AN INVESTIGATION INTO PHYTOPLANKTON BLOOMS IN THE VAAL RIVER AND THE ENVIRONMENTAL VARIABLES RESPONSIBLE FOR THEIR DEVELOPMENT AND DECLINE Final Report to the Water Research Commission by AJH Pieterse and Sanet Janse van Vuuren Department of Plant and Soil Sciences Potchefstroom University for Christian Higher Education Private Bag X6001 Potchefstroom 2520 WRC Report No 359/1/97 ISBN 1 86845 330 8 11 STEERING COMMITTEE Dr G Offringa (Chairman) Water Research Commission, Pretoria Mr AJ Dippenaar Goldfield Water, Bothaville Mr HM du Plessis Water Research Commission, Pretoria Dr S Mitchell Water Research Commission, Pretoria Mr K Morgan Western Transvaal Regional Water Company, Stilfontein Mr G Quibell Department of Water Affairs and Forestry, Pretoria Mr N Rossouw Watertek, CSIR, Pretoria Mr MC Steynberg Rand Water Board, Vereeniging Mr D Huyser & Ms AM du Toit (Secretariat) Water Research Commission, Pretoria Ill CONTENTS Executive summary iv List of Figures viii List of Tables xv 1. Introduction and objectives AJHPieterse 1 2. Phytoplankton production and photosynthetic characteristics in relation to environmental variables in the Vaal River at Balkfontein JC Roos, AJH Pieterse, JF Prinsloo 3 3. Environmental variables, abundance and seasonal succession of phytoplankton populations S Janse van Vuuren & AJH Pieterse 25 4. The effect of increased concentrations of total dissolved salts on algal species from the Vaal River JF Prinsloo & AJH Pieterse 157 5. Nutrient availability: phosphatase and nitrate reductase activity in the Vaal River and its phytoplankton E Coetzee & AJH Pieterse 185 6. Phytoplankton biomass and environmental variables in the Vaal River at Barrage and Stilfontein AJH Pieterse, M Smith & N Matthew 198 7. Modelling algal growth in the Vaal River AHJ Cloot, SW Schoombie, AJH Pieterse & JC Roos 212 8. Summary and conclusions AJHPieterse 227 Acknowledgments 234 Glossary of symbols 235 Glossary of concepts used in the text 237 IV EXECUTIVE SUMMARY AN INVESTIGATION INTO PHYTOPLANKTON BLOOMS IN THE VAAL RIVER AND THE ENVIRONMENTAL VARIABLES RESPONSIBLE FOR THEIR DEVELOPMENT AND DECLINE AJH Pieterse & Sanet Janse van Vuuren Department of Plant and Soil Sciences, PU for CHE, Potchefstroom When compared with similar studies on rivers in other regions of the world, it is clear that the Vaal River, although unique in many ways, also have characteristics in common with the other rivers. An investigation on the phytoplankton assemblages as well as the associated environmental variables and metabolic and other processes in the Vaal River, however, broadens our understanding of rivers in general. For this reason this particular study on the Vaal River is of general significance to all scientists interested in aquatic ecology and the utilisation and management of inland waters. The Vaal River is the most important and the most regulated river in South Africa. It is also a eutrophic system on account of high chlorophyll-a and inorganic nitrogen and phosphorus concentrations as well as high primary productivity rates. Massive developments of phytoplankton are experienced in certain sections, resulting in aesthetic problems, health hazards, interferences with treatment processes and problems in water distribution systems. Consequently, the causes and consequences of the wax and wane of phytoplankton assemblages in the Vaal River were investigated. The study showed that a variety of phytoplankton species (at least 124 species and varieties), occurred in the Vaal River during the study period. Green algae showed greatest species diversity. The greatest diversity of Cyanophyceae species were present at the Barrage, while the greatest diversity of Euglenophyceae species were present at the Balkfontein sampling locality. The phytoplankton community was dominated mainly by diatoms and green algae (which succeeded each other), as well as by blue-green algae during warmer periods. Different species succession patterns were demonstrated for the different sampling positions. At the Barrage, Parys and Balkfontein, different species succeeded one another within relatively small time spans. At Stilfontein a smaller number of species succeeded one another, possibly indicating more stable environmental conditions at this locality in the river. During the summer periods, diatoms tended to dominate in the Vaal River from January to August of each year, while the green algae were dominant from September to December. Dominance of diatoms during January to April and December, can be ascribed to blooms of Melosira granulata. Concentrations of unicellular centric diatoms were usually low during the summer periods, but they often dominated during the cold-water winter periods. Blue-green algae frequently occurred during the mid and late summer months of each year, especially at the Barrage, Stilfontein and Balkfontein sampling localities. Water temperature also affects solubility of silica and oxygen. Higher silica concentrations were usually present during summer periods when the water temperatures were higher, while lower Si concentrations were present during the winter. Indications of pollution and eutrophication were reflected in high chlorophyll-a concentrations in the Vaal River in comparison with other river systems. Periods of maximum chlorophyll-a concentration in the Vaal River occurred from January to March (summer) and again from July to November (winter-spring) of each year. Increased nutrient concentration in the Vaal River apparently increased the capacity of the water to support high production rates and to maintain large standing crops of phytoplankton. Higher discharge resulted in higher total suspended solids (TSS) concentration, higher nutrient concentration (i.e. N, P and Si), lower total dissolved salts (TDS), higher turbidity and thus in lower euphotic zone (Zeu) and underwater light climate (ULC). Very high levels of discharge can be responsible for a complete wash-out of the phytoplankton. The turbidity of Vaal River water at Balkfontein was lower during the period 1984 to 1991 than during previous years. Reduced turbidity might be due to the clearing effect of salinity or possibly to the presence of water hyacinths. Water hyacinths could have served as sediment and plant nutrient traps. Since 1991, however, increases in tubidity were observed. Decreased turbidity resulted in an improved under water light climate (ULC) which, in turn, was associated with reduced daily rates of areal photosynthesis during certain periods. The total dissolved salts (TDS) concentration in the Vaal River is high, with an average annual maximum of about 650 mg I"1 and a minimum concentration of about 400 mg I*1. The mean annual TDS concentration for the study period (1984 - 1993) was about 520 mg I"1 which is approximately four times higher than the global mean salinity of river water. An average increase rate of 25 mg I"1 per annum was calculated for the study period at both Stilfontein and Balkfontein. Based on the results of the present study, the Vaal River can be classified as a mixohaline system. The order of ionic prominence in the Vaal River 2 2+ + 2+ 2 at Balkfontein was SO4 " > > Ca > Cl" > Na > Mg and at Stilfontein it was SO4 - > > Cl > Ca2+ > Na+ > Mg2+. The major ionic contributor to the total dissolved salts of the 2 water was the anion SO4 \ Salinity was not the primary variable influencing algal growth, but dinoflagellate representatives (responsible for red-tides in the ocean) more frequently occurred in high salinity water, while blue-green algae occurred in water with relatively low salinities. Growth and carbon assimilation experiments indicated that different algae showed different sensitivities to TDS. Of the three algal species investigated, Cyclotella meneghiniana (a diatom) was the most sensitive and Monoraphidium circinale (a green alga) the least sensitive to increased dissolved salts. If the total dissolved salts concentration of Vaal River water increases with time, algal species that could persist over a wide range of salinities can be expected to be present most of the time. Of the species investigated, Monoraphidium circinale can be expected to become dominant under VI conditions of increased salinity. Cyclotella meneghiniana and Microcystis aeruginosa on the other hand, can be expected to be excluded from the water under conditions of increased salinity of 250 mg I"1 and above. The recent increased abundance of Oscillatoria simplidssima (another blue-green alga) in the Vaal River can possibly be attributed to temporary decreasing levels of salinity. Reduced annual DIN:DIP ratios suggested that the Vaal River at Balkfontein switched from a generally phosphorus limited system (1986 to 1990) to a potentially nitrogen limited system (1991 and 1993). The decrease in DIN:DIP ratio was caused by relatively low DIN concentrations and relatively high DIP concentrations. Switching inorganic nitrogen and phosphorus limitation influence algal biomass as well as the composition of the algal assemblages. The switching between Chlorophyceae (green algae) and Bacillariophyceae (diatoms) dominance is apparently partly determined by N:P ratios. Blue-green algae were most probably favoured by low DIN:DIP and TN:TP ratios. A shift from blue-green algae to other algal groups occurred if the TN:TP ratio increased. Increased phosphorus concentrations in the Vaal River (with a resultant decrease in DIN:DIP and thus TN:TP to less than five) will probably cause a shift in the algal assemblages from diatom and green algal dominance