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

通过对河原湖水质的野外观测，发现该湖浮游植物生物量以Gonyostomum semen(G.；夏季在中间层生长和聚集密集。为明确影响天蚕聚集层日迁移的因素，对3个中观群落进行了田间试验。一个中间层中下部水体的一部分被转移到另一个中间层中，改变了水质的分层条件。在白天，g.s semen聚集在约5.5m的深度，在夜间聚集在约7.5m的深度。在中观实验中发现，夜间g.s ersemen对营养盐有向下移动的趋势，到达厌氧层后强烈还原，停止向下移动。在白天，g.s rsemen的向上迁移受pH和光子通量密度的影响。此外，通过频繁的野外考察发现，g.s ersemen的繁殖是在强降雨后产生的，因此，NO_3的供应是引发g.s ersemen快速繁殖的触发因素。

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.