SULFATE REDUCTION IN PROFUNDAL SEDIMENTS IN LAKE-KIZAKI, JAPAN

The sulfate reduction rate was measured for almost four years in the p rofundal sediments of Lake Kizaki, a mesotrophic lake in central Japan . The rate was generally highest in the surface layer and decreased wi th depth. Seasonally, sulfate reduction tended to be high in spring an d summer, and then to decrease until the end of stratification (Decemb er) in spite of a constant in situ temperature of around 6 degrees C, although fluctuations were found in every year. The rate also fluctuat ed greatly according to year. The maximum rate of sulfate reduction wa s 0.33 mmol m(-2) d(-1) in May, 1990, and the minimum was 0.004 mmol m (-2) d(-1) in March, 1993. These relatively low rates, compared with t hose reported for freshwater sediments, seem to be due to low concentr ations of sulfate in the sediments (5-23 mu mol l(-1) in the surface l ayer). The rate was highly correlated with the concentration of sulfat e in the sediments. The addition of sulfate stimulated sulfate reducti on in all sediment samples tested, but adding lactate did not. Therefo re, sulfate reduction should be limited mainly by the supply of sulfat e. Measurements of sulfate reduction rates at different concentrations of added sulfate revealed a low concentration of half-saturation cons tant as low as 12 mu mol 1(-1).