Biological pumps of carbon, nitrogen, and phosphorus in the northern South China Sea

Abstract. This paper presents the measured biological pumps (BPs) 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 BP 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 BP. The magnitudes of total C, N, and P fluxes were respectively estimated to be 71.9–347 (mean: 163) mg C m−2 d−1, 13.0–30.5 (mean: 21.6) mg N m−2 d−1, and 1.02–2.97 (mean: 1.94) mg P m−2 d−1, which were higher than most previously reported BPs in open oceans, likely because a quarter of the BPs was contributed from active fluxes that were unaccounted for in BPs previously. Moreover, the passive fluxes dominated the BPs and were estimated as 65.3–255 (mean: 125) mg C m−2 d−1 (76.7 % of total C flux), 11.9–23.2 (mean: 17.6) mg N m−2 d−1 (83.0 % of total N flux), and 0.89–1.98 (mean: 1.44) mg P m−2 d−1 (74.2 % of total P flux). Vertical fluxes of dissolved organic C, N, and P generally contributed to less than 5 % of 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 impacts of anticyclonic eddies and internal-wave events on BP enhancement 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 prediction of active and passive fluxes for unmeasured events. To a first approximation, the SCS may effectively transfer 0.208 Gt C yr−1 into the ocean's interior, accounting for approximately 1.89 % of the global C flux. The internal forcing and climatic conditions are likely critical factors in determining the seasonal and event-driven variability of BP in the NSCS.