Active and passive fluxes of carbon, nitrogen, and phosphorus in the northern South China Sea

Jia-Jang Hung,Ching-Han Tung,Zong-Ying Lin,Yen-Huei Lin,Li-Shan Tsai,Shao-Hung Peng,Yuh-Ling Lee Chen

doi:10.5194/bg-18-5141-2021

Abstract

Abstract. This paper presents the measured active and passive fluxes of carbon (C), nitrogen (N), and phosphorus (P) and their response to seasonal and event-driven oceanographic changes in the northern South China Sea (NSCS). The total vertical flux of carbon (TFC) is defined as the sum of active and passive fluxes of biogenic carbon in the surface layer, which may be considered as the central part of marine carbon cycle. These active and passive fluxes of N and P were also considered to understand stoichiometric flux patterns and the roles of nutrients involved in the TFC. The magnitudes of total C, N, and P fluxes were, respectively, estimated to be 71.9–347 (mean ± SD, 163 ± 70) mgCm-2d-1, 13.0–30.5 (21.2.± 4.9) mgNm-2d-1, and 1.02–2.97 (1.94 ± 0.44) mgPm-2d-1, which were higher than most previously reported vertical fluxes in open oceans, likely because a quarter of the fluxes was contributed from active fluxes that were unaccounted for in vertical fluxes previously. Moreover, the passive fluxes dominated the total vertical fluxes and were estimated to be 65.3–255 (125 ± 64.9) mgCm-2d-1 (77 ± 52 % of total C flux), 11.9–23.2 (17.6 ± 4.2) mgNm-2d-1 (83 ± 28 % of total N flux), and 0.89–1.98 (1.44 ± 0.33) mgPm-2d-1 (74 ± 24 % of total P flux). Vertical fluxes of dissolved organic C, N, and P were small (< 5 %) relative to passive fluxes. The contrasting patterns of active and passive fluxes found between summer and winter could mainly be attributed to surface warming and stratification in summer and cooling and wind-induced turbulence for pumping nutrients into the euphotic zone in winter. In addition to seasonal variations, the impact of anticyclonic eddies and internal-wave events on enhancing active and passive fluxes was apparent in the NSCS. Both active and passive fluxes were likely driven by nutrient availability within the euphotic zone, which was ultimately controlled by the changes in internal and external forcings. The nutrient availability also determined the inventory of chlorophyll a and new production, thereby allowing the estimates of active and passive fluxes for unmeasured events. To a first approximation, the South China Sea (SCS) may effectively transfer 0.208 ± 0.089 Gt C yr−1 into the ocean's interior, accounting for approximately 1.89 ± 0.81 % of the global C flux. The internal forcing and climatic conditions are likely critical factors in determining the seasonal and event-driven variability of total vertical fluxes in the NSCS.

Highlights

It is widely recognized that the global ocean may have absorbed anthropogenic CO2 as large as 50 % of total release to the atmosphere since the industrial revolution began in the middle of 18th century (Sabine et al, 2004)
The oceanographic conditions in the coast-excluded northern South China Sea (NSCS) domains were likely dominated by monsoon-mediated surface circulation and Kuroshio intrusion (Chen et al, 2005; Dai et al, 2013; Hung et al, 2007, 2020; Liu et al, 2002; Zhai et al, 2005, 2013)
A strong northeast monsoon prevails between November and April, and a weak southwest monsoon prevails between June and September, leading to a basin-wide cyclonic circulation being dominant in winter and an anticyclonic circulation being dominant in summer (Shaw and Chao, 1994; Liu et al, 2002; Wong et al, 2007)

Summary

Introduction

It is widely recognized that the global ocean may have absorbed anthropogenic CO2 as large as 50 % of total release to the atmosphere since the industrial revolution began in the middle of 18th century (Sabine et al, 2004). Hung et al.: Active and passive fluxes of C, N, and P in the northern South China Sea tion and grazing processes in the ocean surface (Falkowski, 1998; Ducklow et al, 2001; Sarmiento and Gruber, 2006; Passow and Carlson, 2012; Steinberg et al, 2000; Steinberg and Landry, 2017; Archibald et al, 2019). The investigation of active and passive fluxes in the large marginal sea appears to be important in increasing our understanding of the global context of oceanic carbon cycling and budgets

Methods

Results

Discussion

Conclusion