陸水学雑誌

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1) Field and laboratory studies of the crustacean plankton have been conducted in some Japanese lakes for the purpose of obtaining fundamental as well as practical knowledge concerning them. Two representative plankton species, Acanthodiaptomus pacificus and Daphnia longispina have been taken up. They were collected from Lake Shirakoma (Nagano prefecture, at an altitude of 2, 115 m) and Lake Nanamagari (Saitama prefecture, altitude 100 m), as the representative of mountain and the lowland-lake respectively. Several tolerable as well as culture experiments were made, using a pure culture of bacteria, Escherichia freundii (Braak) as food, and the growth and rate of propagation were observed.
2) The highest temperature during the summer was measured 22.4 °C in Lake Shirakoma and was as high as 31.4 °C in Lake Nanamagari. The mean annual temperature of the former was usually 9.8 °C lower than that of the latter.
Lake Shirakoma usually begins to freeze during the period from late October to early November. On Jan. 11, 1959, the ice was 39. 0 cm thick, being covered with snow 49 cm deep, when the air temperature was as low as -13.5 °C.
3) The colour of water measured by Forel's colour scale ranged between No. 5-No. 8 in Lake Shirakoma, while it was usually yellow green in Lake Nanamagari, though the changes were very irregular. The change of colour in the former lake was caused by the occasional floods of rain water, while in the latter it was chiefly due to the regular change of the abundance of plankton.
4) Owing to the scarcity of phytoplankton and the meagre content of dissolved organic matter, the dissolved oxygen of the water of Lake Shirakoma was usually saturated, scarcely exceeding 100 %, without a marked decrease throughout the year. On the contrary, the oxygen content of the water of Lake Nanamagari showed a remarkable change, exceeding 121% in saturation when phytoplankton enormously increased during the spring and became lesser than 75% when zooplankton decayed during the summer.
5) The pH of the water ranged between 4.8-5.8 in Lake Shirakoma and 6.2-7.7 in Lake Nanamagari, respectively.
6) Dissolved organic matter in terms of permanganate consumption of these lake waters changed in the range of 16.80-42.82 mg/l and 0.65-52.92 mg/l, respectively. The maximum quantity was observed during July-November in Lake Shirakoma and in the summer months in Lake Nanamagari.
7) The differences of physical and chemical factors between the surface and the bottom in Lake Shirakoma were only 3.8 °C in water temperature, 0.1 in pH and 24.3% in oxygen saturation.

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