Spatial Correlation And Seasonal Variation Of Phytoplankton And Environmental Factors In The Central Bohai Sea

In this thesis, spatial distribution and season variation of seawater temperature, salinity, nutrient concentrations, chlorophyll a(Chl-a) concentrations and phytoplankton community structures were investigated in the Bohai Sea in July 2013, May 2014 and November 2014. The results showed significant seasonal differences. We also analyzed the correlation between phytoplankton and environmental factors based on the principal component analysis(PCA) and canonical correspondence analysis(CCA). The result explored the main factors affecting the phytoplankton biomass and community structures in different seasons and provided basic materials for the studies of biological resources and ecological disaster in the Bohai Sea.In spring, influenced by the Yellow River and the Yellow Sea, the Yellow River Estuary(YRE) showed high temperature and low salinity while the Bohai Strait was inverse. The total dissolved inorganic nitrogen(DIN) was much higher in the YRE than other areas. Vertically, the nutrient concentrations of bottom layer were slightly higher than the surface and middle layers. Phosphate and silicate limitations were found according to the nutrient concentrations and structures and the silicate limitation was more serious. The Chl-a concentration was much higher in the YRE and the Luanhe Estuary and the concentration of surface layer was a little bit higher than the middle and bottom. The result of PCA indicated that temperature, salinity, inorganic nitrogen and DIN to phosphate ratio(N/P) were the main factors affecting the distribution of Chl-a. Diatoms were the most important class in the study area and the relative abundance was 57%. And the central area showed lower diatoms biomass than other areas. The high value areas of cryptophytes were located in the YRE and Laizhou Bay and the relative abundance was 29%. Prasinophytes showed high values in strait areas and the relative abundance was 10%. The relative abundances of chlorophytes, chrysophytes, dinoflagellates and haptophytes were between 1% and 3%. Cyanobacteria and prochlorococcus were the fewest classes that the relative abundance was less than 0.3%. Vertically, the relative abundance of diatoms were increased with the depth while others were decreased or had no significant changes. The result of CCA showed that temperature, salinity, DIN and silicate to DIN ratio(Si/N) were the main factors affecting the phytoplankton community structures in spring.In summer, vertical gradients were found with higher temperature, lower salinity and nutrient concentrations in the surface water than the middle and bottom layers. On the horizontal pattern, the YRE and Laizhou Bay were characterized by lower salinity, higher temperature and nutrient concentrations than other areas. The nutrient characteristics showed obvious relative and absolute phosphate limitation in the study area and the silicate supply from river discharge relieved the silicate limitation. High Chl-a concentration was mainly found around the estuaries which illustrated the influence of terrigenous materials. The result of PCA indicated that salinity, phosphate and silicate concentrations, DIN to phosphate ratios and DIN to silicate ratios were major factors related to the Chl-a distribution. Besides, the temperature may have an effect on the vertical variations of Chl-a concentration. Cryptophytes became the dominant class and the relative abundance reached 57%. The high value regions were still in the YRE and Laizhou Bay. While the relative abundance of diatoms dropped to 19% and mainly distributed in the northwestward and strait areas. The relative abundances of prasinophytes, cyanobacteria and haptophytes were all between 5% and 10%. While relative abundances of chlorophytes, chrysophytes and dinoflagellates were between 1% and 5%. The relative abundance of chrysophytes, cyanobacteria and haptophytes of the surface layer were much higher than the middle and bottom layers while diatoms and cryptophytes were opposite. The result of CCA showed that the concentrations and structures of nutrients were the main factors affecting the phytoplankton community structures in summer.In autumn, the most obvious characteristic was vertical mixing uniformity. Small area of the YRE showed low temperature and salinity. While the Bohai Strait showed high salinity and the high salinity water tongue extended to the central area. The nutrient concentrations were much higher while the Chl-a concentration was much lower than spring and summer. The nutrient concentrations and structures indicated that the nutrient limitation almost disappeared in autumn. The result of PCA indicated that temperature and salinity were more important factors affecting the Chl-a distribution. Diatoms and cryptophytes were the dominant classes and the relative abundance were 44% and 37% respectively. And their high values both distributed in the strait and central areas. The distribution of prasinophytes could be described as high in the east and low in the west and relative abundance was 13%. Chlorophytes and haptophytes were between 1% and 3% and cyanobacteria, dinoflagellates and prochlorococcus were about 0.5%. Chrysophytes were the least class that the relative abundance were only 0.01%. The result of CCA showed that temperature, salinity, DIN and(N/P) were the main factors affecting the phytoplankton community structures in autumn.The main factors effecting the phytoplankton biomass and community structures were significantly different. The concentrations and structures of nutrient were the important factors, while the impact of temperature and salinity were more remarkable in summer. Cryptophytes and prasinophytes were found high biomass in the Bohai Sea which should not ignore.