DIEL VERTICAL MIGRATION OF Gonyostomum semen (Raphidophyceae) IN KAWAHARA LAKE

Seasonal changes and the vertical distributions of the water quality in Kawahara Lake were observed during Mar.-Sep. 2001. With progressing of thermal stratification in the lake, DO decreased gradually in the lower layer and the nutrients (NH4-N, PO4-P) released from bottom sediment. On the other hand, in the middle layer of 3-5m depth, phytoplankton biomass grew and aggregated densely in the state of thin layer about 1 m thick. The dominant species of the phytoplankton in that layer is Gonyostomum semen. G. semen has the character of diel vertical migration. Then, its diel vertical migration was investigated by using the mesocosm. As the results, it was able to be confirmed that its diel vertical migration received not only the photon flux density but also the influence of pH.