Influence of precipitation on development of the denitrification process in Lake Fukami-ike.

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Denitrifying activity in Lake Fukami-ike emerged early in April 1986 as well as in 1985. In contrast to 1985, owing to the depletion of nitrate, denitrifying activity in 1986 did not develop after early May, when its mean and maximum value were 0.033 and 0.155μg-at N·1-1d-1, respectively. On the other hand, the denitrifying bacterial population in the lake grew slowly from April (3 cells ml-1) to June (19 cells ml-1). The maximum population level was only one fourth of that in 1985. The difference in the development of the denitrification process between 1985 and 1986 was discussed mainly in terms of the supply of nitrate through precipitation and runoff. From analysis of the correlation between precipitation at Anan Observatory and NO3- concentration in Lake Fukami-ike, it is concluded that precipitation during late February to early March determined the nitrate level in the vernal circulation period of the lake, and it is suggested that the nitrate level affected the extent of initial development of the denitrification in the lake. Moreover, NO3- in the lake was almost completely depleted during the long drought from 20 May to 15 June 1986, and in situ denitrifying activity was also almost completely suppressed. Under these circumstances, the dependence of denitrifying activity on the precipitation was shown by a correlation between maximum denitrifying activity and the amount of the precipitation during the preceding 20 days.