Abstract: Based on the multidisciplinary cruise investigation in the Yangtze River estuary and its adjacent waters in April 2009, the phytoplankton species and
their abundance were analyzed by the Utermöhl method, and the size-fractionated chlorophyll a concentrations were determined. In the meantime, the relationships between the dominant phytoplankton species and environmental physicochemical factors were explored by Canonical Correspondence Analysis (CCA). A total of 3 phyla, 46 genera, and 64 species (not including uncertain species) were found, mostly diatoms and dinoflagellates, with diatoms dominant. Most of these species were temperate and coastal, but a few brackish and oceanic species also presented. There were 33 diatom genera including 45 species. The dominant species were Skeletonema dohrnii, Paralia sulcata, Thalassionema nitzschioides, Pseudo-nitzschia pungens, Melosira granulata var. angustissima, Pseudo-nitzschia delicatissima, and Guinardia delicatula. The phytoplankton cell abundance ranged
from 0.3 to 13447.7 cells·ml^-1, with an average of 1142.385 cells·ml^-1. Concerning the horizontal distribution, cell abundance was the highest in the middle-northern part of the survey area, with S. dohrnii dominant. The phytoplankton cell abundance was high in the surface layer water, and decreased with increasing depth. The Shannon diversity index and Pielou evenness index were consistently low in the middlenorthern part of the survey area, in contrast to the trend of phytoplankton cell abundance. The chlorophyll a concentrations ranged from 0.34 to 29 g·L^-1, with an average of 3.3 g·L^-1, consistent with the cell abundance distribution. Size-fractionated chlorophyll a results showed that the biomass in the middle-northern part of the survey area was mainly composed of microphytoplankton (>20 μm), while that in offshore waters was mainly composed of nanophytoplankton (2-20 μm) and picophytoplankton (<2 μm). The CCA showed that the distribution of predominant species S. dohrnii was mainly affected by the water nitrate content, pH value, and microzooplankton grazing. On the contrary, the distribution of other common species including dinoflagellates was mainly associated with water salinity, and phosphate and silicate contents. The paper also compared the differences in the methods of phytoplankton quantification used by the present study and by the previous studies based on net samples. It was suggested that in future work, the autecological study of genus Skeletonema in the Yangtze River estuary and its adjacent waters should be strengthened.

Key words: phytoplankton, assemblage structure, size-fractionated chlorophyll a, Yangtze River estuary, canonical correspondence analysis, spring, Prionace glauca, North Atlantic Ocean, demographic analysis, sensitivity analysis, age at first capture.