|Budget Amount *help
¥2,200,000 (Direct Cost : ¥2,200,000)
Fiscal Year 1996 : ¥800,000 (Direct Cost : ¥800,000)
Fiscal Year 1995 : ¥1,400,000 (Direct Cost : ¥1,400,000)
In the Ishitegawa Reservoir in Japan, the mean chlorophyll-a concentration at 0.5m depth during July, August and September (C^s, mugl^<-1>) is predictable by the conductivity in May (E_5, mScm^<-1> at 25ﾟC) and the mean air temperature in May (T_5, ﾟC). In this report the authors intended to make clear the actual substances corresponding to E_5 mainly by regression analyzes of 14 years of monthly data 1983 through 1996. The C_s in a logarithm (logC_s) correlated positively with chlorophyll-a concentration (mugl^<-1>) in May in a logarithm (logChl-a_5) (gamma=0.89, P<0.01). Only one factor in May, a ratio of concentrations (mmoll^<-1>) between dissolved magnesium and potassium, (Mg/K)_5, was found to correlate positively both with logChl-a_5 (gamma=0.72, P<0.05) and E_5 (gamma=0.67, P<0.05) among calcium, magnesium, sodium and potassium concentrations and each ratio. Potassium concentration was always low in the reservoir, and that in May correlated negatively with logChl-a_5 (gamma=-0.78, P<0.01), which might be due to the result of potassium absorption by algae, so the positive effects of the magnesium part of Mg/K on algal growth were examined by culture experiments of Chlorella vulgaris under definite potassium, phosphorus and nitrogen concentrations. The results of the culture experiments showed positive effects of magnesium on the growth of C.vulgaris under usual magnesium concentrations in the reservoir. Finally, using (Mg/K)_5, the authors were able to induce a new regression equation predicting the chlorophyll-a concentration in the summer, "logC_s=0.578 (Mg/K)_5 -0.44" (R^2=0.77, P<0.01). The residuals of observation minus prediction showed an interannual trend to become larger in minus values, correlating negatively with summer nitrite and acid-soluble iron concentrations.