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 (>2μm and <2μm) 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 C m-2 d-1 in autumn, 171.4-1000.0 mg C m-2 d-1 in spring and 129.3-4328.3 mg C m-2 d-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 μm phytoplankton biomass to total biomass (p <0.001). The size-dependent euphotic layer depth-integrated Chl a and primary production revealed obviously seasonal variation in the Taiwan Strait. The >2 μm size phytoplankton contributed mostly of depth-integrated Chl a and primary production during autumn, the contribution of <2 μm size phytoplankton increased during spring, moreover, <2 μm 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 μm phytoplankton contribution to total biomass in summer (p <0.01), whereas showed negative relationship in spring (p <0.05). and α 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 and αB were similar in size during autumn, which might correspond to the constrain to PSI and PSII in both size phytoplankton.  and α of pico-phytoplankton were greater than large phytoplankton during spring, which might in related to the package effect that <2 μm phytoplankton own the higher light absorption coefficient. However, The  of >2 μm phytoplankton sustained higher than pico-phytoplankton and α was similar in size in summer, suggesting the absorption of irradiance might not the effect that constrain the photosynthesis and the higher  might associated to the higher PSII efficiency or the higher nutrient storage ability of >2 μm phytoplankton. The Ek of >2 μm 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  and α demonstrated no significantly discrepancy in optical depth, whereas both size of  and α was decreased with optical depth increased in spring and summer (: p <0.05, α: p <0.05, Spring; : p <0.001, α: p <0.001, Summer, Mutil-ANOVA Analysis), suggesting the distinct photoacclimation mechanism in mixing water and stratification water.