Joint effect of freshwater plume and coastal upwelling on phytoplankton growth off the Changjiang River

The Changjiang (Yangtze) River discharges vast amount of unbalanced nutrients (dissolved inorganic nitro- gen and phosphorus with N / P ratio > 80 in general) into the East China Sea in summer. To study nutrient dynam- ics and P-stress potential for phytoplankton, a cruise was conducted in the Changjiang plume during summer 2011. With 3-D observations of nutrients, chlorophyll a (Chl a), and bulk alkaline phosphatase activity (APA), we concluded that the Changjiang Diluted Water and coastal upwelling significantly influenced the horizontal and vertical hetero- geneities of phytoplankton P deficiency in the Changjiang plume. Allochthonous APA was detected at nutrient-enriched freshwater end. Excessive N ( 10 to 112 µM) was observed throughout the entire plume surface. In the plume fringe fea- turing stratification and excess N, diapycnal phosphate sup- ply was blocked and phytoplankton APA was stimulated for growth. We observed an upwelling just attaching to the tur- bidity front at seaward side where Chl a peaked yet much less APA was detected. An external phosphate supply from subsurface, which promoted phytoplankton growth but inhib- ited APA, was suggested to be sourced from the Nearshore Kuroshio Branch Current. In the so hydrographically com- plicated Changjiang plume, phosphate supply instead of its concentration may be more important in determining the ex- pression of APA. Meanwhile, allochthonous APA may also alter the usefulness of APA as a P-stress indicator.