Limnological Study on Ono Reservoir (Second report)

Abstract

Continued to the survey of 1962 (Japanese Journal of Limnology, Vol. 25, No.1), the auther investigated Ono Reservoir limnologically eight times in 1963. It contains the vertical distributions of water temperatures, pH values and amounts of dissolved oxygen.
The results obtained are as follows :
No ephilimnion was recognized, and the metalimnion directly touches the water surface, from spring to summer.
The vertical distributions of the amounts of dissolved oxygen show irregular curves from winter to spring. The surface water contains a large amount of dissolved oxygen, but it decreases remarkably in the hypolimnion, from summer to autumn.
The variation of pH values from the upper layer to the bottom is not large, but their vertical inclinations get larger from summer to autumn.
The auther observed these distinctive features of the water temperature, dissolved oxygen and pH values, in 1962 too.
On February 3rd of this year, the inverse stratification of water temperatures was found, though in 1962 no inverse stratification was seen.
As to the annual succession of zooplankton, much the similar tendencies were recognized as in 1962:
Synchaeta pectinata EHRBG. and Asplanchna priodonta GOSSE increase vigorously in winter and the species number as well as individual number of Rotatoria increase greatly in July and decrease in March.
As to Cladocera, both species number and individual number were less than in 1962, but the period of their appearance is about the same as in 1962. Copepoda also decreased in population, except in June.
The time and duration of the appearance of Asplanchna priodonta GOSSE, Keratella cochlearis GOSSE, Ploesoma truncatum LAVENDER were similar, but not exactly the same as in 1962.
As to phytoplanktons, they show the same tendencies as in 1962. The species number and individual number of Bacillariophyceae are abundant from March to June and in November, and Eudorina elegans EHRENBERG is abundant in June.
The population of the species belonging to Ulothrix, Dinobryon, Oedogonium are less than in 1962. Aphanothece sp., Asterococcus limneticus G. M. SMITH which were not recognized in 1962, appeared in 1963, the former in great number only in June, the latter in June, September and November.
The fact that Oscillatoria tenuis C. A. AGARDH, Synedra ulna (NITZSCH) EHR., Eudorina elegans EHRENBERG are recognized in this reservoir may suggest that the water of this artificial lake is rich in nutrient salts.