Abstract: Diatom was the dominant taxa of phytoplankton with a total phytoplankton density of 66%-83% during the study period (2011-2013). Skeletonema costatum was the dominant species in diatom taxa (Y≥0.1), and the following dominant species was Paralia sulcata. The other major dominant species were nearshore protosalt species， including Cocconeis placentula, Coscinodiscus marginato-lineatus, C. granii, Nitzschia pungens and Thalassionema nitzschioides. There were more dominant species in winter, whose species number were greater than 10, whereas those were less in summer and autumn with a species number of 2-6. The seasonal variations of total nitrogen and total phosphorus concentrations were significant with a higher level in summer and lower level in winter. Analysis of variance indicated the difference of phytoplankton density between three years was significant (P＜0.05, F(2, 69)=6.22). The monthly average density of phytoplankton change each year without stable rule was related to the tidal changes in harbor. Pearson correlation analysis showed the diatom density had significant positive correlation with suspended solids and total phosphorus (P<0.01), thus phytoplankton density was affected by phosphorus greatly. Since the 1980s the density of phytoplankton increased significantly and the increasing rate of phytoplankton density was up to 103, 102 and 10 in 1980s, 1990s and after 2000s, respectively. The number of dominant species in dinoflagellates was increased and the species composition was also changed while the size of dominant species in diatom was minimized gradually. Compared to the historical data, the increment of phytoplankton density and the change of community structure were closely associated with eutrophication, imbalance of nutrient structure and phosphorus deficiency in Yangshan port.

Key words: Yangshan port, phytoplankton, community structure, eutrophication, succession