Coral reefs over the Nansha Islands have characteristic geomorphological patterns; they exist in the form of atolls. The Niu’e (also known as Mckennan) and Ximen (Whitsum) reefs of the Jiuzhang Giant Atoll are two secondary atolls with newly-formed debris deposits, which may represent an important stage of landform evolution from a submerged atoll towards a reef island. Grain-size analysis of the surficial sediment samples collected from the two reefs shows that gravel-sized coral debris is the main component of the deposit. Medium grain sizes of the samples (measured by the long axis) range from 14 to 45 mm. Sorting coefficients range from 4.5 (being poorly sorted) to 31 with a sphericity value of 0.52 to 0.68 (far from a sphere shape). Medium grain sizes show a trend of increasing away from the shoreline. Medium grain sizes of the sediment samples are positively correlated with sorting coefficients, whereas they are more or less negatively correlated with sphericity values. Based on an analysis of historical records and satellite images, these beach ridge- or sand bar-like deposits represent modern accumulation landforms; their formations are related to storm waves and to some extent to the winter monsoon waves. The sand bar on the Niu’e reef had been extending towards the southeast and at the same time moving towards the north. The sand bar on the Ximen reef had an overall migration towards the southeast, with recurved sand spits on both sides. The mechanisms for the formulation of these features, in terms of changes in coral material supply and storm wave pattern, need further investigations.

Marine planktonic ostracods are small crustaceans with a wide distribution, playing an important role in ocean carbon cycle. The ostracods in smaller size are usually underestimated when marine zooplankton samples were collected with 505 μm mesh. Based on the zooplankton samples collected with 505 and 160 μm meshes in the northern South China Sea from July to August 2006 (summer) and from December 2006 to January 2007 (winter), the capture efficiency for the ostracods using two planktonic mesh sizes was compared, and the community structure and the influence of environmental factors on their distributions were analyzed. We found that the abundance of ostracods obtained by the 160 μm mesh net was six times that by the 505 μm net in summer. The community structure of planktonic ostracods collected by the 160 μm mesh was analyzed with the following findings. 1) The species richness of planktonic ostracods of a total of 32 species was identified to increase from nearshore to offshore, and the difference between the areas with water depth shallower and deeper than 100 m was extremely significant (p<0.01). 2) The abundance of planktonic ostracods was higher in summer, when the high abundance area appeared mainly in the coastal waters of the Leizhou Peninsula and northeast Hainan Island higher than that in offshore significantly (p<0.05), and lower in winter, when the distribution of ostracods abundance was homogeneous. 3) Euconchoecia aculeata, dominant species, contributed the most to high abundance in the nearshore. 4) The species richness of ostracods was positively correlated with temperature and water depth; and the abundance was negatively correlated with temperature and salinity, and positively correlated with chlorophyll α concentration. Qiongdong coastal upwelling and Yuexi Coastal Current boosted the abundance of planktonic ostracods up to 1252 ind·m ^-3 in the coastal waters of the Leizhou Peninsula and northeast Hainan Island. Results suggest that smaller mesh net should be used for the overall assessment of community characteristics of planktonic ostracods.

Marine ecosystem dynamics model is an important means to study marine ecological environment. As the model complexity increases, the number and uncertainty of biological parameters increase, which has a great impact on model results; therefore, optimization of model parameters is particularly important. In this paper, a one-dimensional physical-biological model is applied to the northern South China Sea, and the key biological parameters obtained through sensitivity analysis are optimized by using genetic algorithm. The results show that the sensitive parameters in the model are related to phytoplankton growth, zooplankton growth, feeding and death, and detritus sinking. Based on the genetic algorithm optimization of the above-mentioned parameters, we find that the surface and vertical simulation errors of the model are reduced by 27.80% and 21.40%, respectively, by using only surface satellite data; the surface and vertical simulation errors are reduced by 14.90% and 32.70%, respectively, by adding observed profile data. The success of applying genetic algorithm in the one-dimensional model provides the basis for its further application in three-dimensional marine ecosystem models.

We investigate submesoscale characteristics of summer upwelling fronts in the western South China Sea (WSCS) and associated ageostrophic processes by using satellite measurements and high-resolution ROMS simulations. Active submesoscale filaments with a typical horizontal scale of O(1-10) km are detected to be characterized by O(1) Rossby number (Ro) from the fine-resolution satellite images and simulation results. The diagnostic analysis shows that down-front wind forcing drives a net cross-front Ekman transport and advects heavy water over light water. This process at submesoscale tends to reduce the stratification and potential vorticity (PV), exacerbates the frontal instabilities, and forms the cross-front secondary circulation. The high-resolution simulation results show that the maximum vertical velocity in the frontal zone can reach 100m?d ^-1, which significantly enhances vertical material exchange. In this context, active submesoscale processes may contribute to enhanced vertical exchanges of the upper ocean in the summer upwelling front of the western SCS.

Summer (from June to September) time series of 15 years (2002-2016) of chlorophyll-a (Chl a) from the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua products were collected to discuss seasonal and interannual variability of ecological response of the Pearl River Plume in the Northern South China Sea (NSCS). In June, under the influence of southwest monsoon, nutrients from the Pearl River Delta were transported eastward to the shelf area, phytoplankton bloomed and Chla increased. The coverage area over the shelf in the NSCS reached the maximum in July, and decreased in August when the river discharge and wind reduced. In September when the northeasterly wind prevailed in the NSCS, the coverage area with high concentration of Chla (AHCHL) west of the estuary increased. Linear regression analysis suggested that the river discharge was the main reason for driving the AHCHL variability with a time lag of about one month. The combined effect of stronger (weaker) than the multi-year mean river discharge and wind resulted in extremely large (small) AHCHL in 2008 (2004). Because the spatiotemporal variability of Chl a in the NSCS was mainly influenced by the Pearl River Plume, the quality controlled satellite Chl a products can be used to discuss the variability of the Pearl River Plume.

There are many islands and reefs associated with abundant fishery resources in the South China Sea (SCS). Ecological variables such as chlorophyll in these regions were seldom studied. In this paper, merged ocean color data in recent ~20 years are used to analyze the spatial, seasonal and interannual variations of sea surface chlorophyll concentration (CHL) around 38 islands and reefs in the SCS. The results show that CHL is relatively high around these islands and reefs, which is generally reduced to the background level at five equivalent radii from the island center. The CHL anomaly (relative to the background CHL) is consistent with background CHL in terms of spatial distribution. They are relatively high in Xisha and Dongsha regions, and relatively low in Zhongsha and Nansha regions, mainly due to differences in temperatures and monsoon intensities. The CHL has significant seasonal and interannual variations. It rises during the winter monsoon and falls to a minimum before the summer monsoon onset. It declines in the following years of El Ni?o, when the SST increases and the monsoon weaken; and the opposite happens in the following years of La Ni?a. Moreover, the CHL anomaly increases in spring and autumn seasons when the coral reef community is growing vigorously in Xisha and Dongsha regions. It decreased significantly (P=5.05×10 ^-5) during the last 20 years, probably due to the increases in sea surface temperature and its amplitude. These findings provide useful information for the development and management of fishery resources in the SCS.

Age and growth rate are key parameters in fish stock assessment and management. Sthenoteuthis oualaniensis has a short life cycle and a fast growth rate. We employ statolith microstructure to study the difference in age composition, gender and growth of the squid collected from October 2016 to September 2017 by falling-net in the South China Sea in order to study their age and growth. The results show that the age of medium cohort squid ranged from 48d to 125d and that the dominant age was from 61d to 80d. There were significant differences in the composition of female and male ages; age of dwarf cohort squid was between 44d to 95d, and the dominant age was between 51d to 70d. The difference in age composition between male and female individuals is significant. The estimated incubation period for the medium cohort samples was from July 2016 to July 2017; the peak hatching dates were distributed in the months of January and from July to August; the incubation period of the dwarf cohort was from June to October 2016 and from December 2016 to February 2017, and their peak hatching months were January and August. The mantle length, body mass and age of different individuals in the medium cohort could be fit in logarithm and linear relationship. The mantle length and age of different individuals in the dwarf cohort were fit in logarithm and logistic relationship, while the body mass and age were fit in logistic relationship. Our findings indicate that the growth rate of the dwarf group was higher than that of the medium group. The growth rate of the female was lower than that of the male in the dwarf group, but it was the opposite in the medium group. Through the analysis of statoliths growth pattern, we can learn about the age composition of the squid, estimate their hatching time, select their appropriate growth equation, and estimate their growth rate, to provide basic data for fishery biology.

The effect of microorganisms on coral skeletons is more concealed than that by macro-bioerosion, which is visible to the naked eye. The study of micro-bioerosion is rare, and there is no such research in China. In this study, micro-bioerosion research was carried out under scanning electron microscopy on the coral skeletons of the Porites samples collected from Weizhou Island, Daya Bay and Chenhang Island. The “Orange Bands” were formed by microorganisms in the coral skeletons of Weizhou Island and Daya Bay, while there were no similar bands in the samples of Chenhang Island. This phenomenon may be related to the eutrophication of local sea water. Scanning electron microscopy of these "Orange Bands" revealed that this was caused by an endolithic algae—Ostreobium quekettii, a common eroding microorganism in the tropical coral reef area. The structure of Porites skeleton eroded by Ostreobium quekettii is destroyed, the thickness is reduced, the porosity is increased from 1.54% to 6.76%, and the compressive strength is reduced from 27.1% to 51.2%. The microorganism makes the coral skeleton more "loose" and form a layer of "osteoporosis" on the entire surface of the coral. This microorganism acts as a “forerunner,” making corals more susceptible to macro-bioerosion, which in turn promote micro-bioerosion. The “positive feedback effect” explains the increased bioerosion of the coral skeleton in the eutrophic northern South China Sea.

The South China Sea is the largest marginal sea in the northwestern Pacific and the key channel connecting the North Pacific and northern Indian Ocean. When the Kuroshio flows northward along the Philippine Islands, signals from the western Pacific are transmitted to the South China Sea through the Luzon Strait, thus affecting the hydrodynamic environment of the South China Sea. We analyze the spatial distribution and low-frequency variability of subsurface salinity in the South China Sea, and try to explain the relationship between the subsurface salinity in the South China Sea and the Pacific Decadal Oscillation (PDO); we also explore the change of the subsurface salinity in recent years. The results are as follows. 1) Driven by the subsurface cyclonic circulation, subsurface salinity at about 24-26 σ_θ decreases gradually from north to south counterclockwise starting from the Luzon Strait. 2) The low-frequency variability of subsurface salinity is significantly correlated with PDO. When the PDO is in the positive phase, the westward transport in the Luzon Strait is strengthened and the subsurface salinity increases. When the PDO is in the negative phase, the westward transport in the Luzon Strait is weakened and the subsurface salinity decreases, and the salinity change is directly affected by the horizontal circulation. 3) In recent years, the subsurface salinity has shown a trend of refreshing from 2006 to early 2014, and then increasing from 2014 to early 2017, lagging behind the PDO by about 10 months. Since 2017, the salinity decreased again.

The energy accumulation and allocation of somatic and reproductive tissues in the medium form of male Sthenoteuthisoualaniensiswere were analyzed by using the technology of energy density determination combined with residual analysis. The results showed that the energy density of mantle, fins, arms, spermatophoric complex, and testis were 21.16±1.44, 21.98±1.51, 21.44±1.50, 20.83±1.70, and 21.41±1.70kJ·g ^-1, respectively. The energy density for tissue of mantle or spermatophoric complex varied significantly depending on sexual maturity stages, while there were no significant differences between sexual maturity stages for the tissue of fins, arms, or testis. The total energy accumulation for each individual was estimated at 299.31±90.81 kJ, and showed an increasing trend with body size growth. With regards to relative energy accumulation, the somatic tissues, including mantle, fins and arms, accounted for 95.31%~98.04% of the total energy accumulation, but decreased with maturation. The relative energy accumulation in reproductive tissues, being testis and spermatophoric complex, accounted for 1.96%~4.96% of the total energy accumulation, and increased with maturation, in which the maximum value was reached at stage V. The analysis of residuals of relationship between tissue absolute energy accumulation and mantle length, using Model II regression, showed a significantly positive correlation between somatic and gonadal tissues. This evidence suggests that there was no energy remobilization of somatic tissues to fuel reproduction during sexual maturation, and the mode of reproductive investment is more likely to be income breeding in the medium form of male S.oualaniensis. In this study, we preliminary established the progress of energy accumulation and its allocation in soma and reproductive organs for medium form of male S.oualaniensisin the South China Sea. These findings should assist future research on its reproductive strategy, as well as sustainable utilizationof this kind of resource in the area.

A conid species, Conus ferrugineus Hwass in Bruguière, 1792, was recorded from China for the first time based on specimens deposited in the Marine Biodiversity Collections of the South China Sea, Chinese Academy of Sciences, and on samples recently collected from voyages to the South China Sea. The specimens were collected from the Xisha Islands. The shell morphological features of this species were presented, as well as its habitat and distribution. C. ferrugineus is easily confused with its sympatric sibling species C. vitulinus Hwass in Bruguière, 1792. The most remarkable difference is its uniformly white aperture, in contrast to the violet to violet brown base of the aperture in C. vitulinus. The identification characteristics between them were also discussed and summarized in this paper.

A 1050-km long comprehensive geophysical profile (CFT) was acquired across the conjugate margins of the southwestern sub-basin of the South China Sea, which includes 49 OBSs, 6- or 8-km long streamer, gravimeter, and magnetometer. Various refined processing procedures were applied to the aforementioned geophysical data; and a joint reflection and refraction seismic travel time inversion was performed to derive a 2-D velocity model of the crustal structure and upper mantle. Based on this new tomographic model and shipboard gravity data, a comprehensive crustal structure model was created. Finally, some interesting issues including High Velocity Layer (HVL) in the base of crust, an anomalous low density seamount (long-men seamount) along the CFT profile are discussed in this paper. HVL are widely distributed under the northern slope, southwestern sub-basin and Nansha block along the CFT profile, with the velocity varying from 7.0 to 7.5 km·s ^-1, and thickness between 0 and 4 km. HVL in the marginal lower crust might be derived from melting and mixing of lower crust material and mantle material, and HVL in the oceanic crust might have originated from serpentinization by tectonically dominated seafloor spreading.

Intra-seasonal variability (ISV) of sea level anomalies (SLA) and their propagation features in the northern South China Sea (NSCS) are investigated using 25-year (1993-2017) satellite observations. The standard deviation of intra-seasonal SLA reveals that larger ISV exists in the northeastern South China Sea along the continental shelf/slope (200~2000 m) where it extends southwestward from Taiwan to Hainan. The ISV of SLA exhibits obvious seasonality, being strong in winter and weak in summer. By using Complex Empirical Orthogonal Function (CEOF) analysis, the spatial pattern and temporal variability of the ISV, as well as their inter-annual modulation, are studied. We find that there are mainly two types of ISV pattern, showing great agreement with eddy activities in the NSCS. The ISV of SLA is also modulated by seasonal and inter-annual variation. The first mode of CEOF indicates southwestward propagation of the ISV from southwest of Taiwan to east of the Xisha Islands, being especially strong in winter. The second mode of CEOF reveals two westward ISV regions: southwest to Taiwan and south to the Dongsha Islands. The statistical analysis for the seasonal variation of mesoscale eddies indicates that the CEOF results are consistent with the distribution of mesoscale eddy activities in the NSCS.

By using various satellite remote sensing data and statistical analysis method, the contribution of typhoons in summer (from June to September) to the precipitation and freshwater flux in the South China Sea (SCS) and their possible influences on circulation anomalies in the SCS in the past 17 years (2000-2016) were studied. The main conclusions are as follows. Typhoon is one of the crucial factors that impact the precipitation in both northern and middle SCS, and the daily mean precipitation can be increased by 12 mm, which accounts for half of the daily mean rainfall in the SCS during summer. Besides, there are significant differences in the location and intensity of the rainfall distributions between the Northwest Pacific Ocean typhoons (NWP TYs) and SCS typhoons(SCS TYs). In summer, the saline circulation in the SCS induced by freshwater flux shows a weak cyclone that centers in the southwestern part of Hainan island with its magnitude of about -0.15 Sv, which is approximately 10% of wind-induced circulation (about -1.5 Sv) during the corresponding period. Precipitation induced by typhoons can intensify cyclonic saline circulation in the northern-central part of the SCS in summer. And the intensity of the saline circulation caused by NWP typhoons is stronger compared to that by SCS typhoons.

Oithona is one of the most abundant species of small and medium zooplankton in the ocean, and plays an important role in marine biogeographic genetics and ecology research. In this study, we employed 28S rDNA to analyze the population genetic structure and haplotype pattern of the most common species (Oithona setigera) in the South China Sea. A dataset of 792 bp in length sequences was obtained. The average contents of G+C (58.2%) were significantly higher than those of A+T (41.8%) in the fragment. A total of 28 haplotypes were defined from 186 individuals, of which the dominant haplotype H10 was found in 21 populations. The largest distance between two sampling sites harboring this haplotype is more than 1000 km, indicating Oithona setigera can achieve long distance disperse and be affected by ocean currents. The Mantel test showed that there was no linear correlation between the genetic distance and geographical distance (R= -0.04615, P=0.678); the RDA (redundancy analysis) results indicated space factor significantly affect the population genetic structure rather than environmental factors. The population genetic structure of Oithona setigera may be caused by colonization events followed by demographic expansions.

We aim to study the secondary metabolites from the South China Sea soft coral-derived fungus Eupenicillium sp. DX-SER3 (KC871024). The rice fermentation products of the strain were purified by comprehensive chromatography methods of silica gel column, medium pressure preparative liquid chromatography (MPLC), ostade-cylsilane (ODS), and semi- preparative HPLC. The compounds’ structures were identified by nuclear magnetic resonance spectroscopy, mass spectroscopy, and comparison with the reported data. Five known compounds were obtained and identified as helvolic acid, β-adenosine, 2'-deoxythymidine, N-acyltryptamine, and p-hydroxybenzaldehyde. The high yield of the helvolic acid indicates that this strain has the potential to develop an engineered strain of this kind of compound. Helvolic acid was also tested for the activities of phytopathogenic fungus, and the result was not clear.

In this paper, we review latest research on the Zhongnan-Liyue Fault Zone (ZLFZ), and then analyze the spatial distribution and tectonic deformation feature of the ZLFZ based on the geophysical data including topographic, seismic, gravity, and magnetic data. The results show that the ZLFZ in the South China Sea Basin has obvious north-south segmentation characteristics. The north section, which is between northwest sub-basin and east sub-basin, is a NNW trend narrow zone with a width of ~16 km from (18°00'N, 115°30'E) to (17°30'N, 116°30'E). The south section, which is between southwest sub-basin and east sub-basin, is a NNW trend wide zone with a width of 60~80 km from the east of the Zhongsha Bank to the west of the Liyue Bank. The main fault of the ZLFZ is NNW trend along the seamounts’ ridge of Zhongnan. The ZLFZ of transition region is NNE trend from the north section to the south section. On the eastern and western sides of the ZLFZ, the sub-basin’s sedimentary thickness and oceanic crust thickness are obviously different. We speculate that the ZLFZ plays an important role in the geological structure of sub-basin. According to the change of crustal structure, we suspect that the ZLFZ is at least a crustal fracture zone.

Based on the analysis of data obtained from the China Meteorological Administration (CMA) (1949-2016) and State Oceanic Administration (SOA) (1989-2016), the trend of tropical cyclone and storm surge disaster along the northern South China Sea was examined in this study. The result indicated that the annual frequency of typhoons had been decreasing slightly but the influence of powerful typhoons and typhoon intensity showed a rising tendency. The frequency of landfall typhoons had experienced significant transformation in the mid-90s. Eastern Guangdong was frequently affected by high-intensity typhoons, and typhoon intensity in western Guangdong had increased year by year since 2007. Typhoon intensity gave the accordant distribution in the four study areas. Four typical paths of storm surge were summarized, and variations of intensity and regional distributions of typhoons with different incidence angle ranges were also analyzed in the study. We found that both the frequency of strong storm surge disaster and the level of annual maximum storm surge had increasing trends along South China coast in recent years.

The central western South China Sea is one of the main fishing grounds for fishery development. To understand the trophic relationships of major fishery organisms of the central western South China Sea, stable isotope techniques were used to analyze and determine carbon and nitrogen stable isotope ratios of major fishery biological samples in the area. The trophic levels of the main fishery biota were calculated to construct a continuous trophic spectrum of the main fishery organisms in the area. These results show that the major fishery isotope ratios have a wide range of changes in the central western South China Sea. The δ¹³C and δ¹⁵N values of the fish ranged from -20.00‰ to -16.51‰ and from 7.94‰ to 11.81‰, respectively. The δ¹³C and δ¹⁵N values of the cephalopods ranged from -18.84‰ to -17.60‰ and from 10.10‰ to 12.85‰, respectively. The corresponding trophic levels of each species were calculated using zooplankton as the baseline organism. The trophic level of fish ranged from 2.41 to 3.53, and that of cephalopods ranged from 3.03 to 3.84. Among the organisms, the average trophic level of cephalopods is higher than that of fish. Comparison of trophic levels in different lengths (mantle length) of Symplectoteuthis oualaniensis, Thysanoteuthis rhombus, Decapterus russelli, Decapterus lajang, and Thunnus albacares reveals that the trophic level has a correspondingly increasing trend as body length increases. In this study, we preliminarily established the continuous trophic levels’ spectrum of major fishery bio-nutritional levels in the central and western waters of the central western South China Sea, providing a theoretical basis for the food web structure and fishery resource utilization in the area.

In this study, the vertical characteristics of near-inertial oscillations (NIOs) induced by Typhoon Kalmaegi are analyzed by using data from two moorings about 20 km apart in the northeastern South China Sea in Sept. 2014. The results show that the energy of typhoon-induced NIOs, which propagates downward into the ocean interior, increases with depth in the upper layer and becomes maximum in the mid layer before dissipating with depth. However, we find that there is a large difference in vertical characteristics of typhoon-induced NIOs between the two moorings. The NIOs at mooring A had three different vertical phase velocities at different depths while the NIOs at mooring B showed invariable phase velocity. The value and depth of the maximum near-inertial kinetic energy (NIKE) are also different at the two moorings. The maximum NIKE was 15% greater at mooring B than at mooring A. After the passage of Kalmaegi, fD₂ via nonlinear wave interaction between f and D₂ occurred at the two moorings, except that the intensities of fD₂ at the two moorings were different. At mooring A, fD₂ changed with depth and its interaction between f and D₂ was weak. The energy of fD₂ at mooring B enhanced at the full depth, and the kinetic energy and velocities of fD₂ and f had a good correlation for their time-space distributions and variations. The difference of vertical phase velocity of NIOs and fD₂ at the two moorings may be caused by the effects of eddy and different stratifications.

Sea surface wind is not merely a major driving force to the upper-ocean currents; the wind energy input is the main source of mechanical energy to keep these currents moving. To analyze the secular trend of wind energy input into the South China Sea (SCS), we calculate the wind energy input into the surface current, surface geostrophic current and surface ageostrophic current from 1901 and 2010 by using SODA (v 2.2.4) data. The results show that during the past 110 years, the trend of the wind energy input into surface current, surface geostrophic current and surface ageostrophic current decreased on the whole. The reducing amplitude is 56%, 65% and 49%, respectively. The dominant factor is the decline of wind stress (about 35%). Due to the monsoon systems, seasonal variation of wind energy input into the SCS is significant. In winter, the wind energy input is the strongest, mainly in the north and west parts of the basin, and the shape of energy input distribution is like a “boomerang.” Our results have some implication for further understanding SCS circulation in terms of energy.

The basaltic basement in the South China Sea was cored for the first time during International Ocean Discovery Program (IODP) Exp 349. The basalt was collected from Sites U1431 and U1433 close to the fossil spreading ridge in East and Southwest Sub-basins, respectively. Carbonate veins (n=16) within the basalt were investigated under microscope and by the laser Raman spectra. At Site U1431, the carbonate veins consisted of calcite veins or aragonite veins, while at Site U1433 the carbonate veins are composed of either calcite veins, or aragonite veins, and calcite-aragonite mixed veins. Meanwhile, the calcite veins and aragonites alternated, and at ~42.1 m below basement, two calcite veins and one aragonite vein occurred parallel, likely indicating that multiple hydrothermal fluids of different sources have circulated there. At both sites, the textures of aragonite are generally the same-blocky, fibrous, and radiating fibers. However, it is not the case for calcite. At Site U1431, the calcite is clotted blocky, blocky, granule, and fibrous, while at Site U1433 the calcite only occur as blocks. The abundance of carbonate veins at Site U1431 is much higher than that at Site U1433. These suggest that the low-temperature hydrothermal circulation at Site U1431 is stronger than that at Site U1433. Different local geology and environment mostly lead to the differences in hydrothermal activity at these two sites: Site U1431 is close to a giant seamount that serves as a recharge site, while Site U1433 is far away from any recharge or discharge site.

A coupled physical-biological 3D model was established, which is forced by realistic forcing, to quantify the effects and underlying mechanisms of upwelling and river plume on the spatial distribution of phytoplankton biomass during summer in the northern South China sea (NSCS). The model was validated by using cruise data of 2006 and 2008 and remote-sensing observations from 2006 to 2008. The results suggest that the model satisfactorily captures the processes of coastal upwelling and river plume dynamics that determine phytoplankton distribution. Model results indicate that summer phytoplankton in the NSCS were mainly distributed within 50m isobath. In Qiongdong and Shantou, phytoplankton distributed evenly. Upwelling processes accounted for up to 90% of the biomass, and horizontal advection in the upper layer was the main sink, while biological processes were the main sources for the phytoplankton biomass. In the Pearl River and Shanwei, phytoplankton shows surface and subsurface maxima: the river plume contributed 35%~40% (mainly to the upper layer) of the phytoplankton biomass, and upwelling contributed 60%~65% (mainly to the middle and bottom layers). In Yuexi, the phytoplankton biomass was extremely low in the upper layer, and mainly distributed in the middle and bottom layers; in total, upwelling contributed 92% of phytoplankton biomass. In the NSCS, both upwelling and river plume processes stimulated phytoplankton through nutrient supply. In the upwelling process, nutrient supply reflected the combination of along- and across-shore nutrient transports. The thermocline, which alters vertical nutrient transfer, is the key factor affecting the variable contributions of upwelling and river plume processes for phytoplankton in different layers. Overall, the spatial variation of summer phytoplankton is mainly driven by upwelling and river plume processes, and by the circulation-nutrient-biological coupling effect.

Jack mackerel (Trachurus japonicus) is one of the main catches in the northern South China Sea. Based on the survey data of the North China Coastal Fishery Resources Survey (2014-2017) in the South China Sea, we used geostatistics to explore the spatial and temporal distribution characteristics of jack mackerel and related eco-dynamic processes. The results showed that the overall spatial distribution of jack mackerel was mainly associated with low resource density and less high density of resources. The characteristics of seasonal aggregation are obvious and as follows: summer> spring> autumn> winter. We found that the spatial distribution had a strong heterogeneity, and the proportion of spatial structure was above 75%. The spatial variability was dominated by Gaussian distribution, and the spatial correlation distance (variation range) was about 0.52° with obvious seasonal characteristics. Through the research on the correlation between geostatistical parameter values and catch per unit effort (CPUE), we found that the greater the resource density of jack mackerel, the more obvious the spatial heterogeneity was. Through the heterogeneity analysis in all directions, we found that the spatial heterogeneity in the northwest-southeast direction was significant, indicating that the marine dynamic process in this direction had a significant impact on the migration and distribution of jack mackerel. In addition, based on the Kriging interpolation analysis of the spatial distribution of jack mackerel, we found that jack mackerel had a pattern of migratory distribution from southwest to northeast, which had obvious characteristics of patchy distribution. Jack mackerel was also susceptible to extreme climate (El Nino, La Nina, and other extreme events).

Ocean dynamic processes in the South China Sea driven by the monsoon are characterized by seasonal and multi-scale variations, which have significant impacts on the evolution of marine ecosystems. Marine bacteria, as an important component of material cycle and energy flow in marine ecosystems, have active responses to the environmental changes associated with multi-scale dynamic processes. The interdisciplinary research on microbial community structures, their dynamic variation and associated physical processes is one of the hot issues in marine sciences, which combines marine microbiology, microbial ecology and physical oceanography. Based on the primary results of the Key Research Program, "Study on bacteria diversity coupled with the monsoon circulation in the South China Sea and significance in microbial oceanography" supported by the National Natural Science Foundation of China, we discuss in this paper current research progress, including coastal upwelling, seasonal thermal fronts, mesoscale eddy, sub-mesoscale processes, and their potential impacts on the marine bacterial diversity and local ecosystem in the South China Sea.

Chlorophyll a (Chla) concentration is an important indicator of phytoplankton biomass for estimating primary production, which exhibits inhomogeneous vertical distribution. In this study, optical measurements with high vertical resolution were used for studying the spatio-temporal variability of Chla profiles and its influencing mechanism, providing us much insights for understanding marine environmental dynamics in the South China Sea. In this study, an optical inversion method was proposed to retrieve Chla profiles from in situ measurements with high-spectral absorption/attenuation spectrophotometer in the northeastern South China Sea. Based on the in situ data of the Kuroshio Cruise in summer 2015, an empirical relationship between the absorption line height at 676 nm and Chla was developed for inverting Chla with a high accuracy (Chla=49.96×(a_LH(676) (676))^0.9339, the coefficient of determination R²=0.87, the root mean square error RMSE=0.16 mg·m^-3). Moreover, the response characteristics of Chla profiles to different hydrodynamic processes were studied. Results showed that large spatial differences existed in the vertical distribution of Chla. In the nearshore area, surface Chla varied from 0.42 to 1.57 mg·m^-3, which generally followed a decreasing trend with depth. Uniform vertical distribution of Chla in upper ocean was observed in the coastal upwelling region. The Subsurface Chlorophyll Maximum (SCM) layer is nearly ubiquitous in stratified waters of the open ocean, with its vertical structure being influenced by mesoscale processes. The depth of SCM ranged between 34 and 100 m, which showed consistent variability with the isopycnal depth of 1023 kg·m^-3. The SCM layer was uplifted to about 34 m due to the mixing effect in the upper ocean caused by Kuroshio intrusion in the western area of Luzon Island. The uplifting and widening of the SCM layer were also observed in the area affected by a cold eddy, with obviously different features in the eddy center from that on the eddy edge.

Particle size distribution (PSD) describes the relationship between particle concentration and particle size, influencing marine ecosystem environment, the optical properties of sea water, and so on. Based on the in-situ profiles of biological and optical properties during summer 2016 in the South China Sea basin, characterization of PSD was studied. The power-law model was fit to describe the PSD, and the results indicated that the mean coefficient of determination between in-situ PSD and simulated PSD could reach 0.95 in the logarithmic space. The PSD slope (ξ) ranged in [1.27, 7.65] with a mean of 3.93±0.56. The mean of ξ in the surface water of the South China Sea basin was similar to the mean of ξ in global ocean surface water, but higher than that in the surface water of other areas such as the bay. There was a strong negative relationship between ξ and the mean diameter (D_A). Taking section T1 as an example, we analyzed the mean PSD profile of these stations. The features of PSD profiles are as follows. 1) At the surface, there were high values of ξ with low values of D_A because the dominate particle was pico-phytoplankton. 2) The minimum value of ξ appeared in the subsurface chlorophyll maximum layer (SCML) with higher D_A, which may result from the higher proportion of big phytoplankton. 3) In the twilight layer, the values of ξ as well as the values of D_A were between those at the surface and in the SCML. This phenomenon may be related to the process of flocculation, decomposition, and settlement of phytoplankton. The traits of PSD would influence the inherent optical properties (IOP) of seawater. We found that both particulate beam scattering coefficient at 532 nm (b_p(532)) and particulate beam backscattering coefficient at 532 nm (b_bp(532)) would be higher in the SCML because of the increasing chlorophyll-a concentration. However, the lowest mean b_p(532) and the lowest mean b_bp(532) were found in the twilight layer. Furthermore, although the relationship between ξ and the particulate beam attenuation sepctral slope was weak, the model of Boss et al. (2001b) is suitable for estimating the regional range and mean value of ξ roughly.

Our Argo trajectory simulation system for the South China Sea (SCS) contains the high-resolution ambient velocity field, a Lagrangian particle tracking model and the parameterization that represents the vertical motions of profiling Argo floats. This system is applied to simulate both conventional Argo floats (typically parked at 1000 m depth and profiling to 2000 m depth) and Deep Argo floats (parked at 500 m above the seafloor) within the SCS. By conducting the simulation with the counterparts of six core Argo floats serviced in the SCS, we find the displacements of synthetic floats from the simulation system resemble the real float displacements over 100-day time intervals. We therefore judge the simulations for core Argo are robust and further apply the system to simulate a potential Deep Argo array (with the resolution of 2°×2°×30 day). The results explore both the representativeness and the predictability of float displacements, which may provide a basis to understand float displacements in the deep layer as well as to contribute to planning deep Argo array program.

Dongsha area is a critical gas hydrate area of the northern South China Sea, and a submarine slide is extensively developed on its slope. In this paper, we analyzed the sedimentological particle size, species characteristic and stable isotope of benthic foraminifera in the cores 973-4 and 973-5, respectively, recovered from the middle of the slope and the flat area at the base. The results of the distinctly negative δ¹³C values and the heavier δ¹⁸O values in both cores in the Last Glacial period suggested that there were persistent gas hydrate dissociation events in the Dongsha area during this period. The δ¹³C negative gradually disappeared and the δ¹⁸O values decreased since the Last Deglacial period, indicating that gas hydrate dissociation was prevented because of global sea level rise. In the core 974, obvious submarine slide deposits only occurred in 440~600 cm according to the Last Glacial Maximum and the number of Uvigerina spp. and Bulimina spp. sharply increased in this location, which implied the submarine slide was probably caused by an intense methane release event induced by the descending sea level in the Last Glacial Maximum period. A smaller scale submarine slide was also recorded in core 973-5, but the deposition time was later than that of core 973-4.

The seabed pockmark is widely found in the Qiongdongnan Basin in the northern slope of the South China Sea. The formation of pockmark is mainly regarded as related to the seabed fluid seepage in recent studies. However, it is still unclear what the geochemical characteristics of the seabed sediment are and what corresponding activities of the pockmark are. Two sediment cores of C14 and C19 from the Southern Qiongdongnan Basin were recovered and investigated in this study. Here, we examine the contents of total sulfur (TS), total carbon (TC) and total organic carbon (TOC) contents as well as chromium reducible sulfur (CRS) content and its stable isotopes (δ³⁴S_CRS) values. Combined with the ratio of total nitrogen to total organic carbon (TN/TOC) and reduced sulfate content in pore water, we analyze the geochemical characteristics. The results suggest that there are two types of sulfate reduction reaction (SR) at the C14 site - the anoxygenous oxidation of organic matter controlled SR (OSR) at the depth of 0~3.91 meters below seafloor (m bsf) and the anaerobic oxidation of methane (AOM) derived SR at the depth of 3.91~7.55 m bsf, and the depth of 3.91 m bsf is the boundary of the two types of reaction of SR. The contents of TS and TC below 3.91 m bsf are higher than those above 3.91 m bsf, and the concentration of sulfate in pore water below 3.91 m bsf decreases linearly with depth, Therefore, the depth of 3.91 m bsf is a significant indicator for geochemical characteristics in sediment core. Moreover, the SR below 3.91 m bsf is controlled by the seepage of methane-bearing fluid. The contents of TS and TC at the C19 site increase with depth. However, the variation of TN/TOC ratios is almost contrary to the patterns of TS and TC contents, that is, because OSR is the main SR type of the whole sediment core at the C19 site and the organic matter is revealed to be deposited in early diagenetic stage. The δ³⁴S_CRS values from cores C14 and C19 are -50.2‰~-46.9‰ and -50.1‰~-42.0‰, respectively. The negative δ³⁴S_CRS values suggest that the major biogeochemical process is the result of sulfate reduction in a relatively open system. Therefore, the methane-bearing fluid for the pockmark in the Qiongdongnan Basin had leaked and the pockmark is now seemingly in a weak seepage stage and/or even to be inactive.