EXPLICATION ON GROWING FACTORS OF BLOOM OF GONYOSTOMUM SEMEN(RAPHIDOPHYCEAE) AND IMPROVEMENT ON THE WATER QUALITY OF A LAKE

通过对川原湖水质的野外观测，发现夏季浮游植物生物量在中层密集生长和聚集，优势种为异形口藻精液。为了弄清影响棉铃虫精液聚集层迁移的因素，对3个附着体进行了田间试验。为了改变水质分层的状况，将一个中层水体的一部分下部水体转移到另一个中层水体。在白天，精液聚集在约5.5米深处，在夜间聚集在约7.5米深处。通过室内实验发现，精液在夜间向下移动，到达厌氧层后停止向下迁移。在白天，棉铃虫精液的向上迁移受到pH值和光子通量密度的影响。此外，在频繁的野外调查中发现，棉铃虫精液的开花是在暴雨后产生的，因此触发棉铃虫精液快速增殖的触发因素是随着河流流量的流入而来的NO_3的供应。

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. 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 the daytime G.semen aggregated at the depth of about 5.5m and in the nighttime about 7.5m. 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 NO_3 with inflowing of river discharge.