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

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.

Bacteria and Archaea Regulate Particulate Organic Matter Export in Suspended and Sinking Marine Particle Fractions

“…that influences the flux of POM from the surface layer ( 47 , 48 ). For example, recent studies have demonstrated a positive relationship between phytoplankton biomass and the magnitude of POM export ( 49 , 50 ), with our data corroborating these findings. In addition, POM content (labile, semi-labile, recalcitrant, or refractory) rather than concentration is considered the main driver of prokaryotic community structure ( 51 , 52 ), with sinking POM subject to degradation by several FL and particle-attached (PA) prokaryotes ( 44 ) that alter its chemical and biological properties ( 53 ).…”

Prokaryotes Regulate Particulate Organic Carbon Export in Suspended and Sinking Particle Fractions

“…Differences in prokaryotes may explain the divergence in POC and PON concentration in both the sinking and suspended particle pools Recent studies have shown a positive relationship between phytoplankton biomass and the magnitude of POM export [32,33]. These ndings were corroborated by our data showing a positive relationship between MLD integrated chlorophyll and POM ux.…”