Mechanisms of Algal Species Dominance among Cyanobacteria, Diatoms and Green Algae as a Consequence of Phosphorus Reduction Caused by River Water Dilution in Eutrophic Lake

Abstract

This study examined the mechanisms of dominant species transition from Microcystis aeruginosa to mainly Cyclotella sp. and also Scenedesmus quadricauda as a consequence of phosphorus reduction caused by dilution in Lake Tega. The growth potentials and competitive abilities for each species were investigated through a series of culture experiments and the effect of phosphorus fluctuation on the species transition was discussed. The monoculture experiment for each species showed that S. quadricauda possessed the growth superiorities compared to other species under phosphorus-limited condition, representing the highest maximum growth rate (0.32 day^-1) and the highest Chl.a division ability (5.6 times) in phosphorus-free condition. The competitive experiment culturing all species together in the same medium indicated that the condition at PO₄-P = 0.5 mg/L led to the predominance of M. aeruginosa, reflecting the condition before dilution. On the other hand, at PO₄-P = 0.1 mg/L, which represented the condition after dilution, S. quadricauda completely dominated and the predominance of Cyclotella sp. was never observed. Accordingly, it could be concluded that the predominance of Cyclotella sp. was affected by the dilution in Lake Tega and would be attributed not to phosphorus reduction but other factors such as nitrogen, light intensity, dilution rate and/or their interaction.