Abstract: 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 (PO₄^3--P), dissolved oxygen (DO) and total phosphorus (TP), and negatively related to nitrate (NO₃^--N), temperature and transparence.

Key words: biogeochemistry., soil horizon, temperate forest