Controlling Mechanism Of Size-specific Photosynthetic Parameters Of Phytoplankton In The Taiwan Strait

In the marine ecosystem,the phytoplankton photosynthesis in the sunlit layer drive the transmission of substance and energy in the ecosystem.The photosynthesis of phytoplankton play a critical role in global carbon cycles and regulating the earth's climate.Size structure is an important biological feature of the phytoplankton.The study of size structure and photosynthetic physiology of phytoplankton contribute to the further analysis of the structure and function of marine ecosystem.In this study,size-fractionated?>2 ?m and<2 ?m?photosynthesis-irradiance experiments coupling temperature,irradiance,phytoplankton community structure were conducted in the temperate monsoon-driven Taiwan straits during November 2015,March-April 2016,July 2016 and August 2016,respectively.Our study explored the seasonal variability and control on size-dependent photosynthetic parameters of phytoplankton in the temperate monsoon-driven Taiwan Strait.The main results were:?1?The depth-integrated primary production in the euphotic layer ranged from 242.7?1174.1 mg Cm-2d-1 in autumn,171.4?1000.0 mg Cm₂ d-1 in spring and 129.3?4328.3 mg Cm-2d-1 in summer,respectively.The depth-integrated primary production showed positive correlation with the euphotic layer depth-integrated Chl a?p<0.001?.Moreover,the depth-integrated primary production was significantly correlated with the contribution of>2?m phytoplankton biomass to total biomass?p<0.001?.The size-dependent euphotic layer depth-integrated Chi a and primary production revealed obviously seasonal variation in the Taiwan Strait.The>2?m size phytoplankton contributed mostly of depth-integrated Chl a and primary production during autumn,the contribution of<2?m size phytoplankton increased during spring,moreover,<2?m size phytoplankton contributed most of biomass and primary production except the coastal stations which influenced by physical process during summer.?2?The phytoplankton assimilation numbers were significantly correlation with the phytoplankton size structure,assimilation numbers were positive correlated to>2?m phytoplankton contribution to total biomass in summer?p<0.01?,whereas showed negative relationship in spring?p<0.05?.PBm and a showed the highest value during autumn cruise,which might associated to the lowest irradiance during autumn and low-light acclimation accelerating the phytoplankton photosynthetic pigment synthesis and enzymatic reactants?ATP,NADPH?.In addition,the PBm and ?B were similar in size during autumn,which might correspond to the constrain to PSI and PSII in both size phytoplankton.PBm and a of pico-phytoplankton were greater than large phytoplankton during spring,which might in related to the package effect that<2?m phytoplankton own the higher light absorption coefficient.However,The PBmof>2?m phytoplankton sustained higher than pico-phytoplankton and a was similar in size in summer,suggesting the absorption of irradiance might not the effect that constrain the photosynthesis and the higherPBm might associated to the higher PSII efficiency or the higher nutrient storage ability of>2 ?m phytoplankton.The Ek of>2?m phytoplankton was significantly greater than the pico-phytoplankton,which may indicate that causing the package effect,the pico-phytoplankton have lower irradiance requirements than large size class phytoplankton to saturate photosynthesis.In addition,the APP index increased with the irradiance,and Ek showed positive correlation to the APP index.In autumn cruise,the PBm and ? demonstrated no significantly discrepancy in optical depth,whereas both size of PBm and ? was decreased with optical depth increased in spring and summer?PBm:p<0.05,?:p<0.05,Spring;PBm:p<0.001,a:p<0.001,Summer,Mutil-ANOVA Analysis?,suggesting the distinct photoacclimation mechanism in mixing water and stratification water.?3?The variation of PBm and a in related to temperature and irradiance demonstrated distinct tendency.The leas clear variation of a in relate to temperature,indicating that a represent the photochemical process and not direct influenced by temperature.In addition,a revealed size-related difference variation with irradiance,which might associated with the variation with phytoplankton pigments.Temperature or irradiance might be a good indicator of phytoplankton photosynthetic performance in Taiwan Strait.The PBm and a of>2?m phytoplankton showed no significantly correlation with diatom to total Chl a rate in eutrophic autumn,spring and summer stations in our results,however,The PBm and a of<2?m size class in oligotrophic summer stations were significantly negatively correlated with the concentration of prochlorococcus to total Chl a?PBm:p<0.05,a:p<0.05?,suggesting the phytoplankton community structure might be an important control on the phytoplankton photosynthetic performance in Taiwan Strait.