Stimulation of Heterotrophic and Autotrophic Metabolism in the Mixing Zone of the Kuroshio Current and Northern South China Sea: Implications for Export Production

To evaluate the influences of the Kuroshio Current intrusion on the biogeochemistry of the northern South China Sea (NSCS), we conducted field observations of the responses of microbial metabolism to the intrusions of the Kuroshio Current into the NSCS. We used an isopycnal mixing model to quantitatively assess the extent of the Kuroshio intrusion into the NSCS and found that bacterial abundance, production, and growth efficiency were unimodal functions of the fraction of Kuroshio water. Values were maximal at ~60% Kuroshio water and decreased monotonically as the percent of Kuroshio water deviated from 60%. The patterns of gross primary production and nitrate concentration were similar, but the peaks occurred at ~50% Kuroshio water. The Kuroshio intrusion, however, had little impact on bacterial and community respiration. The observed elevation of nitrate concentrations at ~50% Kuroshio water was quantitatively consistent with estimates of the amount of inorganic nitrogen released by remineralization of dissolved organic nitrogen by bacteria. Based on these observations, we hypothesize that dissolved organic matter in the Kuroshio water stimulates bacterial activity, and catabolism of the dissolved organic matter releases inorganic nutrients that stimulate primary production in the NSCS basin. The concomitant elevation of net community production subsequently enhances export production and thereby strengthens the biological pump in the nitrogen-limited NSCS basin. Recognition of this mechanism underscores the role of horizontal currents in regional carbon and nitrogen cycles and broadens understanding of the processes that sustain export production in the oligotrophic ocean.