中文摘要：

针对汉江中下游春季硅藻水华频繁暴发的现状，研究了水华暴发不同时期浮游植物群落结构特征，并结合水体理化因子分析，揭示硅藻水华发生的主要影响因子。结果表明：汉江春季水华暴发期间，共鉴定出浮游植物7门48属64种（含变种），其中硅藻种类所占比例最大（18属23种），不同样点所占比例介于44.13%～51.29%，其次是绿藻类（16属22种），不同样点所占比例介于28.21%～35.14%，蓝藻门和隐藻门种类数在水华期间较少，各样点仅发现2～3种，金藻门和甲藻门种类最少；水华发生期间主要优势种为硅藻门的小环藻，水华暴发初期和中期往往形成单优种，而水华末期随着其优势度的降低，蓝藻门、隐藻门和硅藻门其他种类也会形成优势种；水华暴发期间浮游植物密度最高可达2.34×107 cells·L-1，其中小环藻种群数量最高可达2.06×107 cells·L-1，占浮游植物群落总数的88%；浮游植物多样性指数中，Margalef物种丰富度指数无显著时空差异，Shannon多样性在水华中期较低，而水华初期和末期相对较高，BergerParker指数变化与Simpson指数相反；浮游植物与理化因子的冗余分析（RDA）发现，浮游植物总数量与磷酸盐、溶解氧、总磷呈正相关，与硝酸盐、温度、透明度呈负相关。

英文摘要：

Diatom bloom occurs frequently in the middle and lower reaches of Hanjiang River during spring season. In this paper, we investigated the characteristics of phytoplankton community structure at different bloom stages and analyzed the major physical and chemical factors influencing water bloom. During the spring bloom season, 64 phytoplankton species （including varieties） belonging to 48 genera and 6 phyla were identified in Hanjiang River. The largest proportion was the species of diatom （23 species, 18 genera）, which accounted for 44.12%-51.28% of the total phytoplankton species at different sampling sites. The second was the species of Chlorophyta （22 species, 16 genera）, which accounted for 28.21%-35.14% of the total species. Only 2-3 species from either Cyanophyta or Cryptophyta were found at each sampling site during this season. Species of Chrysophyta and Dinophyta were the least. The absolutely dominant species in this period were found to be Cyclotella sp. （Bacillariophyta）. In the early and middle stages of the bloom season, Cyclotella sp. was the predominant component of phytoplankton while other species of Bacillariophyta, Cyanophyta and Cryptophyta co-dominated in the late stage. During the bloom season, the phytoplankton population density could reach to 2.34×107 cells·L-1, while the density of Cyclotella sp. could reach up to 2.06×107 cells·L-1, accounting for 88% of the total phytoplankton density. Several diversity indices of the phytoplankton were also calculated. The Margalef index had no significant difference in space and time. The Shannon index was lower in the middle stage of the bloom season than in the early and the late stages. On the contrary, the trend of BergerParker index and Simpson’s index were the opposite. Redundancy analysis （RDA） on phytoplankton and environmental factors in Hanjiang River revealed that the total density phytoplankton was closely related to phosphate （PO43--P）, dissolved oxygen （DO） and total phosphorus