SOME GROWTH STUDIES ON THREE FRESHWATER PHYTOPLANKTERS FROM A TROPICAL RESERVOIR

Abstract

Some factors that may contribute to the dominance of Mycrocystis aeruginosa Kutz. amend Elenkin in Seletar Reservoir were investigated. Some physio-chemical variables in Seletar Reservoir were monitored at two to four days intervals over a period of two months. Higher concentrations of dissolved ammonia than nitrite and nitrate were recorded, and the euphotic zone was often phosphate depleted. Thus, phosphate could be an important factor limiting phytoplankton growth. Overturn of the reservoir water column was observed on five occasions. The period between these overturns were 25, 7, 6 and 2 days. Part of the surface phytoplankton was probably redistributed below the euphotic zone by each of the overturns. The concentration of two important nutrients Ð phosphate and dissolved ammonia were reduced after an overturn and the hypolimnion remained oxygenated for about 12 to 14 days. During this period, the reservoir waters were phosphate depleted and the dissolved ammonia content was reduced. The reason for the high incidence of overturns was probably the weak thermal stratification of the reservoir water. Isothermal conditions easily developed in the water after a few days of colder weather. Weather is therefore an important factor governing the incidence and frequency of overturns and hence the growth of phytoplankton in Seletar Reservoir. M. aeruginosa and two green algae (Scenedesmusnanus Chod. and Ankistrodesmusfalcatus (Cords) Ralfs.) were isolated from Seletar Reservoir and grown in axenic cultures. An in vitro study of the buoyancy of these algae showed that the sinking rates of S. nanus and A. falcatus increased with age and were maximal during their stationary phase of growth. The flotation rates of M. aeruginosa increased with age andn were also maximal during the stationary phase of growth. M.aeruginosa cells probably have a relatively longer resident time in the euphotic zone of Seletar Reservoir than S. nanus and A. falcatus cells. The positive buoyancy of M. aeruginosa probably enabled it to re-enter the euphotic zone after an overturn redistributed the surface phytoplankton into the aphotic zone. This could be a factor contributing to its success in Seletar Reservoir. The growth kinetics and nutrient requirements of the three algae species under varying concentrations of nitrate, ammonium-nitrogen, phosphate and different light intensities were studied. Phosphate was the only nutrient that affected the lag phase of these algae. The lag times of M. aeruginosa, S. nanus and A. falcatus at optimal concentrations were about 39, 35, and 50 hours respectively. Under optimal conditions, S. nanus had the fastest growth rates, followed by A. falcatus and M. aeruginosa respectively. However, S. nanus and A. falcatus generally required higher nitrate, ammonium-nitrogen, and phosphate concentrations to achieve maximal growth rates. The nitrate Ks values of M. aeruginosa and A. falcatus were about the same, but S. nanus had a higher Ks value for nitrate. Of the three algae, M. aeruginosa had the lowest Ks value for ammonium-nitrogen. The phosphate Ks values of the three algae were about the same. M. aeruginosa produced higher yields than S. nanus and A. falcatus for all nutrient levels studied. Thus under batch culture conditions, M. aeruginosa would be expected to dominate over S. nanus and A. falcatus at all nitrate, ammonium-nitrogen and phosphate concentrations. Light intensity had no effect on the final yields of these algae. The relative abilities of these algae to grow under the conditions found in Seletar Reservoir were discussed. The buoyancy mechanism of M. aeruginosa and its low nutrient requirements to produce high yields could be two factors contributing to its success in Seletar Reservoir.