Comparison Of Phytoplankton Assemblage Structure In The Yangtze River (Changjiang) Estuary Before And After The Construction Of The Three Gorges Dam

The characteristics of phytoplankton assemblages in the Yangtze River (Changjiang) Estuary and its adjacent water in 2006 are studied in this paper including species composition,cell abundance,dominant species,community diversity and seasonal variation.The purpose of this study is to recognize the impacts of the environment factors on the phytoplankton after the Three Gorges Dam built on the Yangtze River.Besides,comparison of other 9 cruises provides basic founding of state art of phytoplankton assemblages and fundamental data for estuary ecosystem protecting researchs in this region.The results show that total phytoplankton species richness is 210.Of them 57 genus 130 taxa are found in Bacillariophyta,20 genus 69 taxa in Pyrrophyta,4 genus 5 taxa in Chlorophyta,2 genus 2 taxa in Chrysophyta,3 genus 3 taxa in Cyanophyta. Diatoms and dinoflagellates were the main phytoplankton groups in the study area. Most of phytoplankton ecotype in survey area was temperate coastal species,while the warm water species and oceanic species sparely appeared.The dominant species were Paralia sulcata,Thalassiosira excentricus and Thalassiosira rotula in winter; Prorocentrum dentatum,Skeletonema costatum and Karenia mikimotoi in spring; Dactyliosolen fragilissima and Skeletonema costatum in summer;Thalassiosira rotula, Skeletonema costatum and Coscinodiscus oculus-iridis in autumn.Skeletonema costatum was annual dominant specie,with the larger cell abundance existed in onshore in spring.The cell abundance varies between seasons with the maximum 398.51 cells·ml^-1 appearing in spring and 290.92 cells·ml^-1 in summer.Distinctive Shannon-Wiener diversity index and Pielou evenness index are found in different seasons with the least optimum appearing in spring.The maximum Shannon-Wiener diversity index and evenness index respectively appears in winter and autumn.Phytoplankton cell abundance in surface water increased in spring after the Three Gorges Dam built on the Yangtze River,the while Shannon-Wiener diversity indices and Pielou's Evenness indices decreased.Distribution of phytoplankton and dominant species varied in these 4 years.The larger cell abundance existed in the 122°30'E-123°30'E area.Different patterns of phytoplankton vertical distribution are found in 2003,2005 and 2006.Proportion of centricae diatoms reduced from 2003,meanwhile cell abundance of dominant species distinctly increased.Phytoplankton cell abundance and species in surface water rapidly decreased in summer after the Three Gorges Dam built,meanwhile Shannon-Wiener diversity indices and Pielou's Evenness indices increased.The dominant species was Dactyliosolen fragilissima in 2006,while Skeletonema costatum was the dominant specie in other years.Skeletonema costatum cell abundance change obviously among different year.In summer the larger cell abundance of dinoflagellates existed in offshore,phytoplankton distribution mainly described by diatom.Phytoplankton cell abundance and species in surface water distinctly decreased in autumn after the Three Gorges Dam built on the Yangtze River.Shannon-Wiener diversity indices increased,but Pielou's Evenness indices decreased.Cell abundance of Skeletonema costatum decreased yearly.Several dominant species coexitsted and described the ptiytoplankton distribution.The high cell abundance existed in the region of 31°N～31°30'N,1,122°E～123°30'E in 2004,2005 and 2006.No obvious regulation of the distribution of phytoplankton high cell abundance area is found before and after the construction of the Three Gorges Dam.The runoff and ratio of diatoms to dinoflagellates decreased after the construction.The increase of proportion oceanic and warm-water species shows that the assemblage structure has changed in the estuary area.