The blue-green alga Microcystis aeruginosa and the diatom Cyclotella sp. were grown in Wright's 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.

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The formation potentials of 24 disinfection by-products, six indicators of organic matter and the number of algae were measured in raw water once a month for one year from October 2006 to September 2007 in order to investigate the quality of water in the two water purification plants on the Ogasawara Islands. It was estimated that the main source of organic matter in raw water on the Ogasawara Islands was not autochthonous organic matter but allochthonous organic matter that flows from streams into reservoirs, as determined from the correlations between the parameters of water quality, the relationship between rainfall and water quality, UV absorbance: TOC ratio and other factors. The mean annual formation potentials of disinfection by-products in filtered raw water on Chichijima Island and Hahajima Island were as follows: total haloacetic acid: 286, 194 μg·L^-1, total trihalomethane: 227, 190 μg·L^-1, total haloacetonitrile: 13 μg·L^-1, and chloral hydrate: 13, 9 μg·L^-1, respectively. The total trihalomethane formation potentials were at the maximum levels in our country and the formation potentials of other disinfection by-products were also presumed to be their maximum levels. The formation potentials of the brominated disinfection by-products in most months were higher than those of the nonbrominated disinfection by-products. The correlations of total trihalomethane formation potential (mole concentration) with color, TOC, KMnO₄ consumption and UV absorbance were lower than those of the formation potentials of total haloacetic acid, total haloacetonitrile and chloral hydrate in raw water. KMnO₄ consumption and UV absorbance were effective indicators of disinfection by-product formation potential for these high correlations (P<0.01).

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