Net Community Production And Its Associated Controlling Mechanism In The South China Sea And Adjacent Western Pacific Ocean

In the context of the increasing CO2 emission due to the human activity and global climate change,the function of oceanic sequestration of atmospheric CO2 and its role in regulating the climate have become hot spots in the field of the marine biogeochemistry.The biological-mediated transfer and export of organic carbon from the surface to the deep ocean,so-called as the biological pump,is an important pathway for the ocean to uptake CO2.The net community production as well as its associated processes such as autotrophic and heterotrophic metabolism,are the good metrics to characterize the biological pump potential and efficiency.In this study,we combined multiple approaches,including BGC-Argo observations,deck incubations,satellitebased algorithms and numerical models to constrain the variability and the controlling mechanism of net community production in the South China Sea and the region of Pacific western boundary currents.The main findings were as follows:(1)For the first time,the high-resolution net community production(NCP)was estimated over a complete annual cycle in the basin of the South China Sea using oxygen measurements from an BGC-Argo profiling float and an oxygen mass balance model.NCP estimates followed a monsoonal pattern with higher monthly cummulative values in the cold season(0.29 mol C m-2)when northeast monsoon prevailed and low monthly cummulative values(0.1/mol C m-2)in the warm season when this area was dominated by the southwest monsoon.Most of the net heterotrophic events occurred in the warm season.The NCP showed a positive correlation with surface Chl-a concentration and wind speed.The annual NCP from July 2014 to July 2015 was estimated to be 2.7±1.0 mol C m-2 a-1,with the uncertainty of 1 mol C m-2 a-1.Comparison with satellite-derived NCP revealed that the results derived with NPP from Carbon-based Production Model(CbPM)were closer to the Argo measurements than the results derived with NPP from Vertically Generalized Production Model(VGPM)in magnitude;while the VGPM-based approach did a better job in reproducing the seasonal cycle of NCP in this area.This novel approach provides the possibilities to study the carbon cycle in the SCS with a much higher temporal and spatial resolution,as well as more insights for the metabolic state in the oligotrophic subtropical gyres.(2)Using the data collected from a cruise in the northern South China Sea basin during a period when a significant Kuroshio intrusion was detected,we analyzed concurrent measurements of autotrophic and heterotrophic metabolism together with the observations of nutrient pools to construct a more comprehensive picture of the impact of the Kuroshio Current on biogeochemical processes in the northern South China Sea basin.Firstly,we used an isopycnal mixing model to quantify the proportion of water contributed by the Kuroshio in the upper water column(RK).We observed that the microbial metabolism were unimodal functions of RK and there was an "optimum RK" for enhancement of microbial metabolism.The microbial metabolism decreased monotonically as RK deviated from "optimum RK".However,the optimum RK was 50%for autotrophic metabolism(i.e.,gross primary production,net community production and nitrate concentration)and 60%for heterotrophic bacterial variables(i.e.,bacterial abundance,bacterial production,and bacterial growth efficiency).The Kuroshio intrusion,however,had little impact on bacterial and community respiration.The observed elevation of nitrate concentrations at RK=50%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(DOM)in the Kuroshio water stimulates bacterial activity,and catabolism of the DOM releases inorganic nutrients that stimulate primary production in the northern South China Sea basin.The new regeneration of inorganic nitrogen from bacterial decomposition of organic matter transferred horizontally serves as another mechanism to fuel export production and enhance the ability of carbon sequestration in the northern South China Sea basin.Recognition of this mechanism broadens our understanding of the processes that sustain export production in the oligotrophic ocean.(3)We examined the spatial variation of net community production as well as planktonic and bacterial metabolism using in vitro incubations in the region of Pacific western boundary currents along the western Pacific boundary.We also combined the analyses of the microbial metabolism across different approaches to enhance our understanding of the metabolic state of the oligotrophic ocean.The integrated gross primary production(GPP)of the photic zone exhibited higher values in the region of 2-8°N along 130°E and the western Luzon Strait,which is consistent with the regional variability of nutrients and chlorophyll-a concentrations in the different ocean provinces.Spatially,the community respiration(CR)and bacterial production were less variable.At the most stations,the in vitro incubation results suggested that CR exceeded the GPP and the heterotrophic state prevailed in this region.However,the global biogeochemical model predicted the autotrophic state in the same region.The further comparison showed that our observed GPP values were close to those predicted by the model,but the measured CR was significantly higher than the modelled values.We attempted to estimate the respiration rates of major groups based on the GPPs,bacterial production and published growth efficiencies of corresponding groups to provide additional constraints on the CR.We found that most of the in vitro CR estimates were higher than the upper range of the empirical CR estimated from the sum of the contributions of the main trophic groups.Conversely,the estimates of the empirical CR support the rationality of the CR predicted by the biogeochemical model.In general,the cross-validation indicates that systematic net heterotrophy is more likely a result of the overestimation of CR measured by the light-dark bottle incubation experiments.(4)In this study,we developed two mechanistic approaches to reconstruct net community production from the snapshot of the total oxygen.We parameterized the physical oxygen saturation anomaly(?O2[phy])and then retrieved the biological oxygen saturation anomaly from the observed ?O2[total]minus our modeled ?O2[phy].The first algorithm to parameterize the ?O2[phy]was to adopt the model simulations of saturation anomaly of Ar predicted from the global ocean circulation model.The second algorithm derived ?O2[phy]using the histories of satellite-derived physical drivers(i.e.temperature,wind speed,pressure)prior to the sampling day to force an iterative air-sea gas exchange model.We assessed our numerical approaches by comparing model results to simultaneous measurements of O2/Ar and O2 concentration from six independent cruises.Overall,our models' performances in predicting NCP were comparable to other previous global NCP models.The r2 and RMSE for the first model were 0.44 and 11 mmol O2 m-2 d-1,respectively.The r2 and RMSE for the second model were 0.64 and 5.2 mmol O2 m-2 d-1,respectively.Since the second model did a better job in capturing the general variability of measured ?[O2]phy,especially for some high ?[O2]phy signals(i.e.?[O2]phy>3%),predictions of NCP were improved with the second model reflected by reduced RMSE of 7.2 mmol O2 m-2 d-1 compared to the RMSE of 11mmol O2 m2 d-1 for the first model in the case where the ?[O2]phy larger than 3%.After further refining and re-tuning the model,we expect our approach to provide an independent insight into regional,inter-annual and decadal variability of NCP from total O2 observations and advance our understanding in the long-term variation of the biological pump and its responses to climate change.