Articles | Volume 11, issue 2
Biogeosciences, 11, 409–423, 2014

Special issue: Biogeochemistry and ecosystems in the western north Pacific...

Biogeosciences, 11, 409–423, 2014

Research article 28 Jan 2014

Research article | 28 Jan 2014

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

Y.-F. Tseng1, J. Lin2, M. Dai2, and S.-J. Kao1,2 Y.-F. Tseng et al.
  • 1Research Center for Environmental Changes, Academia Sinica, Taipei, China
  • 2State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China

Abstract. The Changjiang (Yangtze) River discharges vast amount of unbalanced nutrients (dissolved inorganic nitrogen and phosphorus with N / P ratio > 80 in general) into the East China Sea in summer. To study nutrient dynamics 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 heterogeneities 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 featuring stratification and excess N, diapycnal phosphate supply was blocked and phytoplankton APA was stimulated for growth. We observed an upwelling just attaching to the turbidity 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 inhibited APA, was suggested to be sourced from the Nearshore Kuroshio Branch Current. In the so hydrographically complicated Changjiang plume, phosphate supply instead of its concentration may be more important in determining the expression of APA. Meanwhile, allochthonous APA may also alter the usefulness of APA as a P-stress indicator.

Final-revised paper