Abstract
By the field observation on water quality in Kawahara Lake, it was found that phytoplankton biomass, of which dominant species was Gonyostomum semen (G. semen), grew and aggregated densely in the middle layer in summer season. To clarify the influence factors on the diel migration of aggregated layer of G. semen, the field experiments with 3 mesocosms was conducted. A part of the lower water body in a mesocosm was transferred into the other mesocosm to change the condition of stratification for the water quality. In daytime G. semen aggregated at the depth of about 53m and in nighttime about 75m. As the results of the mesocosm experiment, it was found that G. semen moved downward for the nutrient salts at nighttime and stopped the downward migration when reached the anaerobic layer in strong reduction. Also, in daytime, the upward migration of G. semen was influenced by pH and the photon flux density.In addition, it was found that the bloom of G. semen was generated after the heavy rainfall from the frequent field researches, so that the trigger of a rapid proliferation of G. semen was the supply of NO3 with inflowing of river discharge.
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