In situ study on photosynthetic characteristics of phytoplankton in the Yellow Sea and East China Sea in summer 2013

• We measured the detailed distribution of photosynthetic characteristics of phytoplankton in the Yellow Sea and East China Sea in summer (June and August) for the first time.
• Fv/Fm of phytoplankton in summer was significantly positively associated with nitrate content (NO2-), which in fact reflects relationship between metabolism and photosynthesis.
• Photosynthetic characteristics and activity of phytoplankton are influenced by its biological characteristics and surrounding ecological factors.

In situ studies on photosynthetic characteristics of phytoplankton were important for the analysis of changes in community structure and for the prediction and control of algal blooms, but such studies of phytoplankton in offshore China were few. In this study, the detailed distribution of photosynthetic characteristics of phytoplankton in the summer of 2013 in the Yellow Sea and East China Sea was measured using Phyto-PAM (Pulse Amplitude Modulation). The phytoplankton community structure and the environmental parameters were also investigated to estimate the relationship between the distribution of the photochemical competence of phytoplankton and ecological factors. The total average Fv/Fm (the potential maximum quantum yield) value of phytoplankton in the Yellow Sea and East China Sea in summer 2013 was less than 0.5, reflecting that the photosynthetic activity of phytoplankton was relatively low. Fv/Fm of phytoplankton in summer was significantly positively associated with nitrate content (NO2−), which reflects relationship between metabolism and photosynthesis of phytoplankton: accompanied by NO2− metabolism, photosynthesis and photosynthetic capacity may be enhanced simultaneously, so the Fv/Fm value would increase with the NO2− released by phytoplankton. Through the in situ study on photosynthetic characteristics of phytoplankton in the Yellow Sea and East China Sea, we come to the conclusion that photosynthetic characteristics and activity of phytoplankton are influenced by its biological characteristics and surrounding ecological factors, such as irradiance, nutrients and phytoplankton community. Meanwhile, the thermally stratified structure and the movement of water masses, such as the Yangtze River diluted water, the Yellow Sea cold water mass and other different water system, also have an important impact on phytoplankton photosynthetic activity and characteristics. Greater understanding of the detailed photosynthetic characteristics of phytoplankton coupled with their community structure and the environmental parameters will encourage new researches and developments in studies of marine ecosystems.