Factors of influence on diel vertical migration of Gonyostomum semen (Raphidophyceae)

By the field observation on the water quality in Kawahara Lake, it was found that the phytoplankton biomass, of which dominant species was Gonyostomum semen (G. semen ), grew and aggregated densely in the middle layer in summer season. The diel migration of aggregated layer of G. semen was observed by using the mesocosm in the field. In the daytime G. semen aggregated at the depth of about 3m and in the nighttime about 6.5m. Then, the laboratory experiment by using the microcosm was conducted to make clear characteristics on diel vertical migration of G. semen. As the results of the microcosm experiment, it was found that G. semen moved downward for the nutrient salts at nighttime, and G. semen stopped the downward migration when reached the anaerobic layer in strong reduction. Also, the upward migration of G. semen was influenced by pH and the photon flux density. In addition, there is a correlation in pH and the photon flux density in the top of G. semen\u27s bloom layer. It was found that the optimum photon flux density for G. semen decreased with the rise of pH