SouthEast Asian Time-series Study Station Research Articles

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.

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.

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.

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.

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.

Nutrient dynamics and N-anomaly at the SEATS station

The distributions of nitrate+nitrite, or (N+N), and soluble reactive phosphate, or SRP, at the SouthEast Asian Timeseries Study (SEATS) station in the northern South China Sea (SCS) at 181N and 1161E were determined in 19 cruises between September 1999 and October 2003. In the deep water below 1500 m, the concentrations of (N+N) and SRP stayed constant with depth and time at 38.470.3 and 2.8470.03mM, respectively. The variability could be accounted for by the analytical uncertainties in the measurements alone. In contrast, variations in the upper water were significantly larger and far exceeded the analytical uncertainties. In the mixed layer, unlike what has been widely reported in other low-latitude waters, there was a distinct seasonal pattern. During most of the year, the concentrations were low and hovered around the detection limits. In the winter, readily detectable concentrations were found as a result of enhanced vertical mixing. In the deep water below 1500 m, the average (N+N)/SRP molar ratio, 13.670.2, was noticeably lower than the Redfield ratio of 16. The corresponding N-anomaly, N * , was � 4.070.6mM. In the upper nutricline, high (N+N)/SRP and N * , even exceeding 16 and 2mM, were found. These elevated values are suggestive of the remineralization of nitrogen-rich organic matter in the upper nutricline. Temporally, while there were considerable inter-annual variations, higher (N+N)/SRP and N * were found mostly between September and April during the period of higher depositional fluxes of atmospheric dust. In the Tropical Water, which was situated within the upper nutricline at 150 m, the average N * were 1.9 and � 0.3mM, and the average (N+N)/SRP were 15.0 and 12.6 in September–April and May–August, respectively. These seasonal patterns suggest that the nitrogen-rich organic matter could have originated from nitrogen fixation, which was enhanced by the availability of iron during seasons of high dust-depositions. A box model calculation indicates that, in the absence of nitrogen fixation, the deficit in combined nitrogen would lead to a rapid escalation in the concentration of SRP in the mixed layer in contradiction to the field observations. If nitrogen fixation is allowed, the deficit in combined nitrogen in the mixed layer could be satisfied and a steady state in (N+N) and SRP in the mixed layer maintained by supporting 3–13% of the net primary production by nitrogen fixation. r 2007 Elsevier Ltd. All rights reserved.

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.

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.

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