||Zooplanktons play a critical role in the biological pump by acquiring organic material in the euphotic zone at night, and release part of the assimilated material including organic and inorganic matters below the euphotic zone during the day. This diet vertical migration drives the active fluxes of carbon (C), nitrogen (N) and phosphorus (P). Active fluxes are composed of respiratory flux, mortality flux, excretion flux and gut flux, which have been reported in the previous studies but are not yet reported in the northern South China Sea (NSCS) where is generally thought as tropical and oligotrophic. This study is the first to explore the active fluxes of CNP in the NSCS, and the results are also compared with those found in the northwest Pacific.|
The active migrant biomass were 376 mg m-2 in regular summer, but varied from 635 mg m-2 in an anticyclonic event to 997 mg m-2 in an internal-waves induced event. Active fluxes of C, N, and P were respectively estimated to be 25.6 mg C m-2 d-1, 2.68 mg N m-2 d-1 and 0.37 mg P m-2 d-1 in regular summer, about 50.0 mg C m-2 d-1, 4.09 mg N m-2 d-1 and 0.57 mg P m-2 d-1 in an anticyclonic event, and about 93.4 mg C m-2 d-1, 7.26 mg N m-2 d-1 and 0.99 mg P m-2 d-1 in an internal-waves induced event. The migrant biomass and active fluxes of CNP were significantly higher in special ocean events than in regular summer. Comparing to other studies at Canary Island in regular summer and anticyclonic event, our results show similar ranges and trends in fluxes. Furthermore, our studies in the northwest Pacific show that the magnitude of migrant biomass (158 mg m-2) and active fluxes of CNP (10.9 mg C m-2 d-1, 1.40 mg N m-2 d-1 and 0.57 mg P m-2 d-1) in regular summer were much lower than those in summer in the NSCS.
The proportion of active CNP fluxes to the sum of biological pump in the NSCS are 34.5±3.75%, 38.6±8.25% and 36.7±2.62%, respectively, in the regular summer, about 28.6%, 31.3% and 26.2%, respectively, in an internal-waves induced event, and about 32.4%, 25.8% and 32.8%, respectively, in the northwest Pacific. The active transport obviously plays an important role in the downward fluxes of CNP in the NSCS.