Study On Phytoplankton Community Structure Of Seamount Areas In The Pacific Ocean

Seamount is the most important and special geographical entity locates in the seafloor.The most significant regulation of seamount to the upper layer of marine ecological dynamic process via changing the hydrodynamic process and then to affect the phytoplankton community structure in the euphotic layer.Due to the differences in elevation,ocean current characteristics,temperature and salinity between seamounts,the driving and maintenance mechanism of this process has not been systematically sorted out,thus limiting our further understanding of seamount ecosystem.This thesis aims at the scientific problem of whether the phytoplankton community structure in the euphotic zone is responsive to the hydrodynamic process.We chose Demao Guyot,Batiza Guyot,Niulang Guyot and McDonnell Guyot in the Western Pacific Ocean and Ciluo Guyot in the Eastern Pacific Ocean,the investigation region of Chinese DY135-45 Cruise and Chinese DY135-50 Cruise,as the research object.A comparative study was carried out on the characteristics and spatial distribution pattern of phytoplankton community structure and biomass in the euphotic zone.The results showed that there was significantly higher Chl a concentration in summit of Guyots in the Western Pacific Ocean.The inventory Chl a in the summit of Demao Guyot and Batiza Guyot,Niulang Guyot,McDonnell Guyot were about 39.62mg m^-2,30.01 mg m^-22 and 31.82 mg m^-2,respectively.These values were about 1.5 to2.5 times of values at the side and bottom.Pico-phytoplankton was the dominant group in the guyots areas,mainly Synechococcus and Prochlorococcus,followed by haptophytes-8.Both diatoms and dinoflagellates contributed less than 10%to the biomass of the phytoplankton.From the profile,there was a rising of high salt water in the subsurface layer and the iso-density surface,so the maximum chlorophyll layer was obviously shallower.Although Demao Guyot and Batiza Guyot iso-density surface were decline,due to the upwelling of the mesoscale anticyclonic process,the biomass of phytoplankton was still high,and showing higher abundance in the west side and lower in the east side.This was mainly due to the influence of eddy on the eastern side and the nutrient supply ould not support the growth of phytoplankton,so it presents obvious spatial distribution characteristics.In Ciluo Guyots,phytoplankton show significant variability while comparing their biomass of the east and west summit and their adjacent areas.The maximum value was at west bottom station,and the minimum value was at east summit station,with an average value of 24.26±7.68 mg m^-2.The phytoplankton biomass at the west summit was about 2.2 times that of the east.Prochlorococcus was the most dominant group in the Ciluo Guyot,contributing about 46.2%to the biomass of total phytoplankton.Followed with Prasinophytes and Synechococcus were the second.Diatoms,chlorophytes and dinoflagellates all contributed less to TChl a in the whole research area.The temperature,salinity and chlorophyll a of the upper layer in Ciluo Guyots in the Eastern Pacific Ocean were characterized by typical stratification.At the summit station of the western guyot,the iso-density surface was slightly uplifted,and the corresponding chlorophyll maximum layer was also uplifted to some extent.The difference between east and west guyots may be due to the terrain blocking on the western side,which leads to the nutrients restriction of phytoplankton growth in the eastern.Comparing the biomass and community structure of phytoplankton in the Eastern and Western Pacific Ocean,it was found that the average inventory Chl a value of the guyots in the Western Pacific Ocean was higher than that of the guyots in the Eastern Pacific Ocean,which may be caused by the presence of a pair of cold eddy and warm eddy during the cruise in the Western Pacific Ocean.These guyots belong to the deep sea,although every summit exsist high chlorophyll a phenomenon,but the maximum its mainly appear in the middle adjcent summit.Compared with the previous studies of shallow seamount and middle seamount,which showing maximum chlorophyll a in the summit caused by Taylor column.we found that seamounts at different depths have different impacts on phytoplankton biomass.On the other hand,the phytoplankton groups showed similar structure in the oligotrophic sea,with pico-phytoplankton as the dominant group.The analysis of environmental factors affecting phytoplankton biomass shows that temperature is the most important factor affecting phytoplankton biomass.A strong correlation between environmental factor parameters could be shown.Basically,there was a significant negative correlation between temperature and all kinds of nutrients,and NO₂^-+NO₃^-?PO₄^3-and SiO₃² showed a very significant positive correlation.There is a good positive correlation between the temperature and Synechococcus,and a good correlation between the prochlorococcus and nutrients.