Northeastern South China Sea Research Articles

Acoustic propagation in the South China Sea: Internal waves and sand dunes

Very large subaqueous sand dunes were discovered on the upper continental slope of the northeastern South China Sea (SCS) in the spring of 2007 during the ONR 322OA-funded NLIWI Acoustics field experiment. The dunes’ formation mechanism is hypothesized to be the internal solitary waves (ISW) which generate from tidal forcing on the Luzon Ridge on the east side of the SCS, propagate west across the deep basin with amplitudes regularly exceeding 125 m, and dissipate large amounts of energy via turbulent interaction with the continental slope, suspending and redistributing the bottom sediment. These internal waves and sand dunes are important acoustical features, as it is expected that they will cause significant anomalies in the acoustical field. Data analysis and modeling are presented to quantify the degree to which these features impact broadband (850-1200 Hz) signals propagating along an acoustic transect oriented perpendicular to the internal wave fronts and sand dune crests.

Light carbon isotope events of foraminifera attributed to methane release from gas hydrates on the continental slope, northeastern South China Sea

In 2013, the China Geological Survey and Guangzhou Marine Geological Survey conducted the second Chinese gas hydrate expedition in the northern South China Sea (SCS) and successfully obtained visible gas hydrate samples. Five of the thirteen drilling sites were cored for further research. In this work, Site GMGS2-08 is selected for the stable isotopic analysis of foraminifera present in the boreholes in order to reveal the carbon isotopic characteristics of the foraminifera and their response to methane release in the gas hydrate geological system. Our results show that the methane content at Site GMGS2-08 is extremely high, with headspace methane concentrations up to 39300 μmol L−1. The hydrocarbon δ13C values, ranging from −69.4‰ to −72.3‰ PDB, distinctly indicate biogenic generation. Based on the δD analytical results (−183‰ to −185‰ SMOW), headspace methane is further discriminated to be microbial gas, derived from CO2 reduction. By isotopic measurement, five light δ13C events are found in the boreholes from Site GMGS2-08, with foraminiferal δ13C values being apparently lower than the normal variation range found in the glacial-interglacial cycles of the SCS. The δ13C values of benthic Uvigerina peregrina are extremely depleted (as low as −15.85‰ PDB), while those of planktonic Globigerinoides ruber reach −5.68‰ PDB. Scanning electron micrograph (SEM) studies show that foraminiferal tests have experienced post-depositional alteration, infilled with authigenic carbonate, and the diagenetic mineralization is unlikely to be related to the burial depths. The correlation calculation suggests that the anaerobic oxidation of organic matter has only weak influences on the δ13C composition of benthic foraminifera. This means that the anomalous δ13C depletions are predominantly attributed to the overprinting of secondary carbonates derived from the anaerobic oxidation of methane (AOM). Furthermore, the negative δ13C anomalies, coupled with the positive δ18O anomalies observed at Site GMGS2-08, are most likely the critical pieces of evidence for gas hydrate dissociation in the geological history of the study area.

Characteristics of mud diapirs and mud volcanoes and their relationship to oil and gas migration and accumulation in a marginal basin of the northern South China Sea

Mud diapir and mud volcano have the same formation mechanism and similar development and evolution characteristics, but different geological features and forms. In this study, the development and characteristics of a mud diapir in Yinggehai Basin (northwestern South China Sea) and a mud volcano in Southwest Taiwan Basin (northeastern South China Sea) were investigated. The geochemical characteristics of their associated natural gases were also analyzed. The results indicated that the Yinggehai Basin mud diapir is mainly distributed in the deep and large depression of the central basin and presents five lines in echelon arrangement along a northwest direction. Its development and evolution characteristics are low density and speed, and abnormally high temperature and overpressure. In contrast, the Southwest Taiwan Basin mud volcano is mainly distributed in a deep water slope area of the south sag and Tainan-Kaohsiung. Onshore mud volcanoes are mainly distributed along the deep and large fault zone, and their morphological characteristics are similar to venting volcanoes. The seabed morphology of offshore mud volcano seems similar, but current research is still insufficient. There are many types of associated hydrocarbon gases in mud diapir and mud volcano, but the high maturity coal-type hydrocarbon gas is the main one. The associated non-hydrocarbon gas is rich in CO2 and N2, which is mainly derived from a CO2 source from inorganic crust. The source of both diapir and volcano is thick marine Miocene and Pliocene age mud with hydrocarbon-generating potential. The mature hydrocarbon generation, mud diapir/mud volcano migration and deposition into an appropriate reservoir, as well as diapir-associated structures, constitute a unique oil/gas migration and accumulation system, which thus adds to our understanding of the distribution of such “mud diapir/volcano-type oil/gas reservoir.”

Distortion and broadening of internal solitary wavefront in the northeastern South China Sea deep basin

AbstractInternal solitary waves (ISWs) with peculiar fronts are frequently observed in the world ocean by satellite images, though with quite few explanations. In this study a distorted and broadening ISW front across the northeastern South China Sea deep basin is presented by using synthetic aperture radar (SAR) image. To illustrate this peculiar front, a nonlinear refraction model is developed to simulate and evaluate the effects of realistic bottom topography, current, and stratification on its transformation. Simulated results in realistic oceanic environments show good agreements with this SAR‐observed front. Based on separate and comparative results in different background environments, we demonstrate that the distortion is actually caused by the strong mesoscale currents at periphery of an anticyclonic eddy. Moreover, the broadening is due to the difference in change of wave half width at different rays, which is associated with the different transformation of ISWs across variable bottom topography in the deep basin.

A weakly coupled data assimilation system of a coupled physical–biological model for the northeastern South China Sea

A weakly coupled data assimilation system was established for a coupled physical–biological model for the northeastern South China Sea (NSCS). The physical model used was the Regional Ocean Modeling System; the biological component was a seven-compartment nitrogen–phytoplankton–zooplankton–detritus ecosystem model; and the data assimilation method was Ensemble Optical Interpolation. To test the performance of the weakly coupled data assimilation system, two numerical experiments (i.e. control and assimilation runs) based on a process-oriented idealized case were conducted, and climatological SST was assimilated in the assimilation run. Only physical variables were adjusted in the weakly coupled data assimilation. The results showed that both the assimilated SST and other unassimilated physical variables had reasonable process responses. Due to the warmer SST observation, the water temperature (salinity) in the assimilation run increased (decreased) in coastal upwelling regions. Both the alongshore and bottom cross-shore currents were reduced, jointly demonstrating the weakening of the upwelling system. Meanwhile, ecosystem variables were also affected to some extent by the SST assimilation through the coupled model. For example, larger phytoplankton (chlorophyll) productivity was found in the upwelling region within the shallow layer due to the warmer waters in the assimilation run. Hence, the application of this data assimilation system could reasonably modify both physical and biological variables for the NSCS by SST assimilation.

Interannual modulation of eddy kinetic energy in the northeastern South China Sea as revealed by an eddy‐resolving OGCM

AbstractInterannual modulation of eddy kinetic energy (EKE) in the northeastern South China Sea (NE‐SCS) is investigated based on outputs of an eddy‐resolving oceanic general circulation model between 1980 and 2014. The EKE displays distinct interannual modulations with periods between 1.5 and 7 years. The maximum peak‐to‐trough amplitude of the interannual modulation occurred during period 2004–2005, which was about 1.5‐fold the time‐mean EKE level. Further analysis suggested that interannual variability of EKE in the NE‐SCS is primarily modulated by the Luzon Strait transport (LST). During high‐EKE years, the LST increases corresponding to a strengthened Kuroshio intrusion. The strengthened Kuroshio intrusion enhances the baroclinic instability of current in the NE‐SCS and thus leads to a strong EKE. The reverse is true during low‐EKE years when LST is smaller. Influences of ENSO and Pacific mesoscale eddies on the interannual modulation of LST are also discussed in this study.

Textural and map contrasts of the subduction-collision boundary between the Philippine Arc and the Sunda margin

The trench-arc-basin system of Southeast Asia is one of the most developed areas in the world, and there is a subduction-collision boundary which is still active nowadays between the Philippine Arc and the Sunda margin. Based on the previous research and the latest geological and geophysical data, the spatial distribution characteristics as well as the deformation characteristics of tectonic units on both sides of the tectonic boundary are systematically analyzed in this paper; we propose that the boundary begins from the east side of Taiwan coast plain in the northeastern South China Sea, then extends southward along the Manila Trench, through the thrust fault zone of Mindoro island, joints to the Negros Trench at the east edge of Sulu Sea, then the boundary goes through the thrust fault zone of Zamboanga and connects with the Cotabato Trench. According to the crust properties and interaction methods of the tectonic units on both sides of the boundary, the tectonic boundary can be divided into three types: arc-continent collision type, trench-subduction type and transitional type, and the different types of tectonic boundary have different significant characteristics in the geological and geophysical data. By the comparative analysis on the structural characteristics of the different sections of the boundary as well as the tectonic units on both sides of the boundary, we suggest that the crust properties on both sides of the boundary and the ancient tectonic position of the micro-continents of the west boundary are the main dynamic mechanism which leads to the significantly subsection characteristics of the tectonic boundary.

Phytoplankton dynamics driven by vertical nutrient fluxes during the spring inter-monsoon period in the northeastern South China Sea

Abstract. A field survey from the coastal ocean zones to the offshore pelagic zones of the northeastern South China Sea (nSCS) was conducted during the inter-monsoon period of May 2014 when the region was characterized by prevailing low-nutrient conditions. Comprehensive field measurements were made for not only hydrographic and biogeochemical properties but also phytoplankton growth and microzooplankton grazing rates. We also performed estimations of the vertical turbulent diffusivity and diffusive nutrient fluxes using a Thorpe-scale method and the upwelling nutrient fluxes by Ekman pumping using satellite-derived wind stress curl. Our results indicated a positive correlation between the integrated phytoplankton chlorophyll a and vertical nutrient fluxes in the offshore region of the nSCS during the study period. We generally found an increasing role of turbulent diffusion but a decreasing role of curl-driven upwelling in vertical transport of nutrients from the coastal ocean zones to the offshore pelagic zones. Elevated nutrient fluxes near Dongsha Islands supported high new production leading to net growth of the phytoplankton community, whereas the low fluxes near the southwest of Taiwan had resulted in a negative net community growth leading to decline of a surface phytoplankton bloom. Overall, phytoplankton dynamics in the large part of the nSCS could be largely driven by vertical nutrient fluxes including turbulent diffusion and curl-driven upwelling during the spring inter-monsoon period.

Deep-water carbonate dissolution in the northern South China Sea during Marine Isotope Stage 3

The production, transportation, deposition, and dissolution of carbonate profoundly form part of the global carbon cycle and affect the amount and distribution of dissolved inorganic carbon (DIC) and alkalinity (ALK), which drive atmospheric CO2 changes during glacial/interglacial cycles. These processes may provide significant clues for better understanding of the mechanisms that control the global climate system. In this study, we calculate and analyze the foraminiferal dissolution index (FDX) and the fragmentation ratios of planktonic foraminifera for the 60–25 ka B.P. time-span, based on samples from Core 17924 and ODP Site 1144 in the northeastern South China Sea (SCS), so as to reconstruct the deep-water carbonate dissolution during Marine Isotope Stage 3 (MIS 3). Our analysis shows that the dissolution of carbonate increases gradually in Core 17924, whereas it remains stable at ODP Site 1144. This difference is caused by the deep-sea carbonate ion concentration ([CO32−]) that affected the dissolution in Core 17924 where the depth of 3440 m is below the saturation horizon. However, the depth of ODP Site 1144 is 2037 m, which is above the lysocline where the water is always saturated with calcium carbonate; the dissolution is therefore less dependent of chemical changes of the seawater. The combined effect of the productivity and the deep-water chemical evolution may decrease deep-water [CO32−] and accelerate carbonate dissolution. The fall of the sea-level increased the input of DIC and ALK to the deep ocean and deepened the carbonate saturation depth, which caused an increase of the deep-water [CO32−]. The elevated [CO32−] partially neutralized the reduced [CO32−] contributed by remineralization of organic matter and slowdown of thermohaline. These consequently are the fundamental reasons for the difference in dissolution rate between these two sites.

Effect of background parabolic current on characteristics and energetics of internal solitary waves by numerical simulation

Based on modifications of the observed background parabolic current in upper layer of the northeastern South China Sea (SCS), the effects of eight kinds of background currents on the characteristics and energy conversion of internal solitary waves (ISWs) are investigated by an Internal Gravity Wave (IGW) model. It is found that, although the background current has little effect on the number of the generated ISWs, it reduces the resulted phase speed of ISW. When the background parabolic current appears with its lower boundary near or above the main thermocline, the ISW amplitude and the depth of the isopycnal undergoing maximum displacement increase; when the background parabolic current curvature is reduced, the ISW amplitude and the ratio of baroclinic to barotropic energy reduce, whilst the phase speed of ISW, the baroclinic energy, and the ratio of baroclinic kinetic energy (KE) to available potential energy (APE) increase; when the lower boundary of background parabolic current extends down to the seabed and the background current curvature is reduced, the ISW amplitude and phase speed decrease, whilst the barotropic kinetic energy, the baroclinic energy and the ratio of KE to APE increase. At a whole depth, when the lower background current curvature is reduced and the upper current curvature is increased, the ISW amplitude, and phase speed, the ratio of baroclinic to barotropic energy, the baroclinic energy, and the ratio of KE to APE all increase.

Galatheid and chirostylid crustaceans (Decapoda: Anomura) from a cold seep environment in the northeastern South China Sea.

Six species of squat lobsters from a cold seep field in the northeastern South China Sea are studied. Two new species, Uroptychus jiaolongae n. sp. and U. spinulosus n. sp., are described, and their distinctions from the related species are detailed. Two species, Munidopsis tuberosa Osawa, Lin & Chan, 2008 and M. verrilli Benedict, 1902, are herein reported for the first time from a cold seep/hydrothermal vent environment. The number of squat lobsters species associated with those chemosynthetic environments now stands at forty-one.

Simulations of Internal Solitary Wave Interactions with Mesoscale Eddies in the Northeastern South China Sea

AbstractWith the combined analysis of synthetic aperture radar image and satellite altimeter data collected in the northeastern South China Sea (SCS), this study found one type of distorted phenomenon of internal solitary wave (ISW) with the long front caused by the oceanic mesoscale eddy. Motivated by these satellite observations, the authors carried out numerical experiments using the fully nonhydrostatic and nonlinear MITgcm to investigate the perturbation of ISWs by an isolated cyclonic or anticyclonic eddy. The results show that the ISW front is distorted by these oceanic eddies due to the retardation and acceleration effects at their two sides. The ISW energy along the front is focused onto (scattered from) the wave fragment where a concave (convex) pattern is formed, and the previously accumulated energy in the focusing region is gradually released after the ISW propagates away from the eddies. The ISW amplitude is modulated greatly by the eddies due to the energy redistribution along the front. Sensitivity results indicate that the magnitude of the modulated ISW amplitude in the focusing region can reach twice the incident ISW amplitude, while in the scattering region it can be reduced by more than a half. These results therefore suggest that models with eddies included, especially the energetic eddies, could further improve the amplitude predictions in the northeastern SCS. Additionally, the internal gravity wave formed behind the energy-focusing region by the anticyclonic eddies can steepen and break with the consequent formation of a secondary trailing ISW packet. Finally, this study shows that the model results of the distorted front and trailing packet are in qualitative agreement with that of the satellite observations in the northeastern SCS.

Sea surface temperature anomalies in the South China Sea during mature phase of ENSO

Based on the 18-year (1993–2010) National Centers for Environmental Prediction optimum interpolation sea surface temperature (SST) and simple ocean data assimilation datasets, this study investigated the patterns of the SST anomalies (SSTAs) that occurred in the South China Sea (SCS) during the mature phase of the El Nino/Southern Oscillation. The most dominant characteristic was that of the outof-phase variation between southwestern and northeastern parts of the SCS, which was influenced primarily by the net surface heat flux and by horizontal thermal advection. The negative SSTA in the northeastern SCS was caused mainly by the loss of heat to the atmosphere and because of the cold-water advection from the western Pacific through the Luzon Strait during El Nino episodes. Conversely, it was found that the anomalous large-scale atmospheric circulation and weakened western boundary current during El Nino episodes led to the development of the positive SSTA in the southwestern SCS.

A winter chlorophyll bloom analyzed using Lagrangian diagnoses in the northeastern South China Sea

The Finite Time Lyapunov Exponent technique is a highly effective way to visualize and analyze flow fields. Its ridges are referred to as Lagrangian Coherent Structures which reveal the transport barriers of fluid systems, or their hidden skeletons. This allows us to investigate the complex mesoscale and submesoscale ocean processes on a Lagrangian view. In this article, we use the Lagrangian technique to investigate one chlorophyll bloom event in the northeastern South China Sea and discuss the impact of horizontal transport on the distribution of surface chlorophyll. Our results demonstrate that horizontal transport barriers uncovered by Lagrangian Coherent Structures can shape the vortex and filament in a simple and distinct way.

Observed enhanced internal tides in winter near the Luzon Strait

AbstractSeasonal characteristics and nonlinear interaction of internal tides (ITs) near the Luzon Strait in the northeastern South China Sea are investigated using 285 day, in situ observation data. It is found that ITs, which are dominated by the first‐mode wave throughout the year, are the strongest in subsurface layer. Baroclinic incoherent diurnal (semidiurnal) variance accounts for about 85.7% (78.3%) of diurnal (semidiurnal) ITs. The amplitude and seasonal variation of the diurnal ITs are more prominent than those that are semidiurnal, e.g., the largest kinetic energy densities of diurnal and semidiurnal baroclinic tidal currents are 2.81 and 0.83 KJ/m2 in winter, respectively. It is considered that there are two reasons for the significantly enhanced ITs in winter: (1) it may be due to the Kuroshio intrusion, and (2) the enhanced diurnal ITs may be due to the enhanced diurnal barotropic tidal currents, while the enhanced semidiurnal ITs may be caused by the strong nonlinear interaction between diurnal IT constituents O1 and K1 due to their high vertical shears in subsurface layer. Thus, harmonic semidiurnal constituent D2 with a similar frequency of constituent M2 is induced; it subsequently enhances the semidiurnal ITs in this subsurface layer, and the associated energy is carried downward to enhance the semidiurnal ITs in the upper and lower layers.

Changes in monsoon-driven upwelling in the South China Sea over glacial Terminations I and II: a multi-proxy record

Upwelling intensity in the South China Sea has changed over glacial-interglacial cycles in response to orbital-scale changes in the East Asian Monsoon. Here, we evaluate new multi-proxy records of two sediment cores from the north-eastern South China Sea to uncover millennial-scale changes in winter monsoon-driven upwelling over glacial Terminations I and II. On the basis of U/Th-based speleothem chronology, we compare these changes with sediment records of summer monsoon-driven upwelling east of South Vietnam. Ocean upwelling is traced by reduced (UK'37-based) temperature and increased nutrient and productivity estimates of sea surface waters (delta C-13 on planktic foraminifera, accumulation rates of alkenones, chlorins, and total organic carbon). Accordingly, strong winter upwelling occurred north-west of Luzon (Philippines) during late Marine Isotope Stage 6.2, Heinrich (HS) and Greenland stadials (GS) HS-11, GS-26, GS-25, HS-1, and the Younger Dryas. During these stadials, summer upwelling decreased off South Vietnam and sea surface salinity reached a maximum suggesting a drop in monsoon rains, concurrent with speleothem records of aridity in China. In harmony with a stadial-to-interstadial see-saw pattern, winter upwelling off Luzon in turn was weak during interstadials, in particular those of glacial Terminations I and II, when summer upwelling culminated east of South Vietnam. Most likely, this upwelling terminated widespread deep-water stratification, coeval with the deglacial rise in atmospheric CO2. Yet, a synchronous maximum in precipitation fostered estuarine overturning circulation in the South China Sea, in particular as long as the Borneo Strait was closed when sea level dropped below -40 m.

Effect of surface mesoscale eddies on deep-sea currents and mixing in the northeastern South China Sea

Recent studies suggest that deep-reaching surface-generated eddies result in anomalous current velocities in the deep sea, and ultimately lead to energy transfer from mesoscale to small-scale motions. Here we examine the influence of mesoscale eddies on deep-sea subinertial and near-inertial currents, and on possible enhanced oceanic mixing in the deep South China Sea (SCS). We analyzed current velocity data for nearly a full water column. Data were obtained using acoustic Doppler current profilers and recording current meters on a deep-sea mooring system at a depth of 2100m in the northeastern SCS from October 2012 to May 2013. A highly nonlinear southwestward-propagating anticyclonic eddy was detected via a resolved sea-surface-level anomaly. This eddy induced pronounced subinertial currents with a characteristic time scale of 1–2 months and a maximum velocity of up to 0.2ms–1 at the subsurface and 0.1ms–1 at great depth. Near-inertial energy co-occurring with subinertial flows showed a distinctive vertical propagation trend during strong subinertial oscillations in the deep sea. During periods of strong subinertial and near-inertial kinetic energy, estimates of diapycnal diffusivity in the deep ocean showed approximately 10-fold enhancement, with a mean value of 1.2×10–3m2s–1 compared to the background value of 1.4×10–4m2s–1. The results provide observational evidence of the effect of surface-observed mesoscale motions on benthic currents and ocean mixing in the deep SCS.

BSRs, estimated heat flow, hydrate-related gas volume and their implications for methane seepage and gas hydrate in the Dongsha region, northern South China Sea

To investigate the relationship among methane seepage, subsurface gas hydrate, and heat flow and estimate hydrate-related gas amount in the Dongsha region, northeastern South China Sea, in this study we first identified Bottom Simulating Reflector (BSR) using multi-channel seismic data collected in 2004. Subsequently, the thickness of gas hydrate stability zones (GHSZ) was calculated from the observed BSR using time–depth conversion, ranging from 60 to 300 m in seawater depths of 600–1200 m. Additionally, a seismic feature of double BSRs is locally observed in this region, which is possibly caused by either the presence of mixed gas source (thermogenic and biogenic) or migration of a pre-existing base of GHSZ. Geothermal gradient, heat flow and hydrate-related gas volume are calculated based on the thickness of GHSZ. The estimated geothermal gradient and heat flow varies from 20 to 100 °C/km with an average of 48 °C/km, and from 20 to 120 mW/m2 with an average of 54 mW/m2, respectively. Both estimated geothermal gradient and heat flow are generally consistent with those from measurements. High geothermal gradient (up to 60 °C/km) and heat flow anomaly (up to 80 mW/m2) are present in one methane seepage site, where BSR is clear and reliable. High heat flow could result in thin GHSZ in the continental slope. In this case, gas hydrate could form in the shallow sediments, making dissolved gas from hydrate and/or free deep-seated gas easy to migrate upwards to seafloor through faults, and thus resulting in the occurrence of methane venting. The hydrate-related gas volume is estimated as ∼6 × 1011 m3 over an area of ∼400 km2 with possible occurrence of BSR. This finding would shed lights on understanding potential amount of subsurface methane in the Dongsha region and place useful constraints for modeling GHSZ from measured heat flow data, especially in regions where no BSRs are present.

Temporal variability of the current in the northeastern South China Sea revealed by 2.5-year-long moored observations

The current in the northeastern South China Sea (NSCS) has been extensively studied by both observations and numerical models, but its vertical structure and temporal variability are still not well understood because of the lack of long-term and full-depth direct observations. From August 2010 to March 2013, the current velocity in the NSCS was directly observed by Acoustic Doppler Current Profilers from a subsurface mooring at 117°52.07′E, 21°06.68′N. The observed current velocity was coherent in the vertical direction and nearly showed the same sign in the nearly full water column (960 m). The observed current presented strong intraseasonal variabilities (ISVs) within periods between 30 and 90 days. Analysis of the altimeter observation suggested that the ISVs were closely related to the propagating mesoscale eddies. Corresponding to the seasonally varying circulation in the NSCS, the ISV-removed current roughly showed a semiannual cycle. The seasonal U (V) averaged over 80–200 m depths reached the peak in March and October (January and July) and the trough in July and February (April and August), respectively, with a peak-to-trough amplitude of 0.14 m/s (0.19 m/s). Dynamical modes analysis suggested that the observed current velocity showed dominated energy in the barotropic mode during most of the observation period but enhanced energy in the first baroclinic mode during eddy events because of the enhanced vertical shear of velocity.

Distribution of radionuclides in a marine sediment core off the waterspout of the nuclear power plants in Daya Bay, northeastern South China Sea

A sediment core was collected and dated using 210Pbex dating method off the waterspout of nuclear power base of Daya Bay, northeastern South China Sea. The γ-emitting radionuclides were analyzed using HPGe γ spectrometry, gross alpha and beta radioactivity as well as other geochemical indicators were deliberated to assess the impact of nuclear power plants (NPP) operation and to study the past environment changes. It suggested that NPP provided no new radioactivity source to sediment based on the low specific activity of 137Cs. Two broad peaks of TOC, TC and LOI accorded well with the commercial operations of Daya Bay NPP (1994.2 and 1994.5) and LNPP Phase I (2002.5 and 2003.3), implying that the mass input of cooling water from NPP may result into a substantial change in the ecological environment and Daya Bay has been severely impacted by human activities.