Nutrients, dissolved gases and pH in the Vaal River at Balkfontein, South Africa

Abstract

Seasonal variation and interrelationships between environmental variables and nutrients as well as dissolved gases and pH in the Vaal River at Balkfontein were investigated between 1985 and 1989. Photosynthesis and temperature are major factors that influence the concentration and dynamics of O 2 and CO 2 in the river water. The average daytime oxygen concentration in the river (9.3 mg1 −1 ) rendered the water supersaturated (115 %) which suggests that photosynthesis exceeded respiration and that the river at Balkfontein was autotrophic. Minimum oxygen concentrations and low pH values were recorded during flood periods, coinciding with high turbidity, high temperature and low algal biomass. Vaal River water, with relatively high pH levels (average = 8.1), is a well-buffered system (average alkalinity = 89 mg CaCO 3 1 −1 ) with only the HCO 3 - species in the CO 2 -HCO 3 -CO 3 system of quantitative importance. Total suspended solids (TSS) is an important transport agent for phosphorus and nitrogen. The phosphate phosphorus (PO 4 -P) concentration was low (average = 16 μg 1 −1 ), but total phosphorus (TP) concentration was high (average = 202 μg 1 −1 ). Theretore, to sustain high algal growth, the organisms must have depended on recycling mechanisms to make dissolved inorganic phosphorus (DIP) available from TP. The TN: TP ratio in the Vaal River was relatively low (average = 11), but the DIN: DIP ratio was relatively high (average = 57), suggesting the occurrence of phosphate limitation in the Vaal River. Factors governing fluctuations in NO 3 -N and NH 4 -N concentrations (and therefore DIN-DIP ratios) in the Vaal River were discharge, decomposition of organic matter and algal blooms. The SiO 2 -Si concentration follows the average monthly water temperature, 'and probably plays an important role in diatom blooms