Abstract
This paper deals with the influences of lightness and pH on the diel vertical migration of the phytoplankton (Gonyostomum semen). By the field observation on the water quality in Kawahara Lake, it was found that the phytoplankton biomass, of which dominant species was Gsemen, grew and aggregated densely in the middle layer in summer season. The diel migration of aggregated layer of Gsemen was observed by using the mesocosm in the field. In the daytime Gsemen aggregated at the depth of about 3m and in the nighttime about 6.5m. The light density of the aggregated layer in the daytime was fairly lower then the optimum photon flux density for Gsemen. Accordingly the laboratory experiment by using the microcosm was conducted to make clear the controlling factor other than lightness. As the results of the microcosm experiment, it was found the diel vertical migration of Gsemen was influenced by higher pH than about 7.8.
