Study On The Sinking Rate Of Phytoplankton In The South China Sea

The South China Sea is the largest and deepest offshore area in China,but there are few studies on the sinking rates and carbon flux of phytoplankton in this area so far.In this paper,the structure of phytoplankton community,the sinking rate and the carbon flux of phytoplankton in the South China Sea were studied through the the voyages of the western South China Sea in autumn 2016,the Central South China Sea in spring 2017 and the northeastern South China Sea in summer 2017.Firstly,Utermohl method was used to investigate the phytoplankton community.The results showed that the cell abundance of phytoplankton in the central South China Sea was higher than that in the northeastern and western regions,while Bacillariophyta and Dinophyta were the main phytoplankton groups and Cyanophyta had the highest cell abundance in the whole South China Sea.Secondly,SETCOL method was used to study the sinking rate of phytoplankton.It was found that the sinking rate was the highest in the western South China Sea in autumn 2016,as that was the lowest in the northeastern South China Sea in summer 2017.Their sinking rates were in the range of 0.12-3.17 m d-1,-0.09-2.60 m d-1 and-0.79-4.23 m d-1,respectively,and their average value was corresponding to 0.72 m d-1,0.72 m d-1 and 0.22 m d-1.The correlation analysis with environmental factors was performed and the results indicated that the sinking rates of phytoplankton for the western South China Sea in autumn 2016 and the Central South China Sea in spring 2017 were negatively correlated to temperature,but positively correlated to phosphate and silicate.Meanwhile the sinking rate in the northeastern South China Sea in summer 2017 was significantly correlated to temperature and silicates.Finally,the carbon flux of phytoplankton was calculated based on the results of sinking rate.The results showed that the carbon flux in the western South China Sea in autumn 2016,the central South China Sea in spring 2017 and the northeastern South China Sea in summer 2017 were within the range of 2.41×10-6-0.0062 mg C m-2 d-1,3.75×10-6-0.0008 mg C m-2 d-1 and-1.21×10-5-4.14×10-5 mg C m-2 d-3,respectively,whose average value was corresponding to 0.0127 mg C m-2 d-1,0.0012 mg C m-2 d-1 and 8.24×10-6 mg C m-2 d-1.The highest carbon flux of phytoplankton was found in the western South China Sea in autumn 2016,while the lowest was in the northeastern South China Sea in summer 2017.Above all,due to the significant influence of physical or chemical factors such as temperature and nutrients,the distribution of phytoplankton communities,the sinking rate and carbon flux of phytoplankton in the South China Sea were obvious different in time and space.And the carbon flux of the western South China Sea was the strongest.These would provide useful insight into the process of carbon cycling in this area and lays a data foundation for further study of the carbon cycle in the South China Sea.