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

Y.-F Tseng,J Lin,M Dai,S.-J Kao

doi:10.5194/bg-11-409-2014

Abstract

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.

Highlights

Rapid urbanization in past decades along coastlines has introduced massive loadings of nutrients and organics into rivers, causing dramatic changes of nutrient status in adjacent estuarine and coastal ecosystems (Jickells, 1998)
We presented 3-D structures of hydrographic parameters, nutrients, chlorophyll a (Chl a), and applied alkaline phosphatase activity (APA) assays to probe whether phytoplankton physiologically endured P stress in such a N-replete and hydrographically complicated CJ plume region
The region we focused on was mainly influenced by the Changjiang Diluted Water (CDW) (CDW is defined as the water with salinity < 31; Gong et al, 1996; Fig. 2b) with high nitrate concentrations and N / P ratios (Shen and Liu, 2009)

Summary

Introduction

Rapid urbanization in past decades along coastlines has introduced massive loadings of nutrients and organics into rivers, causing dramatic changes of nutrient status in adjacent estuarine and coastal ecosystems (Jickells, 1998). These rivers are well known to carry nutritional water with high ratios of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP), or N / P ratios. Such massive yet disproportionate riverine nutrient input has caused an obvious “excess N” (the concentration of DIN in excess of DIP) phenomenon in nearby estuarine ecosystems (Galloway et al, 2004). Ecosystems’ response to nutrient loading has been demonstrated; the information about the dynamic nutrient pattern of excess N versus the influence of nutrient stoichiometry on phytoplankton growth around riverine plumes and adjacent coastal ecosystems requires more exploration

Methods

Results

Conclusion