大野ダム湖の陸水生物学的研究 (第3報)

抄録

Succeeding to the investigations in 1962 and 1963, the author has carried on the limnological investigations of the Ono Reservoir nine times in 1964. This report deals with vertical distributions of water temperature, the pH values, the amounts of dissolved oxygen and the plankton in the water. The results are as follows :
No epilimnion developed in summer, a slight metalimnion being recognized just below the water surface. In September, however, a thin epilimnion developed down to the depth of two meters.
In autumn and winter, the dissolved oxygen was measured in about equal amount from the surface to the bottom, but it was especially rich in February. In summer, the oxygen in the deeper layers decreased remarkably. In 1964 this phenomenon occurred already in June, earlier than in the preceding years (1962, 1963). On September 27, oxygen was found as much in the deeper layers as in the epilimnion. Both on June 21 and September 6 the oxygen a little increased in the layer shallower than 20 meter deep. This phenomenon seems to be due to the density current of cold inflowing river water after rain.
In 1964, the low pH value was measured in the hypolimnion in summer, as it was recognized in the two previous years, but it slightly increased on the bottom of the water as in 1962. The difference of pH was scarcely seen throughout the layers, except in summer. In winter it remained unchanged.
The zooplankton decreased, compared with the results of the previous years, in the number of species and in individual numbers. Their occurrence shows a tendency similar to that in the former years. Synchaeta pectinata occurred in a few numbers in the seasons other than in summer. Keratella cochlearis, Ploesoma truncatum, Polyarthra sp. were also fewer in individual numbers than in the previous two years, and the time of their appearance was somewhat different. In 1964 there was found Chonochiloides dossuarius, which appeared also in 1962 and not in 1963.
Only two species, of Cladocera, Bosmina longirostris and Bosminopsis deitersi were found. The time of appearance of the former differed from that in the previous two years. The individual numbers of Cyclops sp. decreased, the time of appearance being the same as in 1963.
Some species of phytoplankton regarding the time of their appearance and in the number of species as well as in individual numbers showed the same tendency as in 1962, 1963 and 1964, but others showed a different tendency in the time of their appearance.
In 1964 the following four species appeared for the first time : Pediastrum tetras, two species belonging to Scenedesmus and Staurastrum sp. The occurrence of three species of algae, Oscillatoria tenuis, Synedra ulna and Eudorina elegans suggest that the water of this artificial lake is rich in nutrient salts.