A large scale of harmful algal bloom occurred in Zhu jian g River Estuary on South China Sea Coast From March to April in 1998,and made a great economic loss on the local fish farming. In the last two papers,we described th e morphological and biological features of the seven causative organisms,which a ll belonged to unarmored dinoflagellates,sampled from the most hit area--a tan ked fish farm near Guishan Island located on the west side of Zhujiang River E stuary,China,on April 15th 1998, when the bloom was on its peak. In the preset a rt icle,the morphological descriptions of three unarmored dinoflagellates Gyrodin ium instriatum Freudenthal et Lee,Pheopolykrikis hartmannii (Zimmerman n) Matsuoka et Fukuyo and Gymnodinium catenatum Graham (sampled from She nzhen Bay of Zhujiang River Estuary on April 24th 1998, when the bloom was going to its end) are described. rnThe dominant causative organism of the bloom was identified to be Gyr. instr iatu m with the highest density of 3.6×107cells/L,followed by Skeletonema cost atum (1.6×107 cells/L),Prorocentrum (6.1×106cells/L),Gym. mikimoto i (1.1×106cells/L) and Ceratium furca. The red tide organisms also incl uded Gym. sanguineum,Polykrikos schwartzii,P. hartmannii and Gym. catenatu m. Apparently,the bloom occurred in Shenzhen Bay was the same as that in Guish an Island,for almost all of the dominant causative organisms of Guishan Island b loom were found in the Shenzhen Bay bloom. However, diatom S. costatum was t he s econd dominant in Shenzhen Bay bloom,indicating that the unarmored dinoflagellat e bloom occurred in Zhujiang River Estuary would end. The percentages of diatom increased while the total densities of the bloom samples decreased, showing that the normal diatom community was replacing the unarmored dinoflagellate communit y when the bloom was going to its end.%对1998年4月22—24日采集于广东珠江口大规模赤潮 海域之一的深圳湾的活体水样和福尔马林固定水样进行赤潮原因生物的形态分类和群落构成 研究。结果发现，深圳湾赤潮水样里仍以甲藻为主，但硅藻也具相当的数量。其中，最占优 种为无纹螺沟藻(Gyrodinium instriatum),其最高密度为3.6×107cells/L；其次是 中肋骨条藻(Skeletonema costatum)的1.6×107cells/L、原甲藻(Prorocentrum )的 6.1×106cells/L。另外，三宅裸甲藻(Gymnodinium. mikimotoi)、叉角藻(Cerat ium furca)和红色裸甲藻(Gym. sanguineum)也占有一定的数量。其中，无纹螺沟藻 、哈曼褐色多沟藻(Pheopolykrikos hartmannii)和链状裸甲藻(Gym. catenatum) 为我 国首次记录。赤潮样品中浮游植物总数量与甲藻所占百分比成正比，而与硅藻所占比例成反 比。这说明深圳湾赤潮仍属于1998年春珠江口大规模裸甲藻赤潮的一部分，同时标志着此次 珠江口大规模赤潮已开始走向消亡，其特征就是裸甲藻占绝对优势的赤潮群落开始被硅藻和 其它甲藻所取代。
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