heterotrophy indicating a possible adaptive significance of these factors in Swartvlei.

Introduction: Phosphorus is an especially scarce element in the aquatic environment, yet it is of vital importance to living organisms: the pyrophosphate bond is the basis of biochemical energy transfer, and phosphate groups make up the backbone of the nucleic acids (Wetzel, 1975). An understanding of phosphorus movements and transformations is thus essential to a study of the biological functioning of a lake. The present report describes a study of phosphorus dynamics inSwartvlei, a lake with an extremely low pelagic primary production rate, in the order of 0,15 gC.m⁻².d⁻¹ (Robarts, 1973; Howard-Williams and Allanson, 1978). The littoral phosphorus dynamics of Swartvlei have been discussed in some detail (Howard-Williams, 1977; Howard-Williams and Allanson, 1978), and what follows is an attempt to quantify the changes in phosphorus concentration occurring in the deeper parts of the lake, which comprise some 57% of the total area.