China Shelf Research Articles

Spatial and Temporal Characteristics of Winter Diel Vertical Migration of Zooplankton and Nekton in the East China Shelf Seas Based on Multi‐Frequency Echosounder

AbstractThe diel vertical migration (DVM) of plankton and nekton plays an important role in the energy flow and material cycling of marine ecosystems. Especially in shelf seas, seasonal environmental variations may distinguish DVM characteristics in winter mixed water from summer pattern. Therefore, to investigate the patterns and influencing factors of winter DVM in East China Shelf Seas (ECSS), we carried out comprehensive research based on high‐resolution multi‐frequency acoustic data during the winter of 2019/2020. The results showed that: (a) acoustic and zooplankton sampling data can effectively capture the widely existence of winter DVM of different biological community in ECSS and their spatio‐temporal variabilities. (b) In East China Sea, the migration was mainly dominated by the Gas‐bearing class (GB), which had fast migration speed, compared to the Yellow Sea where the migration was dominated by the Fluid‐like class (FL). In addition, GB descends earlier than FL. (c) For large scale factors, during the full moon, the GB will aggregate deeper and the FL shallower, compared to the new moon, and regional temperature is an important factor affecting the distribution of organisms at meridional direction; for small scale processes, cold and warm currents and ocean fronts can also affect migration behavior. In the muddy area with full moon and spring tide, GB and FL show different inconsistent migration compared to normal relatively consistent pattern. These results will help us to better understand the physiological, ecological and environmental elements that control winter DVM, facilitating the management of ecosystem‐based fisheries resources.

Winter storms drive offshore transport and modulate phytoplankton blooms in Northern Taiwan, China

The East China Shelf Sea (ECSS) is subject to high-frequency storms during winter and spring, with these storm processes serving as a significant driving factor for initiating the outward diffusion of materials from the inner shelf. Inner shelf waters tend to be rich in nutrients, thus their horizontal diffusion is crucial to the marine primary productivity of the outer shelf. However, our understanding of the processes that govern the offshore water mass transport under winter storm oscillations and the mechanisms driving their impact on phytoplankton is currently limited. In this study, we used satellite and reanalysis data from 2003 to 2020 to investigate the modulation mechanism of winter offshore currents (WOC) on phytoplankton bloom (PB) stimulated by storms in Northern Taiwan (NTW). The results revealed that at synoptic scales, winter storms result in higher sea surface height (SSH) and potential energy on the inner side of the front. However, during periods of weakened storms (for example, during north wind relaxation or a shift to southerly winds), a strong WOC is produced in the top 30–50 m of the ocean due to continuous SSH adjustments caused by changed pressure gradients and induced by topography. PB occurs in the offshore waters of the front, driven by a continuous nutrient supply and an increase in photosynthetically available radiation (PAR). On an interannual scale, offshore transport is controlled by the storm's duration and the extent of its weakening. As the climate warms and winter winds in NTW have consistently weakened over the past 20 years, lower turbulent kinetic energy (TKE) and a shallower mixed layer depth (MLD) will promote phytoplankton growth. The results suggest that water level fluctuations caused by high-frequency changes in winter storms are strongly linked to the offshore sediment transport from near-shore waters, impacting the material transport and the health of the ecosystem on the continental shelf.

Spatiotemporal Variations of Ocean Upwelling and Downwelling Induced by Wind Wakes of Offshore Wind Farms

Offshore wind farms (OWFs) generate large-scale wind wakes, which might lead to upwelling/downwelling. Understanding the vertical marine response to the wake effects is crucial for assessing the ecological impacts of OWFs and optimizing their co-deployments with mariculture. In this study, we employ a high-resolution ocean model to investigate the spatiotemporal variations of upwelling and downwelling induced by the wind wakes of OWFs through idealized numerical experiments. We have two main findings. First, the wind-wake-induced upwelling and downwelling are not balanced in the north–south direction, resulting in a net effect of thermocline rising. Second, the thermocline depth changes caused by wind wakes develop nonlinearly over time. Specifically, when the elevated thermocline approaches the sea surface, the upwelling slows down significantly. The spatially asymmetric pattern of the upwelling is attributed to horizontal Ekman transport, while its temporal nonlinear evolution is caused by stratification changes. By utilizing the simulated change law of thermocline depth, we calculate the ocean response of OWF wakes in China’s adjacent waters. The results suggest that baroclinic theory overestimates the ocean response in the Bohai Sea, the Yellow Sea, and the nearshore waters of the East China Sea. However, in the open seas and the South China Shelf, the upwelling/downwelling is expected to be close to the theoretical calculations. This study provides a foundation for conducting regional simulations with high resolutions in areas where OWFs will be constructed.

Effect of land–sea air mass transport on spatiotemporal distributions of atmospheric CO2 and CH4 mixing ratios over the southern Yellow Sea

Abstract. To reveal the spatiotemporal distributions of atmospheric CO2 and CH4 mixing ratios and regulation mechanisms over the China shelf sea, two field surveys were conducted in the southern Yellow Sea in China in November 2012 and June 2013, respectively. The results observed showed that mean background atmospheric CO2 and CH4 mixing ratios were 403.94 (±13.77) ppm and 1924.8 (±27.8) ppb in November 2012 and 395.90 (±3.53) ppm and 1918.0 (±25.7) ppb in June 2013, respectively. An improved data-filtering method was optimised and established to flag atmospheric CO2 and CH4 emission from different sources in the survey area. We found that the spatiotemporal distributions of atmospheric CO2 and CH4 mixing ratios over the southern Yellow Sea were dominated by land–sea air mass transport, which was mainly driven by seasonal monsoon, while the influence of air–sea exchange was negligible. In addition, atmospheric CO2 and CH4 mixing ratios over the southern Yellow Sea could be elevated remarkably at a distance of approximately 20 km offshore by land-to-sea air mass transportation from the Asian continent during the early-winter monsoon.

Corrigendum: Early diagenesis and benthic fluxes of redox-sensitive metals in eastern China shelf sediments

Corrigendum: Early diagenesis and benthic fluxes of redox-sensitive metals in eastern China shelf sediments

Glacial changes in sea level modulated millennial-scale variability of Southeast Asian autumn monsoon rainfall

Most paleoclimate studies of Mainland Southeast Asia hydroclimate focus on the summer monsoon, with few studies investigating rainfall in other seasons. Here, we present a multiproxy stalagmite record (45,000 to 4,000years) from central Vietnam, a region that receives most of its annual rainfall in autumn (September-November). We find evidence of a prolonged dry period spanning the last glacial maximum that is punctuated by an abrupt shift to wetter conditions during the deglaciation at ~14 ka. Paired with climate model simulations, we show that sea-level change drives autumn monsoon rainfall variability on glacial-orbital timescales. Consistent with the dry signal in the stalagmite record, climate model simulations reveal that lower glacial sea level exposes land in the Gulf of Tonkin and along the South China Shelf, reducing convection and moisture delivery to central Vietnam. When sea level rises and these landmasses flood at ~14 ka, moisture delivery to central Vietnam increases, causing an abrupt shift from dry to wet conditions. On millennial timescales, we find signatures of well-known Heinrich Stadials (HS) (dry conditions) and Dansgaard-Oeschger Events (wet conditions). Model simulations show that during the dry HS, changes in sea surface temperature related to meltwater forcing cause the formation of an anomalous anticyclone in the Western Pacific, which advects dry air across central Vietnam, decreasing autumn rainfall. Notably, sea level modulates the magnitude of millennial-scale dry and wet phases by muting dry events and enhancing wet events during periods of low sea level, highlighting the importance of this mechanism to autumn monsoon variability.

Projected Sea Bottom Temperature Variability in the East China Shelf Seas by 2100

Existing research has proven the increase in sea surface temperature (SST) due to global warming. However, the sea bottom temperature (SBT) may exhibit different characteristics in various regional seas. The East China Shelf Seas (ECSSs), which are important shelf seas in the Western Pacific, hold ecological significance when analyzing their SBT variations in a warming future. This article investigates both the interannual and interdecadal SBT variations from 2006 to 2100, utilizing the projection results from phase 5 of the Climate Model Intercomparison Project (CMIP5) sponsored by the Intergovernmental Panel on Climate Change (IPCC). We conducted an analysis of the interdecadal variation by comparing the SBTs from the 2030s, 2060s, and 2090s to the SBT observed in the 2010s. Our findings reveal a significant increase in SBT in the ECSSs. By 2100, the region is projected to experience enhanced warming of 1.18 °C. The springtime warming intensity of the Bohai Sea, reaching 1.92 °C, can be twice the rate of global ocean warming. The outer shelf of the ECSSs also exhibits significant increases in SBT. Through an analysis of the correlation between SBT and ocean currents, we investigate the potential mechanisms behind these observations. This paper provides insights into future SBT variations from both an interannual and interdecadal perspective, explaining the causes and the projected increase in environmental stresses on the benthic ecosystem over the next eighty years.

Early diagenesis and benthic fluxes of redox-sensitive metals in eastern China shelf sediments

Thirteen Short sediment cores (30-50 cm) were collected from Bohai Sea, Yellow Sea and Changjiang Estuary in China, and the early diagenesis of several redox sensitive metals (Fe, Mn, Mo, U and V, referring to as RSMs) in sediment were studied. The recycling process of Mo and Mn was closely correlated with each other, generating benthic fluxes diffusing upward from sediment to overlying water column, and the flux rates are related to the organic carbon oxidation rates. The recycling of U and V were more tightly coupled with Fe oxides, generating benthic fluxes going downward into the sediment in most cores. Significant authigenic accumulation of U, in contrary to little to no accumulation of Mo and V, were found in the study region, even in Changjiang Estuary where hypoxic condition was often found during summer. Benthic diffusive fluxes were compared with authigenic mass accumulate rates (MAR), which indicated that, besides the benthic diffusion process, there are other processes controlling the authigenic accumulation of the RSMs. The close relationships between authigenic accumulation of RSMs with OCburial and OCburial with Sburial, indicating the authigenic accumulation of RSMs is a consequence of redox environment in shelf sediment, which directly influencing the organic carbon degradation process. Compared with other continental margin, moderate enrichment of U was found in China continental sediment. The authigenic U accumulation in BS and NYS sediments accounted for 20 - 68% of the Yellow River input, whilst in SYS sediments accounted for ~ 64% of the Yellow River and Changjiang River input, which acting as important U sinks that cannot be ignored.

Characteristics of Surface Ozone and Nitrogen Oxides over a Typical City in the Yangtze River Delta, China

The Yangtze River Delta (YRD) is the most developed region in China. Influenced by intensive and complex anthropogenic activities, atmospheric pollution in this region is highly variable, and reports are sparse. In this study, a seven-year history of the atmospheric O3 and NOx mixing ratios over a typical city, Hangzhou, was presented to enrich the studies on air pollution in the YRD region. Our results revealed that the diurnal variation in NOx corresponded to traffic rush hours, while O3 was mainly impacted by photochemical reactions in the daytime. The weekend effect was significant for NOx, but inapparent for O3. Two O3 peaks in May and September were caused by seasonal atmospheric stability and climatic conditions. The lower NOx and higher O3 levels observed suggested direct effects from traffic restrictions and large-scale industrial shutdowns during the COVID-19 lockdown in 2020 compared with those in the periods before and after lockdown. The model simulation results showed that O3 mixing ratios were not only related to regional anthropogenic emissions but were impacted by air mass transportation from surrounding provinces and the China shelf seas. The NOx mixing ratios showed a decreasing trend, while the O3 mixing ratios showed the opposite trend from 2015 to 2021, which is indicative of the implementation of the Air Pollution Prevention and Control Acton Plan issued by the Chinese government in 2013.

Contamination, sources and health risks of potentially toxic elements in the coastal multimedia environment of South China

Coastal ecosystems are vulnerable to the accumulation of potentially toxic elements (PTEs), which pose a threat to marine ecosystems and human health. In this study, the concentrations of eight PTEs in a typical area of South China were analysed, and their distributions, seasonal variations, pollution degrees, potential health risks and sources in seawater, sediment and organisms were evaluated. The comprehensive pollution index (CPI), pollution load index (PLI), potential ecological risk index (PERI) and target hazard quotient (THQ) were applied to assess seawater, sediment and organism quality, respectively. The annual mean concentrations of Zn, Hg, Cr and As in the bottom seawater were higher than those in the surface water while those of Pb, Mn and Cu were higher in the surface seawater. The mean content of Hg was higher than the corresponding background value of that in China Shelf Sea sediment. Marine organisms have a high enrichment capacity for Cu, Zn, Cr, Hg, As and Mn in seawater. Based on CPI, the seawater was generally not polluted by PTEs. The PLI and PERI results demonstrated that Hg was the main contamination element in surface sediment. The total target hazard quotient (TTHQ) analysis illustrated that long-term consumption of some fish by children poses a noncarcinogenic health risk, while that risk to adults is negligible. Natural sources, agricultural activity sources, coal burning and industrial emission sources were the main sources of the PTEs in surface sediments according to positive matrix factorization (PMF) model.

Cross-Front Transport Triggered by Winter Storms Around the Shandong Peninsula, China

Ocean fronts are widespread on the eastern China shelf, especially in winter and early spring. Previous studies have shown that sediments on the northern coast of the Shandong Peninsula could penetrate the coastal fronts and deposit on the southern coast of the Liaodong Peninsula, and this cross-front transport process is closely related to winter storms. However, the detailed mechanisms of cross-front transport remain unclear. In this study, through realistic numerical modeling and physical mechanism analysis, a dynamic mechanism involving cross-front transport triggered by winter storms was proposed, and two locations with the highest probability of frontal instability around the Shandong Peninsula were identified. In detail, forced by winter storms, the Lubei Coastal Current significantly intensifies first and causes frontal inertial instability in the early stage of the winter storm, favoring cross-front exchange between both sides of the front. Then, serving as a compensation current, the Yellow Sea Warm Current intensifies in the late stage of the winter storm, leading to the northward transport of the materials out of the front. Furthermore, the intensity, distance, and path of the cross-front transport under varying winter storms are also investigated. The winter storm, with longer duration, greater wind speed, and westward wind direction, tends to trigger stronger cross-front transport and longer transport distance. Transport paths also differ under different winter storms. This study provides a theoretical explanation for cross-front transport and analyzes the role of winter storms in it, providing some new ideas for nearshore materials dispersal to the sea, which is of great significance for future studies on pollutant diffusion, nutrient distribution, and sediment transport in coastal areas.

Methane emission via sediment and water interface in the Bohai Sea, China

Sediment is recognized as the largest reservoir and source of methane (CH4) in the ocean, especially in the shallow coastal areas. To date, few data of CH4 concentration in sediment have been reported in the China shelf seas. In this study, we measured CH4 concentration in sediment and overlying seawater columns, and conducted an incubation experiment in the Bohai Sea in May 2017. CH4 concentration was found to be ranged from 3.075 to 1.795 μmol/L in sediment, which was 2 to 3 orders of magnitude higher than that in overlying seawater columns. The surface sediment was an important source of CH4, while bottom seawater acted as its sink. Furthermore, the net emission rate via sediment water interface (SWI) was calculated as 2.45 μmol/(m2∙day) based on the incubation experiment at station 73, and the earthquake may enhance CH4 release from sediment to seawater column in the eastern Bohai Sea.

Neglected role of continental circulation in cross-shelf sediment transport: Implications for paleoclimate reconstructions

Coastal currents play a key role in regulating alongshore sediment transport, and their relationships with winter storms (burst of the East Asian winter monsoon) and formation of the coastal mud deposits on the eastern China shelf have been established, based on which the evolution of the East Asian winter monsoon has been widely explored. Unlike that of coastal deposits, the formation of offshore deposits on the eastern China shelf is very complex and highly debated, leading to enormous challenges on paleoclimate reconstructions based on these deposits. In this study, cross-front sediment transport under a variable Yellow Sea Warm Current (YSWC) was explored using remote sensing imagery and sedimentary records (seismic profile and sediment grain size) from the North Yellow Sea (NYS). The results indicate that, although the offshore mud deposit in the western NYS is formed by winter storms through triggering cross-front transport of coastal sediment around the Shandong Peninsula, the winter storm signal was completely obscured by that of the YSWC, which might determine the flux of cross-front sediment transport on a millennial scale. The 1500 y sub-orbital climate variability cycle could also be observed in distal muds after 2.8 ka. By comparing the sediment composition within the same mud deposit or between adjacent mud deposits in the NYS, we found that the response of cross-front sediment transport to the YSWC varied spatially, which may be a reason for discrepancies observed in previous winter monsoon reconstructions on the eastern China shelf. Therefore, as indicated by this study, offshore deposits on the eastern China shelf are not suitable for inversions of the winter monsoon. In addition, mud deposits on the eastern China shelf are also dramatically affected by other dynamic factors, including typhoons and river flooding; thus, deeply exploring the dynamic characteristics and formation mechanism of these deposits are prerequisites for paleoclimate reconstruction.

CMIP5-Based Projection of Decadal and Seasonal Sea Surface Temperature Variations in East China Shelf Seas

The East China Shelf Seas, comprising the Bohai Sea, the Yellow Sea, and the shelf region of East China Sea, play significant roles among the shelf seas of the Western North Pacific Ocean. The projection of sea surface temperature (SST) changes in these regions is a hot research topic in marine science. However, this is a very difficult task due to the lack of available long-term projection data. Recently, with the high development of simulation technology based on numerical models, the model intercomparison projects, e.g., Phase 5 of the Climate Model Intercomparison Project (CMIP5), have become important ways of understanding climate changes. CMIP5 provides multiple models that can be used to estimate SST changes by 2100 under different representative concentration pathways (RCPs). This paper developed a CMIP5-based SST investigation framework for the projection of decadal and seasonal variation of SST in East China Shelf Seas by 2100. Since the simulation results of CMIP5 models may have degrees of errors, this paper uses hydrological observation data from World Ocean Atlas 2018 (WOA18) for model validation and correction. This paper selects seven representative ones including ACCESS1.3, CCSM4, FIO-ESM, CESM1-CAM5, CMCC-CMS, NorESM1-ME, and Max Planck Institute Earth System Model of medium resolution (MPI-ESM-MR). The decadal and seasonal SST changes in the next 100 years (2030, 2060, 2090) are investigated by comparing with the present analysis in 2010. The experimental results demonstrate that SST will increase significantly by 2100: the decadal SST will increase by about 1.55 °C, while the seasonal SST will increase by 1.03–1.95 °C.

Holocene paleoenvironmental changes in mud area southwest off Cheju Island, East China Sea: Evidence from benthic foraminiferal assemblages and stable isotope records

A sediment core retrieved from the mud area southwest off Cheju Island (MASCI) was analysed for benthic foraminifera, grain size, and stable isotopes and constrained by AMS14C dating. Holocene paleoenvironmental changes in this broad shallow shelf were investigated, with special focus on the evolution of the MASCI and its related ocean eddy system. The results indicate that the East China Sea shelf has undergone four major environmental evolution stages during the Holocene. Prior to 9.8 cal. Kyr BP, the benthic foraminiferal assemblages were dominated by low salinity, shallow-water species [Ammonia beccarii (Linnaeus), Elphidium advenum (Cushman), and Cribrononion subincertum (Asano)], reflecting a near-coast-deposit environment. A noticeable transition of the benthic foraminifera, from an A. beccarii dominated to Ammonia compressiuscula (Brady) dominated assemblage, taking place at approximately 9.8 cal. Kyr BP together with a contemporaneous increase in δ18O and δ13C, provides evidence that a rapid rise in sea level occurred at this time in the East China Shelf. A gradual increase in δ18O between 9.8 and 6 cal. Kyr BP indicates a steady strengthening influence of open ocean water on the area under study. A sudden increase in the abundance of benthic foraminifera along with a decrease in the coarse fraction imply that an early eddy circulation system was established since almost 6 cal. Kyr BP in the MASCI. This eddy circulation system was quite weak but gradually strengthened between 6 and 3 cal. Kyr BP, possibly due to the intrusion of the Yellow Sea Warm Current (YSWC). A more stable mud deposition period, characterised by a high sedimentation rate of fine-grained sediment and lighter value of δ13C, starting at approximately 3 cal. Kyr BP, indicates that the MASCI was formed mainly during the late Holocene. We believe that the strengthened YSWC since 3 cal. Kyr BP might increase the trap efficiency of the eddy system, thereby accounting for the high sedimentation rate of mud deposits during this period.

Indication of size distribution of suspended particulate matter for sediment transport in the South Yellow Sea

The Yellow Sea is a key area for studying source-to-sink sediment transport systems in East China shelf seas. However, the sediment transport and dispersal patterns in the Yellow Sea are not fully understood because of the significant seasonal variation in ocean circulation. In this study, we identify the ocean circulation patterns in the South Yellow Sea based on temperature, salinity and depth data collected from 52 hydrographic stations during the winter of 2016. The mass concentrations and particle size distributions of suspended particulate matter are also determined to decipher the sediment transport pattern in the South Yellow Sea. The results show that particles from the Yellow Sea Coastal Current and Shandong Peninsula Coastal Current display a bimodal size distribution, with peaks at approximately 2.5 μm and 32–64 μm, while particles from the Yellow Sea Warm Current and residual Yellow Sea Cold Water Mass display a trimodal size distribution, with peaks at 2.5 μm, 32–64 μm and 391 μm. The fine and medium particles (<256 μm) are mainly inorganic particles, while the coarse particles (>256 μm) include organic materials. The peak at 391 μm is dominated by organic particles and reflects the influence of the Yellow Sea Warm Current in the surface layer. The Yellow Sea Warm Current enhances the nutrient and temperature conditions in the South Yellow Sea, which facilitates phytoplankton growth under appropriate light conditions. The content of inorganic particles with sizes of 64–159 μm decreases sharply in the central South Yellow Sea, which reflects the transport of suspended particulate matter from the southern study area to the central Yellow Sea mud area. The synthesized evidence suggests that the Yellow Sea Warm Current, as the main driving force, transports particles from the Jiangsu coast or the Changjiang River into the central Yellow Sea mud area in winter.

Impacts of Currents and Waves on Bottom Drag Coefficient in the East China Shelf Seas

AbstractHigh‐frequency measurements of waves, currents, and turbulence were made using the bottom‐mounted tripod equipped with the Nortek 6‐MHz acoustic Doppler velocimetry at eight mooring stations in the East China Shelf Seas. The observational data are analyzed to estimate the bottom drag coefficient and also the contributions from currents and waves. Variations of bottom drag coefficient caused by currents show no obvious relationship with water depth. Generally, the current‐induced drag coefficient decreases with the strengthening currents, and the wave‐induced drag coefficient increases with the enhancing waves. Synthesis analyses of the observational data reveal a scale relationship between the current‐induced drag coefficient and the turbulent Reynolds number and that between the wave‐induced drag coefficient and the bottom wave orbital velocity. The analyses show no significant impacts of waves on turbulent Reynolds stress, and the total bottom drag coefficient is the sum of that due to currents and waves. The empirical formula for bottom drag coefficient derived from this study may help to improve the simulation of tides, storm surges, and sediment transport in coastal seas.

Evolution of the Southern Yellow Sea Cold Water Mass during the last 7 kyr from benthic foraminiferal evidence

The Southern Yellow Sea Cold Water Mass (YSCWM) is closely related to the modern circulation system of the east China shelf seas, which has significantly influenced regional marine environmental changes. The study of the Holocene evolution of the YSCWM will greatly improve our understanding of the mechanisms of regional environmental change. Benthic foraminifera are sensitive to bottom water environmental changes and can serve as useful indicators in bottom water environmental reconstruction. In this study, benthic foraminifera were analyzed in core N02 from the northwestern margin of the southern Yellow Sea Mud to decipher the phase evolution of the YSCWM during the last 7 kyr. Benthic foraminifera census counts and Q-mode factor analysis indicate that the Holocene sedimentary environment can be divided into three stages: From 6.9–5.0 ka, the fauna was dominated by Ammonia ketienziensis, indicating that the YSCWM was at its strongest during the last 7 kyr, while the Yellow Sea Coastal Current (YSCC) had a weak influence on the bottom water of the study area. From 5.0–2.9 ka, the relative abundance of Hanzawaia nipponica remarkably increased while the abundance of A. ketienziensis decreased significantly, reflecting that the strength of the YSCWM was relatively weak and the range of the YSCWM might have contracted. The influence of the YSCC on the bottom water might have slightly increased, although its influence was still weak during this time. A notable increase in low-temperature and low-salinity species, such as Protelphidium tuberculatum and Buccella spp. has occurred since 2.9 ka, indicating that the YSCC has had a strong influence on bottom water during this period, while the strength of the YSCWM has been at its weakest during the last 7 kyr. Generally, the influence of the YSCWM and the YSCC on the bottom water properties of the study area show an obvious seesaw pattern, with one’s influence increasing while the other’s influence decreases and vice versa. The fluctuations in the strength of YSCWM during the Holocene may be caused by the different effect allocations of regional climatic factors (i.e. El Nino Southern Oscillation, East Asian Winter Monsoon, summer insolation in the northern hemisphere, etc.) acting on the circulation system during different periods.

Phosphorus speciation, transformation, and preservation in the coastal area of Rushan Bay

Phosphorus (P) speciation, burial, and transformation are poorly constrained under low-oxygen conditions. Sequential chemical extraction techniques, in-situ incubation, and laboratory incubation were employed to explore P cycling in the low-oxygen area of coastal Rushan. The study determined that the total P concentrations in the coastal area of Rushan Bay were higher than those of other China shelf seas, and largely affected by anthropogenic activities. The phosphate (DRP) fluxes in the study area calculated using an incubation method (0–1960μmolm−2day−1) and measured based on pore water gradients (1.5–50.4μmolm−2day−1) were both highly correlated with oxygen conditions. Sediment incubations showed that DRP diffusion from the sediment mainly originates from Fe–P and Auth-P dissolution and that Org-P recycling contributed only a small portion of the total released P pool. The benthic phosphate flux can be 60 times higher under low bottom-water oxygen levels of 63–150μmolL−1 than under oxygen levels exceeding 150μmolL−1 in the study area. The P accumulation rates and burial efficiencies in this study area ranged from 16.5–33.3μmolcm−2year−1 and 81.1–83.4%, respectively, and were regulated by the oxygen level and diffusive DRP flux. This study indicates that low oxygen levels between 63 and 150μmol significantly govern P transformation and preservation in the sediment and P pools in the water column.

Molecular phylogeography and population genetic structure of the planktonic copepod Calanus sinicus Brodsky in the coastal waters of China

Planktonic copepod Calanus sinicus is the dominant meso-zooplankton in the Northwest Pacific Ocean. To better understand its population dynamics and phylogeographic patterns, 243 C. sinicus individuals were collected from seven locations across the shelf waters of China and its population genetics was studied by mitochondrial DNA cytochrome oxidase I (mtCOI) sequences analyses. Thirty-nine different sequences, or haplotypes, were detected with moderate haplotype diversity (h=0.749) and low nucleotide diversity (π=0.003) for all populations. The evolutionary divergence between geographic populations varied from 0.24% to 0.37%, indicative of very limited genetic differentiation. Visualized minimum spanning network (MSN) and phylogenetic analysis of all the detected haplotypes did not reveal any clear phylogeographic pattern. Furthermore, AMOVA data showed no significant spatial population differentiation existed among the individuals collected across China shelf waters. Pairwise F ST values showed that population collected from northwest of the East China Sea (ECS) displayed a low difference to other populations. Mismatch distribution analyses and neutrality tests indicated that C. sinicus might undergo a demographic/population expansion. No significant population genetic structuring was detected, indicating an extensive gene flow among the C. sinicus populations. Our results provide molecular evidence for the hypothesis that C. sinicus in the northwestern South China Sea in winter is transported from the East China Sea and the Yellow Sea by the China Coastal Current during the northeast monsoon period.