Effects of ammonium supply rates on competition between Microcystis novacekii (Cyanobacteria) and Scenedesmus quadricauda (Chlorophyta): simulation study

Competition between the cyanobacterium Microcystis novacekii and the green alga Scenedesmus quadricauda was studied, using a simulation model where nitrogen-limited chemostat systems were considered. The effects of dilution rate and ammonium concentration were examined. The dilution rate determined the dominant species and its final biomass. M. novacekii dominated at lower dilution rates (0.65 d −1). Ammonium concentration in the influx culture medium determined the final biomass of the dominant species. However, it exerted no influence on the determination of the dominant species and also the displacement time. Initial cellular N:C ratio that is an index of initial nutrient status of algae and initial biomass did not affect the determination of dominant species or its final biomass. Initial biomass affected the displacement time. This result explains in part the occurrence of blooms of Microcystis in nutrient-rich and slow-flowing waters.