2024 Volume 60 Issue 3 Pages 63-71
Although total nitrogen (TN) and total phosphorus (TP) mass ratio (TN/TP mass ratio) has been recognized as an indicator of the cyanobacterial bloom occurrence, it has been still unclear whether the change in TN/TP mass ratio is a result or a cause of the blooms. This study examined how the TN/TP mass ratio changes accompanying the occurrence of Microcystis blooms via semi-continuous monoculture experiments. BG－11 medium was used with a fixed phosphorus concentration of 0.11 mg L－1 and the initial TN/TP mass ratio varied in the range of 1 to 100. With the growth of Microcystis sp., the TN concentration showed a decreasing trend and the TP concentration exhibited an increasing trend, resulting in lowering TN/TP mass ratio. It was also indicated that Microcystis sp. was more likely to form blooms at the initial TN/TP mass ratios of 18 and 50. Furthermore, the TN/TP mass ratio was strongly influenced by increasing phosphorus-rich extracellular polysaccharides (EPS) and approaching the intracellular nitrogen and phosphorus mass ratio (Nc/Pc mass ratio) of 13.2－20.3 with the increase in Microcystis sp. cells. These results suggest that cyanobacterial blooms could be suppressed by controlling the TN/TP mass ratio to below 18 or above 50.
