In order to make clear the growth characteristics of musty-odor producing algae (Oscillatoria tenuis, Phormidium tenue and Anabaena macrospora), we examined the correlation between the maximum growth number (MGN) of each alga and nutrient concentrations, with the unialgal culture method using a 0.45μm pore size membrane-filtered lake water. We found that the MGN of Lake Biwa water was limited by soluble phosphorus concentration.

In order to verify the results, we examined out the relation between the maximum standing crop (MSC) of musty-odor producing algae in the southern basin of Lake Biwa and nutrient concentrations of our 12-year (1983-1994) observation data. The MSC of P. tenue showed a positive correlation with the N:P ratio. On the other hand, The MSC of O. tenuis showed a strong positive correlation with the phosphorus concentration, and a positive correlation with the iron and soluble phosphorus concentration, but a strong negative correlation with the N:P ratio. When the phosphorus concentration is over 0.030 mg/l, O. tenuis will probably be over 10 filaments/mL. The optimum N:P ratio range was 10 to 20.

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