Abstract
Yubara-ko is a reservoir which was constructed in damming up the upper course of the Asahi-gawa River at the town of Yubara, Maniwa-gun in Okayama prefecture, for the purpose of storaging water to hydro-electric power stations. It was filled with water in March, 1955, and five years have passed after the appearance of this artificial lake up to the beginning of 1959. The writer began the investigations of this lake, concerning its water quality, the vertical distribution of plankton in summer, and the annual succession of net-plankton during the period from September 1958 to August 1959. The results obtained are as follows :
The metaliumion developed in the layers of 4-20 metres deep in Aug. ; the pH values were 6.0-7.2, the amounts of nutritive salts for phytoplankton (NO3-N, NH4-N and PO4-P) being 0.01-0.02 mg/l, 0.01-0.03 mg/l, and 0. 005-0.006 mg/l respectively. This lake seems to be remained still at an oligotrophic condition. The plankton reached the maximum in numbers (1006/l) in the layer of 10 m. deep, and the minimum (41/l) in the layer of 45 m deep (bottom layer). Ceratium hirundinella was the dominant species throughout the layers. In general, the plankton were rich in individual numbers in the layer of 10 m deep, i. e. the middle layer of the rnetalimnion where the dissolved oxygen and the pH values decreased remarkably.
The annual succession of plankton showed that the number of species and the amount of plankton were rich in summer and poor in winter. During the period from September 1958 to August 1959, there occurred 41 species of plankton animals and plants, of which 17 were the phytoplankters and 24 were the zooplankters. The dominant species of plankton animals and plants in each month were as follows :
September Difflugia corona
October Synedra sp.
November-December-January Bosmina longirostris
February Synchata stylata
March Asplanchna priodonta
April Ploesoma truncaturn
May Keratella cochlearis
June Bosminopsis deitersi
July-August Ceratiuin hirundinella
