Contrasting Photophysiological Characteristics of Phytoplankton Assemblages in the Northern South China Sea

The growth of phytoplankton and thus marine primary productivity depend on photophysiological performance of phytoplankton cells that respond to changing environmental conditions. The South China Sea (SCS) is the largest marginal sea of the western Pacific and plays important roles in modulating regional climate and carbon budget. However, little has been documented on photophysiological characteristics of phytoplankton in the SCS. For the first time, we investigated photophysiological characteristics of phytoplankton assemblages in the northern South China Sea (NSCS) using a real-time in-situ active chlorophyll a fluorometry, covering 4.0 × 105 km2. The functional absorption cross section of photosystem II (PSII) in darkness (σPSII) or under ambient light (σPSII’) (A2 quanta-1) increased from the surface to deeper waters at all the stations during the survey period (29 July to 23 August 2012). While the maximum (Fv/Fm, measured in darkness) or effective (Fq’/Fm’, measured under ambient light) photochemical efficiency of PSII appeared to increase with increasing depth at most stations, it showed inverse relationship with depth in river plume areas. The functional absorption cross section of PSII changes could be attributed to light-adapted genotypic feature due to niche-partition and the alteration of photochemical efficiency of PSII could be attributed to photo-acclimation. The chlorophyll a fluorometry can be taken as an analog to estimate primary productivity, since areas of higher photochemical efficiency of PSII coincided with those of higher primary productivity reported previously in the NSCS.

浮游植物的生长及其所驱动的海洋初级生产力，取决于浮游植物细胞响应环境变化的光生理性能。南海（South China Sea, SCS）是西太平洋最大的边缘海，在调节区域气候与碳收支中发挥着重要作用。然而，目前针对南海海域浮游植物光生理特征的研究记载甚少。本研究首次采用实时原位活性叶绿素a荧光法，对南海北部（northern South China Sea, NSCS）的浮游植物群落光生理特征开展调查，调查覆盖面积达4.0×10^5平方千米。2012年7月29日至8月23日的调查期间，所有站位的黑暗条件下光系统II（photosystem II, PSII）功能吸收截面（σPSII）或环境光下光系统II功能吸收截面（σPSII’，单位：A2 光量子^-1）均随水深增加而升高。多数站位中，光系统II的最大光化学效率（黑暗条件下测定的Fv/Fm）或有效光化学效率（环境光下测定的Fq’/Fm’）随水深增加而升高，但在河流羽状流区域，该参数与水深呈负相关关系。光系统II功能吸收截面的变化可归因于生态位分化带来的光适应型遗传特征差异，而光系统II光化学效率的改变则源于光适应过程。由于光系统II光化学效率较高的海域与此前报道的南海北部高初级生产力海域高度吻合，因此叶绿素a荧光法可作为估算初级生产力的有效替代手段。