Seasonal Changes Of The Phytoplankton Community In The East China Sea In2011

In this paper, three multidisciplinary comprehensive investigations were carried out in the area (25°00’～33°30’N,120°00’～127°30’E) of the East China Sea from May to November2011. The characteristics of phytoplankton assemblages were analyzed by means of Utermohl method. The results further enriched the ecological study of phytoplankton in the East China Sea, and providing fundamental information for a long-period variation of phytoplankton assemblages. The major results were as followed:A total of248species which belonged to74genera of4phyla were identified in the East China Sea from May to November2011. Bacillariophyta and Pyrrophyta were the dominant phytoplankton groups in the survey area. Most of the phytoplankton ecotypes were eurythermal species and cosmopolitan species, while the warm-water species and oceanic species were occasional found. The species richness of spring ranked first, followed by summer, and the species richness of autumn ranked last.The spatial-temporal variation of phytoplankton cell abundance in the East China Sea was very obvious. The phytoplankton cell abundance varied between seasons and the average of cell abundance was decreased in turn from summer (85.002×103cells/L), spring (66.937x103cells/L) to autumn (5.811×103cells/L) in the surveyed area. The red tides were caught by Prorocentrum dentatum and Skeletonema costatum, Thalassiosira sp. were found in individual sampling stations of spring and summer. Horizontally, the surface distribution of phytaplankton cell abundance was relatively high in the inshore area and decreased slightly from inshore area to offshore area in spring and summer. On the contrary, the cell abundance was relatively high in the northeast of the surveyed area and increased slightly from inshore area to offshore area in autumn. Vertically, the cell abundance of phytoplankton was concentrated in surface water and decreased slightly with depth from surface to bottom in spring and summer, while the vertical distribution of phytoplankton cell abundance wasn’t obvious with depth change in autumn.The composition of dominant species had obvious seasonal variation. In spring, the dominant species were Prorocentrum dentatum, Pseudo-nitzschia delicatissima, Skeletonema sp., Paralia sulcata and Prorocentrum minimum; in summer, the dominant species were Skeletonema costatum, Thalassiosira sp., Chaetoceros sp., Coscinodiscus sp. and Paralia sulcata; in autumn, the dominant species were Paralia sulcata, Coscinodiscus sp., Pseudo-nitzschia delicatissim, Nitzschia sp. and Thalassiosira sp.. Paralia sulcata was the common dominant species in all three cruises. The succession trend of the phytoplankton community in this survey changed from Dinophyta-Bacillariophyta type in spring, Bacillariophyta-Dinophyta type in summer to Bacillariophyta-Cyanophyta type in autumn. Namely, the phytoplankton dominant species was mainly composed of Dinophyta (Prorocentrum dentatum, Prorocentrum minimum etc.) and coexisted with a small amount of Bacillariophyta(Pseudo-nitzschia delicatissim, Skeletonema sp. etc.) in spring. In summer, it was mainly composed of Bacillariophyta(Skeletonema costatum, Thalassiosira sp. etc.) and coexisted with Dinophyta (mainly included Ceratium fusus and Ceratium furca). In autumn, it was mainly composed of Bacillariophyta (Paralia sulcata, Coscinodiscus sp. and Pseudo-nitzschia delicatissim etc.) and coexisted with Cyanophyta (Only included Trichodesmium thiebaultii).The phytoplankton cell abundance had obvious diurnal fluctuation in three cruises in the East China Sea. In spring and summer, the diurnal fluctuation of cell abundance changed significantly in the upper water column and changed gentle in the lower water column. In D3and Jl stations in autumn, the diurnal fluctuation of cell abundance in various water levels changed gentle except D3station surface water and J1station bottom water.The divisity index of phytoplankton community also showed obvious seasonal variation. The surface distribution of Shannon-Wiener diversity index and Pielou evenness index was generally consistent, and the diversity indexes were low in the area with high cell abundance. The maximum of these two kinds of index was all appeared in autumn and decreased in turn from autumn, summer to spring.The relationship between the phytoplankton assemblages and environmental factors showed that the Pearson correlation between phosphate and total phytoplankton cell abundance were significant in all three cruises, whereas its Pearson correlation with other environmental factors had some differences in different seasons. The CCA analysis between phytoplankton cell abundance and environmental factors showed that different relationship in dominant species in the surveyed area was mainly induced by the ecological habits of phytoplankton dominant specis and the complexity environment.