藍藻類Microcystis aeruginosaと珪藻類Cyclotella sp.の増殖に及ぼすN/P比および温度の影響

概要

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The blue-green alga Microcystis aeruginosa and the diatom Cyclotella sp. were grown in Wrights cryptophytes medium with various mass ratios of total dissolved nitrogen to phosphorus (N/P = 1, 10, 50 and 100) at different temperatures (15, 20 and 25°C). Similar experiments at lower nitrogen and phosphorus concentrations but using the same N/P ratio were also carried out. At high nutrient concentrations (N = 0.5-50 mg·L-1 and P = 0.5 mg·L-1), Cyclotella sp. was dominant at all N/P ratios at 15°C, but M. aeruginosa became a superior competitor at 20 and 25°C at N/P≠ 1. Generally, Microcystis blooms tend to occur at a low mass ratio of total nitrogen (TN) to total phosphorus (TP), i.e., TN/TP ratio. However, this study clearly showed that a high N/P ratio favored the growth of M. aeruginosa. Moreover, the growth of both algae was controlled significantly at all N/P ratios at low nutrient concentrations (N = 0.05-5 mg·L-1 and P = 0.05 mg·L-1). Therefore, it could be concluded that M. aeruginosa dominance occurs at temperatures above 20°C and is promoted at a certain level of nutrient concentrations (N > 0.5 mg·L-1 at P = 0.5 mg·L-1 and/or P > 0.05 mg·L-1 at N = 0.05-5 mg·L-1) rather than N/P ratio.