SouthEast Asian Time-series Study Research Articles

Partitioning of carbon export in the euphotic zone of the oligotrophic South China Sea

Abstract. We conducted samplings of total and particulate 234Th, along with particulate organic carbon (POC), in the summer of 2017 to examine nutrient-dependent structures of export productivity within the euphotic zone (Ez) of the oligotrophic basin of the South China Sea (SCS). Nitrate concentrations throughout the study area were below detection limits in the nutrient-depleted layer (NDL) above the nutricline, while they sharply increased with depth in the nutrient-replete layer (NRL) across the nutricline until the base of the Ez. Based on our vertical profilings of 234Th–238U disequilibria, this study estimated for the first time POC export fluxes both out of the NDL and at the horizon of the Ez base. The total 234Th deficit relative to 238U was determined in the NDL at all study sites. By contrast, 234Th was mostly in equilibrium with 238U in the NRL, except at the northernmost station, SEATS (SouthEast Asian Time-series Study; 18∘ N, 116∘ E), where the 234Th deficit was also observed in the NRL. By combining 1D steady-state 234Th fluxes and POC/234Th ratios, we derived vertical patterns of POC export fluxes. The POC export fluxes at station SEATS were 1.6 ± 0.6 mmol C m−2 d−1 at the NDL base, accounting for approximately half of that at the base of the Ez. For the rest of the sampling sites, the POC export fluxes at the NDL base (averaged at 2.3 ± 1.1 mmol C m−2 d−1) were comparable with those at the base of the Ez (1.9 ± 0.5 mmol C m−2 d−1), suggesting rapid export of POC out of the NDL. This finding fundamentally changes our traditional view that the NDL, being depleted in nutrients, would not be a net exporter of POC. Furthermore, our results revealed a significant positive correlation between POC export fluxes at the NDL base and the potential of subsurface nutrient supplies, indicated by nutricline depth and nutrient concentrations obtained from both in situ measurements and numerical modeling. POC export fluxes (averaged at 3.4 ± 1.2 mmol C m−2 d−1) at the NDL base at stations with shallow nutriclines and high levels of subsurface nutrients approximately doubled those (averaged at 1.6 ± 0.5 mmol C m−2 d−1) at other stations. We subsequently used a two-endmember mixing model based on the mass and 15N isotopic balances to evaluate the relative contribution of different sources of new nitrogen that support the observed particle export at stations SEATS and SS1, located respectively in the northern and southern basin of the SCS with different hydrological features. We showed that more than 50 % of the particle flux out of the NDL was supported by nitrate sources likely supplied from depth and associated with episodic intrusions other than atmospheric deposition and nitrogen fixation. However, the exact mechanisms and pathways for subsurface nutrients to support the export production from the NDL merit additional careful and dedicated studies.

New constraints on biological production and mixing processes in the South China Sea from triple isotope composition of dissolved oxygen

Abstract. The South China Sea (SCS) is the world's largest marginal sea, playing an important role in the regional biogeochemical cycling of carbon and oxygen. However, its overall metabolic balance, primary production rates and links to East Asian Monsoon forcing remain poorly constrained. Here, we report seasonal variations in triple oxygen isotope composition (17Δ) of dissolved O2, a tracer for biological O2, gross primary production (GP; inferred from δ17O and δ18O values) and net community production (NP; evaluated from oxygen–argon ratios) from the SouthEast Asian Time-series Study (SEATS) in the SCS. Our results suggest rather stable mixed-layer mean GP rates of ∼ 1500 ± 350 mg C m−2 d−1 and mean NP of ∼ −13 ± 20 mg C m−2 d−1 during the summer southwest monsoon season. These values indicate, within uncertainties and variabilities observed, that the metabolism of the system was in net balance. During months influenced by the stronger northeast monsoon forcing, the system appears to be more dynamic and with variable production rates, which may shift the metabolism to net autotrophy (with NP rates up to ∼ 140 mg C m−2 d−1). Furthermore, our data from the deeper regions show that the SCS circulation is strongly affected by monsoon wind forcing, with a larger part of the water column down to at least 400 m depth fully exchanged during a winter, suggesting the 17Δ of deep O2 as a valuable novel tracer for probing mixing processes. Altogether, our findings underscore the importance of monsoon intensity on shifting the carbon balance in this warm oligotrophic sea and on driving the regional circulation pattern.

Comparison of Primary Production Using in situ and Satellite-Derived Values at the SEATS Station in the South China Sea

Satellite-based observations of primary production (PP) are broadly used to assess carbon fixation rate of phytoplankton in the global ocean with small spatiotemporal limitations. However, the remote sensing can only reach the ocean surface, the assumption of a PP vertically exponential decrease with increasing depth from the surface to the bottom of euphotic zone may cause a substantial and potential discrepancy between in situ measurements and satellite-based observations of PP. This study compared euphotic zone integrated PP derived from measurements based on ship-based in situ incubation (i.e., PPin situ) and those derived from the satellite-based vertically generalized production model (VGPM; PPVGPM) for the period 2003∼2016 at the South East Asian Time-series Study (SEATS) station. PP values obtained during the NE-monsoon (NEM: Nov∼Mar; PPin situ = 323 ± 134; PPVGPM = 443 ± 142 mg-C m–2 d–1) were ∼2-fold higher than those recorded during the SW-monsoon (SWM: Apr∼Oct; PPin situ = 159 ± 58; PPVGPM = 250 ± 36 mg-C m–2 d–1), regardless of the method used for derivation. The main reason for the higher PP values during the NEM appears to have been a greater abundance of inorganic nutrients were made available by vertical advection. Note that on average, PPin situ estimates were ∼50% lower than PPVGPM estimates, regardless of the monsoon. These discrepancies can be mainly attributed to differences from the euphotic zone depth between satellite-based and in situ measurements. The significantly negative relationship between PP measurements obtained in situ and sea surface temperatures observed throughout this study demonstrates that both methods are effective indicators in estimating PP. Overall, our PPin situ analysis indicates that a warming climate is unfavorable for primary production in low-latitude open ocean ecosystems.

Impact of physical and biogeochemical forcing on particle export in the South China Sea

This study examines temporal variations of 234Th-based particle export based on time-series observations conducted from 2004 to 2014 at the South-East Asian Time-series Study (SEATS) site in the basin area of the Northern South China Sea. It shows significant variability in particle export at both seasonal and interannual time scales. A significant feature of our data as observed by prior studies is that particle export was high during the NE monsoon in winter, but low during the inter-monsoon and SW monsoon. Particle flux increased with increasing mixed layer depth (MLD), primarily in winter due to the presence of a strong NE monsoon, in addition to influence by mesoscale features quantified by eddy kinetic energy values. Particle export was generally elevated when phytoplankton biomass was abundant, as shown by the significant correlation between particle fluxes and concentrations of chlorophyll (Chl) a and/or particulate organic carbon (POC) between 0 and 100 m. This is also supported by the negative correlation between 234Th residence time and Chl a concentration, suggesting that increased phytoplankton biomass accelerated downward particle sinking. The correlation disappeared however at Chl a values < 0.11 mg m−3, implicating other factors (e.g., phytoplankton community composition) as playing a more important role when biomass is relatively low. Satellite Chl a-based POC export estimated using various established algorithms agreed well with our measured fluxes when the export horizon was defined as the bottom of the mixed layer, but differed compared to estimated fluxes at 100 m depth. This is likely due to the fact that satellite-derived Chl a measurements only reflect surface values, suggesting that reliable algorithms must be used after considering the biological processes in subsurface waters.

Diel variability of vertical distributions of chlorophyll a at the SEATS and ALOHA stations: implications on remote sensing interpretations

ABSTRACTThe effects of the diel (involving a 24 hour period) variations in the surface concentrations of chlorophyll a (C) on the use of once-daily remotely sensed C as the diel average were assessed from the diel records in the derived depth-weighted C (Cd) that should be detected by remote sensing and the in situ surface C at two time-series stations in the North Pacific: the SEATS (SouthEast Asian Time-series Study) station in the northern South China Sea and the ALOHA (A Long-Term Oligotrophic Habitat Assessment) station in the North Pacific subtropical gyre. In situ surface C varied by a factor of about 2.0 and 1.3 over a diel cycle, and by ±20% and ±9% over the diel average at the SEATS and ALOHA stations, respectively. As the overpass-times of the different satellites were not identical, Cd was satellite-dependent. While the Cd corresponding to MODerate resolution Imaging Spectroradiometer on Aqua (MODIS-Aqua) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) overpass-times agreed to ±10%, the Cd corresponding to MEdium Resolution Imaging Spectrometer (MERIS) overpass-time could differ from the other two by −22% to +28% at the SEATS station and −1% to +12% at the ALOHA station. In addition, Cd corresponding to the overpass-times of the three satellites deviated from the observed diel average in situ surface C by −19% to +32% at the SEATS station and by −6% to +13% at the ALOHA station. These results indicate that, as a result of diel variations, neither a one-time remotely-sensed nor a one-time observed in situ surface C can represent the diel average in situ surface C accurately. Furthermore, diel variations are an inherent source of uncertainty when data from multiple satellites are pooled for use. The magnitudes of these discrepancies can be comparable to the commonly claimed uncertainties in remotely sensed C and thus should be taken into consideration in its interpretation and use.

One-dimensional ocean model with three types of vertical velocities: a case study in the South China Sea

In this research, three vertical velocities were included in a one-dimensional (1D) ocean model for a case study of the SouthEast Asian Time-Series Study station in the South China Sea. The vertical velocities consisted three processes, i.e., Ekman pumping (WEK), Eddy pumping (WEP), and the background upwelling (WBK). The quantification of WEK followed the classical Ekman pumping theory. The WEP, whose underlying mechanism was consistent with the baroclinic modes (dominated by the first mode), was quantified by Argo observation and altimetry data. The WBK, related with the background circulation, was estimated from the long-term heat budget balance. The skill assessment indicated that the case with all three processes performed best. The study confirmed the capability of the 1D model with three types of vertical velocities, which can reproduce the general structure and variation of temperature in vertical direction.

Sinking fluxes of 210Pb and 210Po in the deep basin of the northern South China Sea

Vertical fluxes of total mass (Fmass), particulate organic carbon (FPOC), particulate inorganic carbon (FPIC), 210Pb (FPb-210), and 210Po (FPo-210) were determined by sediment traps deployed at two depths, 2000 m and 3500 m, at SEATS (South East Asian Time-series Study, 116°00°E, 18°00°N) in the northern South China Sea during June 2008–June 2009. The Fmass ranges from 12.2 to 55.1 mg m−2 d−1 and from 89.3 to 250.8 mg m−2 d−1, at 2000 m and 3500 m, respectively, and shows seasonal and inter-annul variation. The temporal variation of FPOC, FPIC, and FPb-210 were in phase with the Fmass, which was coupled with the seasonal cycles of primary production in the euphotic layer. The FPb-210 ranges from 5 to 48 dpm m−2d−1 and from 38 to 105 dpm m−2d−1, at 2000 m and 3500 m, respectively. Contrasting with 210Pb, the FPo-210 shows poor correlation with Fmass. The FPo-210 ranges from 3 to 146 dpm m−2d−1 and from 50 to 309 dpm m−2d−1, at 2000 m and 3500 m, respectively. Episodic events of the settling of biological particles from the surface layer and the regeneration processes the deep layer control the 210Po removal in the water column of the South China Sea. Strong correlations of the flux and source ratio of 210Pb, (F/P)Pb-210, and the particulate carbon fluxes were found, which give relationships of FPOC (μg cm−2y−1) = 26.8 + 371.0 (F/P)Pb-210 and FPIC (μg cm−2y−1) = −1.4 + 533.1 (F/P)Pb-210.

Seasonal distributions and fluxes of &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Pb and &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Po in the northern South China Sea

Abstract. Vertical distributions of dissolved and particulate 210Pb and 210Po in the water column at the SouthEast Asian Time-series Study (SEATS, 18°00´ N and 116°00´ E) station in the northern South China Sea were determined from four cruises between January 2007 and June 2008. A large deficiency of 210Pb, 379 ± 43 × 103 dpm m−2, from the secular equilibrium was found within the 3500 m water column. On the other hand, a smaller deficiency of 210Po, 100 ± 21 × 103 dpm m−2, relative to 210Pb was found in the water column. Time-series data showed insignificant temporal variability of the 210Pb and 210Po profiles. To balance these deficiencies, the removal fluxes for 210Pb and 210Po via particle settling ranging from 45 to 51 dpm m−2d−1 and from 481 to 567 dpm m−2d−1, respectively, are expected at 3500 m. The 210Pb removal flux is comparable with, whereas the 210Po removal flux is much higher than, the flux directly measured by moored sediment traps. The discrepancy between the modeled 210Po flux and the measured flux suggests that sporadic events that enhance 210Po removal via sinking ballast may occur in the water column at the site.

On the consistency of MODIS chlorophyll $a$ products in the northern South China Sea

Abstract. Chlorophyll a (Chl) concentrations derived from satellite measurements have been used in oceanographic research, for example to interpret eco-responses to environmental changes on global and regional scales. However, it is unclear how existing Chl products compare with each other in terms of accuracy and consistency in revealing temporal and spatial patterns, especially in the optically complex marginal seas. In this study, we examined three MODIS (Moderate Resolution Imaging Spectroradiometer) Chl data products that have been made available to the community by the US National Aeronautics and Space Administration (NASA) using community-accepted algorithms and default parameterization. These included the products derived from the OC3M (ocean chlorophyll three-band algorithm for MODIS), GSM (Garver–Siegel–Maritorena model) and GIOP (generalized inherent optical properties) algorithms. We compared their temporal variations and spatial distributions in the northern South China Sea. We found that the three products appeared to capture general features such as unique winter peaks at the Southeast Asian Time-series Study station (SEATS, 18° N, 116° E) and the Pearl River plume associated blooms in summer. Their absolute magnitudes, however, may be questionable in the coastal zones. Additional error statistics using field measured Chl as the truth demonstrated that the three MODIS Chl products may contain high degree of uncertainties in the study region. Root mean square error (RMSE) of the products from OC3M and GSM (on a log scale) was about 0.4 and average percentage error (ε) was ~ 115% (Chl between 0.05–10.41 mg m−3, n = 114). GIOP with default parameterization led to higher errors (ε = 329%). An attempt to tune the algorithms based on a local coastal-water bio-optical data set led to reduced errors for Chl retrievals, indicating the importance of local tuning of globally-optimized algorithms. Overall, this study points to the need of continuous improvements for algorithm development and parameterization for the coastal zones of the study region, where quantitative interpretation of the current Chl products requires extra caution.

Inter-annual variation of chlorophyll in the northern South China Sea observed at the SEATS Station and its asymmetric responses to climate oscillation

Abstract. It is widely recognised that the variation of average surface chlorophyll a concentration (Chl) in the South China Sea (SCS) is closely related to wind forcing, especially during the intense winter monsoon. In this study, we demonstrate that after removal of the seasonal cycles, the variation of Chl showed strong asymmetric responses to wind speed under El Niño or La Niña conditions. The analysis was based on a time-series of Chl in the study area (115–117° E, 17–19° N) around the SEATS (South-East Asian Time-series Study) station located in the central northern SCS from September 1997 to the end of 2011, which was constructed by merging the SeaWiFS data (1997–2006) and MODIS data (2003–2011). The merged daily data were validated by shipboard observations at the SEATS station. The non-seasonal variations of monthly mean Chl, wind speed, sea surface height (SSH) and sea surface temperature (SST) were examined against the multivariate ENSO index (MEI). The analysis reveals strongly asymmetric correlations of Chl and SST with positive MEI (El Niño) or negative MEI (La Niña). Under El Niño conditions, both showed significant correlations with MEI or wind speed; under La Niña conditions, both showed weak or insignificant correlations. The contrast was more pronounced for Chl than for SST. The subdued responses of Chl to wind forcing under La Niña conditions were attributable to a deepened thermocline, for which wind driven nutrient pumping is less efficient. A deeper thermocline, which was observed during the 1999–2000 La Niña event and inferred by positive SSH anomalies during other La Niña events, was probably caused by reduced SCS throughflow under La Niña conditions. Intrusion of the nutrient-depleted Kuroshio water in the surface layer as observed during the 1999–2000 La Niña could be partially responsible for the suppressed Chl response.

Silicon isotope constraints on sources and utilization of silicic acid in the northern South China Sea

The stable silicon isotopic composition (δ30Si) of waters and diatoms has increasingly been used to investigate the biogeochemical cycling of Si in the major ocean basins. Here we present the first Si isotope data set from the northern South China Sea (NSCS), a large marginal sea system in the western North Pacific to examine sources and utilization of silicic acid (Si(OH)4). During two cruises in July–August 2009 (summer) and January 2010 (winter), samples for isotope measurements of dissolved Si(OH)4 (δ30SiSi(OH)4) and of biogenic silica (δ30SiBSi) in suspended particles were collected along a transect perpendicular to the coast from the inner shelf to the deep-water slope, as well as at the South East Asian Time-series Study (SEATS) station located in the NSCS basin. Surface δ30SiSi(OH)4 generally increased from values ∼+2.3‰ on the inner shelf to ∼+2.8‰ above the deep basin, suggesting an increasing utilization of dissolved Si(OH)4 reflecting the transition from eutrophic to oligotrophic conditions. The δ30SiBSi values were systematically lower than the corresponding δ30SiSi(OH)4 in the euphotic zone (above 100m) on the shelf and slope. In contrast at station SEATS in the NSCS basin, δ30SiBSi signatures in both seasons were within error equal to δ30SiSi(OH)4 in the surface mixed layer (above 50m) and δ30SiBSi in waters below were significantly higher than the corresponding δ30SiSi(OH)4. By comparing the field data with the Si isotope fractionation revealed by the Rayleigh or the steady state models, we demonstrate the existence of variable Si(OH)4 origins in different areas of the NSCS. Surface waters on the inner shelf were largely fed by nutrients from the Pearl River input. While the primary source of Si(OH)4 for the euphotic zone on the outer shelf and slope was upwelling or vertical mixing from underlying waters, the Si(OH)4 in the surface mixed layer of the NSCS basin might have originated from horizontal mixing with other highly fractionated surface waters. As a consequence, the Si isotope dynamics in the NSCS are largely controlled by variable biological fractionation of Si in waters from different sources with different initial Si isotopic compositions rather than any single source water.

Isotope constraints on particulate nitrogen source and dynamics in the upper water column of the oligotrophic South China Sea

Particulate nitrogen (PN) dynamics in the oligotrophic northern South China Sea (around the SouthEast Asian Time‐series Study (SEATS) station) was explored by examining the isotopic compositions of suspended PN in the top 200 m over 3 years and sinking PN collected by sediment traps. The PN inventory (IPN) in the upper 100 m is larger than in the lower 100 m, exhibiting stronger seasonality. Both layers reveal significant seasonality in meanδ15NPN, yet, the mean in the upper 100 m (2.0 to 5.3‰) is consistently smaller than that in the lower 100 m, implying the occurrence of vertical biological fractionation and/or an addition of 15N‐depleted N from the atmosphere. Theδ15NPN surges in winter, when the mixed layer is deeper, indicate an intensified nitrate supply from thermocline, during which relatively stronger downward transfer efficiency was inferred by a small IPN gradient. The largest vertical gradient in IPN appeared during intermonsoon periods, corresponding with weak vertical mixing, low δ15NPN, and high N* values. N fixation is likely the cause for the intermonsoon δ15NPN lows. The δ15NPNvalues of trapped material at 374 m and 447 m range from 3.3 to 7.3‰ with a flux‐weighted mean of 5.6‰ resembling theδ15NO3of upwelled sources. By using a mass‐isotope balance model under the assumption of no atmospheric N deposition, we obtained an N fixation input of ∼20 ± 26 mmol N m−2 yr−1. This value accounts for only ∼5–10% of the new production on an annual basis.

Seasonal and interannual variability of carbon cycle in South China Sea: A three-dimensional physical-biogeochemical modeling study

The South China Sea (SCS) exhibits strong variations on seasonal to interannual time scale, and the changing Southeast Asian Monsoon has direct impacts on the nutrients and phytoplankton dynamics, as well as the carbon cycle. A Pacific basin-wide physical-biogeochemical model has been developed and used to investigate the physical variations, ecosystem responses, and carbon cycle consequences. The Pacific basin-wide circulation model, based on the Regional Ocean Model Systems (ROMS) with a 50-km spatial resolution, is driven with daily air-sea fluxes derived from the National Centers for Environmental Prediction (NCEP) reanalysis between 1990 and 2004. The biogeochemical processes are simulated with the Carbon, Si(OH)4, Nitrogen Ecosystem (CoSINE) model consisting of multiple nutrients and plankton functional groups and detailed carbon cycle dynamics. The ROMS-CoSINE model is capable of reproducing many observed features and their variability over the same period at the SouthEast Asian Time-series Study (SEATS) station in the SCS. The integrated air-sea CO2 flux over the entire SCS reveals a strong seasonal cycle, serving as a source of CO2 to the atmosphere in spring, summer and autumn, but acting as a sink of CO2 for the atmosphere in winter. The annual mean sea-to-air CO2 flux averaged over the entire SCS is +0.33 moles CO2 m−2year−1, which indicates that the SCS is a weak source of CO2 to the atmosphere. Temperature has a stronger influence on the seasonal variation of pCO2 than biological activity, and is thus the dominant factor controlling the oceanic pCO2 in the SCS. The water temperature, seasonal upwelling and Kuroshio intrusion determine the pCO2 differences at coast of Vietnam and the northwestern region of the Luzon Island. The inverse relationship between the interannual variability of Chl-a in summer near the coast of Vietnam and NINO3 SST (Sea Surface Temperature) index in January implies that the carbon cycle and primary productivity in the SCS is teleconnected to the Pacific-East Asian large-scale climatic variability.

Anomalous hydrographic and biological conditions in the northern South China Sea during the 1997–1998 El Niño and comparisons with the equatorial Pacific

It is demonstrated that weakened wind mixing and strengthened water column stratification resulted in the anomalously low sea surface chlorophyll in the northern South China Sea during the 1997–1998 El Niño event. Remotely sensed sea surface temperature, wind and chlorophyll, which were validated by shipboard observations at the SouthEast Asian Time-series Study (SEATS) station (18°N, 116°E) in the northern South China Sea (SCS) provided the basis for this study. During the 1997–1998 winter at the SEATS station, the sea surface temperature was elevated by about 2 °C above the climatological mean, while the wind speed of the northeast monsoon was reduced from a climatological mean of 9.4 to 6.8 m/s. The concentration of surface chlorophyll- a dropped from 0.2 to 0.1 mg/m 3. The monthly area-averaged integrated primary production estimated for the northern SCS area (112–119°E, 15–21°N) was reduced by about 40% of the normal winter value. Under the anomalously high sea surface temperature and weak monsoon, the mixed-layer depth would have been reduced from an average of 65 to 45 m and the nutrients in the mixed layer would have been reduced by half, according to observations at the SEATS station in more recent years. During the 1997–1998 El Niño event, the onset of warming in the northern SCS lagged behind that in the eastern equatorial Pacific by about 5 months and lingered for 11 months. This course of change resembled that of the western Pacific warm pool region. However, contrary to the northern SCS, the sea surface chlorophyll was enhanced in the warm pool region during the event, probably mainly because of the uplifted nutricline. Unlike the eastern equatorial Pacific, the dramatic recovery of biological production did not happen in the SCS in the summer of 1998. These distinctive biogeochemical responses reflect fundamental differences between the SCS and the equatorial Pacific in terms of upper water column dynamics.

Anomalous biogeochemical conditions in the northern South China Sea during the El‐Niño events between 1997 and 2003

Anomalous biogeochemical conditions were observed at the SouthEast Asian Time‐series Study (SEATS) station in the northern South China Sea (SCS) during the 1997–98 and 2002–03 El Niño events. The time‐series records showed decreases of monthly mean sea surface chlorophyll‐a (S‐chl) (and integrated primary production, IPP) by 42% (and 42%) and 13% (and 10%), respectively, below the climatological mean in the winter months (DJF) of the two events. The negative anomalies in S‐chl and IPP corresponded to elevated sea surface temperature by 1.2°C and 0.4°C, respectively, above the climatological mean, while the mean wind speed was reduced by about 20% and 11%, respectively. Statistical analysis demonstrated the reduction in S‐chl and IPP during El Niño events was caused by the diminished vertical mixing and strengthened stratification. Regional anomalies in hydrographic and biological conditions in the northern SCS (15–21°N and 112–119°E) were consistent with those found at the SEATS site.

The SouthEast Asian Time-series Study (SEATS) and the biogeochemistry of the South China Sea—An overview

Abstract As part of the time-series program in the Joint Global Ocean Flux Study (JGOFS), the major objective of the multidisciplinary and multi-institutional SouthEast Asian Time-series Study (SEATS) is to investigate through direct observations the processes that influence the carbon cycle and bioactive elements in the intra-annual to decadal time scale. The SEATS site is situated at 18.3°N and 115.5°E in the tropical oligotrophic northern South China Sea. Among the active time-series stations, it is located at the lowest latitude and it is the only one located in a marginal sea. The major defining environmental characteristics at this site include: oligotrophic water, outside the influence of major rivers, high surface temperature with little seasonality, strong and persistent stratification, thin mixed layer, shallow top of the nutricline depth, deep photic zone, distinct monsoonal seasons, frequent traverses of tropical cyclones, and a high depositional flux of aerosols. Given these environmental conditions, a site-specific hypothesis in SEATS is that the interactions between the upper waters and the atmosphere constitute a major controlling mechanism of the biogeochemistry in the northern South China Sea. The SEATS station has been occupied at approximately seasonal intervals since 1999. The results obtained primarily between 1999 and 2003 in the initial phase of this study are presented here. In contrast to the generally accepted notion that seasonal variations in the tropical waters are minimal, well defined, and regular seasonal patterns were observed in the carbon cycle, the nutrient dynamics and the biological community structure in the northern South China Sea. In the carbon cycle, superimposed on the large seasonal signals were the more subtle inter-annual variations of progressive increases in the mixed-layer CO2 fugacity (fCO2) and total dissolved CO2 (TCO2) that also were observed in other time-series stations. This suggests that the absorption of CO2 by the oceans in response to the increase in atmospheric CO2 in modern times may well be globally manifested. The nutrient dynamics indicate that N2 fixation could play a key role in keeping the nutrient cycle in balance in the northern South China Sea.

Seasonal variability of coccolithophore abundance and assemblage in the northern South China Sea

Abstract Seasonal variations of coccolithophore abundances and assemblages and their causal factors were studied in the shelf and the basin of the northern South China Sea (SCS). Coccolithophores were counted and identified using scanning electron microscopy on samples from: (A) four cruises between March 2002 and July 2003 to survey the spatial and temporal fluctuations of coccolithophore communities in surface (5 m) water of 12 stations in the SCS shelf and basin; (B) a sediment trap deployed between August 2004 and January 2005 at 515-m depth near the South East Asian Time-series Study (SEATS) station in the SCS basin to estimate downward fluxes of coccolithophores; and (C) three cruises to the SEATS station during the period of sediment trap deployment to study the flux coupling. Abundances of coccolithophores in the surface-water ranged between 1.98 and 127.43×103 coccospheres l−1, and showed clear seasonal and shelf-basin variations. More coccolithophores appeared in cold (winter and spring) months when the northeastern monsoon prevailed than in warm (summer and autumn) months when the southwestern monsoon prevailed. Coccolithophore abundance was greater in the basin than on the shelf during cold months; the reverse was true during warm months. While Emiliania huxleyi, Gephyrocapsa oceanica and Gephyrocapsa ericsonii were predominant in cold months, oligotrophic Umbellosphaera irregularis, Umbellosphaera tenuis and Discosphaera tubifera predominated during warm months. Surface coccolithophore abundances were positively (P

Depth distributions of alkalinity, TCO2 and at SEATS time-series site in the northern South China Sea

Abstract In this study, measurements of titration alkalinity (TA), total dissolved carbon dioxide (TCO2), and δ13C of TCO2 ( δ 13 C TCO 2 ) throughout the water column at the SouthEast Asian time-series study (SEATS) site were investigated in order to understand better the fundamental processes controlling their vertical distributions in the South China Sea (SCS). The linear correlations between TA and salinity in the shallow waters, as identified by the mixing line between the surface water and salinity maximum water suggested the predominant control of physical mixing on the variability of TA. In contrast, TCO2 and δ 13 C TCO 2 showed the non-conservative behavior in the respective TCO2 and δ 13 C TCO 2 vs. salinity plot due to the effect of biological production. A stoichiometric model further showed that the depth profile of NTA (=TA×salinity/35) largely reflects the increase of preformed NTA in the shallow waters, whereas carbonate dissolution was responsible for the continuous increase of NTA in the deep waters. A one-dimensional diffusion–advection model further revealed that the carbonate dissolution could account for 28% of NTCO2 (=TCO2×salinity/35) increase in deep waters, and the remaining 72% of NTCO2 was from organic decomposition. Calculation of excess TA further showed that it emerged well above the aragonite and calcite saturation depths at 600 and 2500 m, respectively, indicating that some biologically, chemically, and physically-mediated processes must be involved to provide excess TA into the shallow waters. The decrease in δ 13 C TCO 2 with depth primarily resulted from organic decomposition. The influence of anthropogenic CO2 throughout the water column was assessed with the carbon chemistry and the isotope-based approach in this study. Both methods obtained nearly the same results in which the signal of anthropogenic CO2 decreased exponentially with depth, and its penetration depth were found to be at ∼1000 m. The inventory of anthropogenic CO2 in the water column was estimated to be ∼16.6 mol C m−2, which was less than that reported in the northwest Pacific at the same latitude, presumably due to the enhanced upwelling in the SCS. Such an anthropogenic CO2 penetration had led to decreases of the saturation levels of aragonite and calcite by 17% and 14%, respectively, in the surface water, and an upward migration of aragonite saturation depth by ∼100 m since industrial revolution.

The significance of phytoplankton photo-adaptation and benthic–pelagic coupling to primary production in the South China Sea: Observations and numerical investigations

The primary production in the South China Sea (SCS) has been assessed by a coupled physical–biogeochemical model with a simple NPZD ecosystem [Liu et al., 2002. Monsoon-forced chlorophyll distribution and primary production in the SCS: observations and a numerical study. Deep-Sea Research I 49(8), 1387–1412]. In recent years there have been an increasing number of observations in the SCS that may be used to check the validity of the previous approach. The coupled model of the SCS mentioned above employs a photo-adaptation scheme for the phytoplankton growth and uses the simplest bottom boundary condition of an inert benthic layer. These adopted schemes are checked against observations at the South-East Asian Time-series Study (SEATS) Station in the northern SCS and in the Gulf of Thailand. Numerical experiments with or without photo-adaptation or active benthic processes are carried out in this study. Additional experiments are performed with different parameters used for these processes. The observations at the SEATS Station provide direct evidence for the variable chlorophyll-to-nitrogen ratio in phytoplankton as required by photo-adaptation. It is concluded that a photo-adaptation scheme is critical to the phytoplankton growth, especially for the development of the subsurface chlorophyll maximum (SCM). Without photo-adaptation, the average value of the vertically integrated primary production (IPP) over the whole SCS domain would be 35% lower. It is noted that, the modeled SCM occurs at depths shallower than observations due to physical as well as biological processes employed by the model. Increasing the upper limit of the chlorophyll-to-nitrogen ratio, as suggested by observations, enhances chlorophyll level in the lower part of the euphotic zone and raises primary productivity in areas with rich nutrient supply. The observed values of the IPP in the Gulf of Thailand clearly demonstrate the importance of the benthic–pelagic coupling to the nutrient cycle. Without benthic nutrient regeneration the model grossly underestimates primary production due to failure to build up the nutrient reserve in the Gulf. On the other hand, a fully regenerated flux of particulate organic nitrogen at the sea floor without denitrification produces too strong a primary production. The improved model uses a higher upper limit for the chlorophyll-to-phytoplankton ratio of 3.5 g Chl/mol N and adopts benthic processes of a coupled nitrification–denitrification scheme with denitrification consuming 14% of the detritus flux at the bottom. The model predicts a mean annual IPP value of 406 mg C m −2 d −1 for the SCS, which is 44% higher than that predicted by the original model. The increase can be broken down to 39% attributed to the benthic nutrient regeneration and 5% to the enhanced photo-adaptation. The average IPP is 390 mg C m −2 d −1 for the basin region (>200 m) and 429 mg C m −2 d −1 for the shelf region (<200 m), both of which compare favorably with observed mean values. The model also predicts a mean nitrogen removal flux of 0.16 mmol N m −2 d −1 during denitrification for the shelf region.

Temporal variations in the carbonate system in the upper layer at the SEATS station

Abstract The distributions of total carbon dioxide (TCO 2 ) and alkalinity (TA) in the upper layer at the SouthEast Asian Time-series Study (SEATS) station at 18°N, and 116°E in the northern South China Sea (SCS) were determined on 19 cruises between September 1999 and October 2003. The variations in the concentrations of TCO 2 and TA in the mixed layer, which ranged between 1860 and 1920 μmol kg −1 , and 2170 and 2230 μmol kg −1 , respectively, followed a distinct intra-annual pattern like that of salinity. The maximum concentrations were found in the winter as enhanced vertical mixing brought the subsurface Tropical Water, which was more saline and elevated in TA and TCO 2 , to the mixed layer. There was an even more well-defined and consistent intra-annual pattern in the variations in the associated fugacity of CO 2 , fCO 2 , that fluctuated between 340 and 400 μatm. However, the variations followed the temporal pattern in temperature more closely than that in salinity as fCO 2 rose systematically towards a maximum in the summer and then fell progressively to a minimum in the winter. The intra-annual variations in TA could be accounted for largely by the variations in salinity. Once TA was normalized to the average salinity of 33.5 in the mixed layer, the variations in the resulting NTA were only slightly larger than the analytical uncertainty and they did not follow a consistent intra-annual pattern. On the other hand, consistent intra-annual variations remained evident in NTCO 2 , TCO 2 normalized to a salinity of 33.5, and NfCO 2 , fCO 2 normalized to the average temperature of 27.6 °C in the mixed layer. In fact, the patterns in the intra-annual variations in NTCO 2 and NfCO 2 mimicked each other closely. From the late winter through the summer (February–August), the uptake of carbon in primary production and the evasion of CO 2 to the atmosphere led to a drawdown in NTCO 2 and a decrease in NfCO 2 . From the late summer to the early winter (August–December), variations in NTCO 2 and NfCO 2 were small. The variations in TCO 2 and fCO 2 could be explained largely by changes in salinity and temperature, respectively. In the winter (December–February), both NTCO 2 and NfCO 2 were at a maximum, indicating that the effects of the net invasion of atmospheric CO 2 to the SCS and the enhanced vertical mixing of the surface waters with the subsurface Tropical Water dominated over the effect from the higher primary production during this season. Atmospheric fCO 2 was less than fCO 2 in the mixed layer from April through October and exceeded the latter from November through March. For the year as a whole, there was a net invasion of CO 2 of 0.02 mol C m −2 yr −1 , a value that was indistinguishable from zero, indicating that the northern SCS was neither a significant source nor a significant sink of atmospheric CO 2 . Inter-annually, there were indications that NTCO 2 and fCO 2 in the mixed layer were increasing with time at rates of ∼1.5 μmol kg −1 yr −1 (or ∼0.1% yr −1 ) and ∼4 μatm yr −1 (or ∼1% yr −1 ), respectively between 1999 and 2003.