Trophic State and Hypolimnetic Nitrogen Metabolism of Lake Hibara.

This is the first attempt to elucidate the seasonal changes in some physical and chemical parameters in a water column of Lake Hibara. The observation extended from spring to early winter of 1991.In the hypolimnion, nitrification started between 13 June and 10 July. The nitrification rate in this period was 0.21-0.44μg atom N⋅1-1⋅day-1. The dissolved oxygen (DO) level of about 0.6-0.7 mg⋅1-1 appeared critical in impeding efficient nitrification. The gross ammonia production rate was estimated to be 0.05-0.15 μg atom N⋅1-1⋅day-1. Denitrification seems to have taken place in the lower hypolimnion and/or on the surface of the bottom sediment when DO decreased to 2-3%(0.2-0.3 mg⋅1-1). The levels of TN, TP, Chl. a and transparency in the autumn circulation period indicate that the lake is on the boundary between oligo- and mesotrophic, though the epilimnetic water in summer was oligotrophic. The relative areal oxygen deficit in the hypolimnion (0.055-0.061mg⋅cm-2⋅day-1) was at the lower limit of eutrophy. The question of why, in spite of the lake's overlying oligotrophic epilimnion, the hypolimnetic DO consumption rate was at a eutrophic level remained to be solved. The hypolimnetic DO consumption rate seems to have been accelerating for the last 20 years. To elucidate the extent of the fluctuation of the hypolimnetic processes from year to year is important for future study.