Yellow Sea Cold Water Mass Research Articles

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.

Influence of the northern Yellow Sea Cold Water Mass on picoplankton distribution around the Zhangzi Island, northern Yellow Sea

Picoplankton distribution around the Zhangzi Island (northern Yellow Sea) was investigated by monthly observation from July 2009 to June 2010. Three picoplankton populations were discriminated by flow cytometry, namely Synechococcus, picoeukaryotes and heterotrophic prokaryotes. In summer (from July to September), the edge of the northern Yellow Sea Cold Water Mass (NYSCWM) resulting from water column stratification was observed. In the NYSCWM, picoplankton (including Synechococcus, picoeukaryotes and heterotrophic prokaryotes) distributed synchronically with extremely high abundance in the thermocline (20 m) in July and August (especially in August), whereas in the bottom zone of the NYSCWM (below 30 m), picoplankton abundance was quite low. Synechococcus, picoeukaryotes and heterotrophic prokaryotes showed similar response to the NYSCWM, indicating they had similar regulating mechanism under the influence of NYSCWM. Whereas in the non-NYSCWM, Synechococcus, picoeukaryotes and heterotrophic prokaryotes exhibited different distribution patterns, suggesting they had different controlling mechanisms. Statistical analysis indicated that temperature, nutrients (NO3– and PO43–) and ciliate were important factors in regulating picoplankton distribution. The results in this study suggested that the physical event NYSCWM, had strong influence on picoplankton distribution around the Zhangzi Island in the northern Yellow Sea.

Exploring seasonal acidification in the Yellow Sea

The Yellow Sea on the western continental margin of the North Pacific Ocean is of major ecological and economic importance. Four field surveys were conducted during May and November 2012, August 2015, and January 2016, investigating seasonal variations in dissolved oxygen and carbonate system parameters of this marginal sea. Results showed that the Yellow Sea cold water mass accumulated respiration-induced CO2 in subsurface and bottom waters in summer and autumn, leading to acidified seawaters with critical carbonate saturation states of aragonite (Ωarag) of less than 1.5. These seriously acidified seawaters occupied one third of surveyed areas in summer and autumn, likely affecting local calcified organisms and benthic communities. In a future scenario for the 2050s, in which the atmospheric CO2 mole fraction increases by 100 μmol mol−1, half of the Yellow Sea benthos would be seasonally covered by acidified seawater having a critical Ωarag of less than 1.5. The corresponding bottom-water pHT would be around 7.85 in summer, and 7.80 in autumn. Of the China seas, the Yellow Sea cold water mass represents one of the ecosystems most vulnerable to ocean acidification.

Century-scale high-resolution black carbon records in sediment cores from the South Yellow Sea, China

Black carbon (BC) has received increasing attention in the last 20 years because it is not only an absorbent of toxic pollutants but also a greenhouse substance, preserving fire-history records, and more importantly, acting as an indicator of biogeochemical cycles and global changes. By adopting an improved chemothermal oxidation method (WXY), this study reconstructed the century-scale high-resolution records of BC deposition from two fine-grained sediment cores collected from the Yellow Sea Cold Water Mass in the South Yellow Sea. The BC records were divided into five stages, which exhibited specific sequences with three BC peaks at approximately 1891, 1921, and 2007 AD, representing times at which the first heavy storms appeared just after the termination of long-term droughts. The significant correlation between the times of the BC peaks in the cores and heavy storms in the area of the Huanghe (Yellow) River demonstrated that BC peaks could result from markedly strengthened sedimentation due to surface runoff, which augmented the atmospheric deposition. Stable carbon isotope analysis indicated that the evident increase in carbon isotope ratios of BC in Stage 5 might have resulted from the input of weathered rock-derived graphitic carbon cardinally induced by the annual anthropogenic modulation of water-borne sediment in the Huanghe River since 2005 AD. Numerical calculations demonstrated that the input fraction of graphitic carbon was 22.97% for Stage 5, whereas no graphitic carbon entered during Stages 1 and 3. The obtained data provide new and important understanding of the source-sink history of BC in the Yellow Sea.

Centennial-scale records of total organic carbon in sediment cores from the South Yellow Sea, China

Global carbon cycling is a significant factor that controls climate change. The centennial-scale variations in total organic carbon (TOC) contents and its sources in marginal sea sediments may reflect the influence of human activities on global climate change. In this study, two fine-grained sediment cores from the Yellow Sea Cold Water Mass of the South Yellow Sea were used to systematically determine TOC contents and stable carbon isotope ratios. These results were combined with previous data of black carbon and 210Pb dating from which we reconstructed the centennial-scale initial sequences of TOC, terrigenous TOC (TOCter) and marine autogenous TOC (TOCmar) after selecting suitable models to correct the measured TOC (TOCcor). These sequences showed that the TOCter decreased with time in the both cores while the TOCmar increased, particularly the rapid growth in core H43 since the late 1960s. According to the correlation between the Huanghe (Yellow) River discharge and the TOCcor, TOCter, or TOCmar, we found that the TOCter in the two cores mainly derived from the Huanghe River and was transported by it, and that higher Huanghe River discharge could strengthen the decomposition of TOCmar. The newly obtained initial TOC sequences provide important insights into the interaction between human activities and natural processes.

Diurnal Forcing Induces Variations in Seasonal Temperature and Its Rectification Mechanism in the Eastern Shelf Seas of China

AbstractThis study investigates the seasonal variation in temperature induced by diurnal forcing in the eastern shelf seas of China (ESSC) using a high‐resolution Regional Ocean Modeling System model forced by the National Center for Environmental Prediction and the National Center for Atmospheric Research re‐analysis data for surface fluxes with both 6 h and daily frequencies, respectively. The comparison between two experiments revealed a modification of the variation in seasonal temperature by diurnal forcing, which also increases the mixed‐layer depth (MLD) in August by 26% and reduces the volume of the Yellow Sea Cold Water Mass (YSCWM) by 25%. Sensitivity experiments using different forcing variables indicated that diurnal wind can explain over 80% of the variability in seasonal temperature induced by diurnal forcing. Diurnal wind led to an increase in the net heat flux into the ocean by about 13 in summer and a decrease by about 15 in winter. Diurnal wind also generated an additional downward heat transport of 21 over the ESSC that contributed to variability in the mean MLD and YSCWM in August. Experiments changing the temporal interval of wind forcing suggested that the increase in the forcing temporal interval gradually enhanced the reproduction of the variability in seasonal temperature generated by diurnal wind; a 6 h wind forcing can capture 70% of this type of variability given by 1 h wind forcing, while a 3 h or shorter wind forcing can capture 90%.

Air-sea heat flux control on the Yellow Sea Cold Water Mass intensity and implications for its prediction

The Yellow Sea Cold Water Mass (YSCWM), which occurs during summer in the central Yellow Sea, plays an important role in the hydrodynamic field, nutrient cycle and biological species. Based on water temperature observations during the summer from 1978 to 1998 in the western Yellow Sea, five specific YSCWM years were identified, including two strong years (1984 and 1985), two weak years (1989 and 1995) and one normal year (1992). Using a three-dimensional hydrodynamic model, the YSCWM formation processes in these five years were simulated and compared with observations. In general, the YSCWM began forming in spring, matured in summer and gradually disappeared in autumn of every year. The 8°C isotherm was used to indicate the YSCWM boundary. The modelled YSCWM areas in the two strong years were approximately two times larger than those in the two weak years. Based on the simulations in the weak year of 1995, ten numerical experiments were performed to quantify the key factors influencing the YSCWM intensity by changing the initial water condition in the previous autumn, air-sea heat flux, wind, evaporation, precipitation and sea level pressure to those in the strong year of 1984, respectively. The results showed that the air-sea heat flux was the dominant factor influencing the YSCWM intensity, which contributed about 80% of the differences of the YSCWM average water temperature at a depth of 50m. In addition, the air-sea heat flux in the previous winter had a determining effect, contributing more than 50% of the differences between the strong and weak YSCWM years. Finally, a simple formula for predicting the YSCWM intensity was established by using the key influencing factors, i.e., the sea surface temperature before the cooling season and the air-sea heat flux during the cooling season from the previous December to the current February. With this formula, instead of a complicated numerical model, we were able to roughly predict the YSCWM intensity for the following summer by using the data available online in winter.

Cruise observation of shallow water response to typhoon Damrey 2012 in the Yellow Sea

Typhoon Damrey 2012 has been the strongest typhoon to strike the area north of the Yangtze River since 1949. Two cruise observations to examine the shallow water response to typhoon were carried out just before and after the passage of Damrey in the shallow water near the Jiaozhou Bay on 1 and 5 August 2012. The observation results indicate that, after the passage of Damrey, the thermocline was deepened and weakened significantly. The water temperature below the surface increased by about 4–10°C. A significant temperature gradient was generated at the bottom of the water column. The salinity decreased throughout the water column. The surface heat flux and precipitation data indicate that the temperature increase and salinity decrease of the bottom water were mainly caused by the intrusion of warm, low-salinity water from the surface (depth < 7m). The model results suggest that the mechanism that drive the variations as follows: the warm low-salinity surface water was driven by wind and flowed onshore, accumulated and sank as it encountered the coast, which forced the cold bottom water of the Yellow Sea Cold Water Mass (YSCWM) to flow offshore.

Spatio-temporal distribution pattern of Calanus sinicus and its relationship with climate variability in the northern Yellow Sea

Abstract The spatio-temporal distribution pattern of Calanus sinicus, a key copepod species, was examined in the northern Yellow Sea (YS). Compared with 1959 and 1982, there was a significant increase in the abundance of C. sinicus in the spring, summer, and autumn of 2011 and winter of 2014. The percentage of C. sinicus in the zooplankton assemblages ranging from 45.6 to 75.8% in different seasons of 2011-2014 was significantly higher than that in 1982. Two different spatial distribution patterns of C. sinicus were observed, with higher abundance occurring nearshore during the cold season (e.g. January) and offshore in the central portion of the northern YS in the warm season (e.g. May, July, and October). The YS Cold Water Mass in the central portion of the northern YS likely provides an important over-summering site for the species. Additionally, a greater increase of the abundance of C. sinicus was found in the northern portion (the northern YS) of its spatial distribution during the past half century. This study has an implication on the climate-driven shifts in zooplankton community in the northern YS, highlighting the importance of C. sinicus in the warm-temperate ecosystem of Chinese coastal seas.

Occurrence and spatial distribution of organophosphorus flame retardants and plasticizers in the Bohai and Yellow Seas, China

Seawater samples from 50 sites in the BS and YS were collected to investigate the spatial distribution of 7 OPs. Concentrations of the total OPs (ƩOPs) in the BS and YS ranged from 8.12ngL−1 to 98.04ngL−1 with a geometric mean (GM) of 23.70ngL−1. Tris(1-chloro-2-propyl) phosphate (TCPP) was the dominant compound, followed by tris(2-chloroethyl) phosphate (TCEP) and triphenylphosphine oxide (TPPO). The ƩOPs together with the most commonly detected individual OPs (TCPP, TCEP, tris(1,3-dichloro-2-propyl) phosphate (TDCPP), tri-iso-butyl phosphate (TiBP), triphenyl phosphate (TPhP), and TPPO) tended to decrease from nearshore to offshore and from the surface to the bottom seawaters, indicating major land-based sources. Furthermore, the Yellow Sea Cold Water Mass (YSCWM), Changjiang Diluted Water (CDW), Taiwan Warm Current (TaWC), and Subei Coastal Water (SCW) influenced the horizontal and vertical distributions of the OPs in the study area.

Long-term variation in the salinity of the Southern Yellow Sea Cold Water Mass, 1976\u20132006

This paper discusses the long-term variation in the salinity of the Southern Yellow Sea Cold Water Mass (SYSCWM) and examines factors influencing the SYSCWM based on hydrographic datasets of the China National Standard section and the Korea Oceanographic Data Center. The mean salinity at the center of the SYSCWM showed a decreasing long-term trend. In empirical orthogonal function (EOF) analysis, the second EOF mode showed a similar long-term trend. The mean salinity of the center of the SYSCWM was related to the intrusion of saline water from the Yellow Sea Warm Current (YSWC), the salinity of the source area of the YSWC, the evaporation minus precipitation (E–P) flux, and discharge from the Changjiang River. The decreasing salinity trend to the southwest of Cheju Island produced a freshening trend in the YSWC, resulting in a reduction in the salinity of the SYSCWM. The freshening trends of the water from the northwest Pacific and the South China Sea were seen as the reason for the decreasing salinity trend from the intrusion of water into the Yellow Sea (YS). The freshwater flux influenced the surface salinity and was brought to deep layers by strong mixing in winter. The mean E–P flux signal and Changjiang River discharge signal lagged the first principal component of the SYSCWM by approximately 5 months.

Long-term temperature variation of the Southern Yellow Sea Cold Water Mass from 1976 to 2006

This paper discusses the long-term temperature variation of the Southern Yellow Sea Cold Water Mass (SYSCWM) and examines those factors that influence the SYSCWM, based on hydrographic datasets of the China National Standard Section and the Korea Oceanographic Data Center. Surface air temperature, meridional wind speed, and sea surface temperature data are used to describe the seasonal changes. Mean temperature of the two centers of the SYSCWM had different long-term trends. The temperature of the center in the west of the SYSCWM was rising whereas that of the center in the east was falling. Mean temperature of the western center was related to warm water intrusion of the Yellow Sea Warm Current, the winter meridional wind, and the winter air temperature. Summer process played a primary role in the cooling trend of temperature in the eastern center. A decreasing trend of salinity in the eastern half of the SYSCWM showed that warm water intrusion from the south might weaken, as could the SYSCWM circulation. Weakened circulation provided less horizontal heat input to the eastern half of the SYSCWM. Less lateral heat input may have led to the decreasing trend in temperature of the eastern center of the SYSCWM. Further, warmer sea surface temperatures and less heat input in the deep layers intensified the thermocline of the eastern SYSCWM. A stronger thermocline had less heat flux input from upper layers to this half of the SYSCWM. Stronger thermocline and weakened heat input can be seen as two main causes of the cooling temperature trend of the eastern center of the SYSCWM.

Effects of Elevated Temperature and Food Supply on the Termination of Over-Summering and Subsequent Development of the Calanoid Copepod Calanus sinicus: Morphology, Physiology and Gene Expression.

The copepod Calanus sinicus Brodsky dominates the zooplankton in the Yellow Sea, China, and undergoes over-summering within the Yellow Sea Cold Water Mass (YSCWM). Termination of over-summering and subsequent development are regarded as key processes in population recruitment, and are probably linked to environmental variations in the YSCWM. In this study, we examined the effects of temperature (9 and 18°C) and food conditions (0.1 μg C mL-1 and unfed) on metabolic rates, morphological characteristics, and relative gene expressions of six genes involved in molting, gonad development, lipid catabolism, and stress tolerance processes of C. sinicus during termination of over-summering and subsequent development. Both elevated temperature and external food supply rapidly ended over-summering of C. sinicus, accompanied by up-regulation of the ecdysteroid receptor (EcR) gene expression and increased metabolic rates. These environmental conditions resulted in irreversible termination of over-summering and ensure the success of molting. During subsequent development, the lipid reserve in oil sacs could permit only early gonad development. The food supply might be a trigger to activate the final maturity of gonad by up-regulating expression of the vitellogenin receptor (VgR) gene. Thus, food played an indispensable role in population recruitment after termination of over-summering, whereas the elevated temperature accelerated these physiological processes. This study revealed the first dynamic profiles of physiological processes involved in over-summering termination and the subsequent development of C. sinicus using morphological, physiological and molecular methods simultaneously, confirmed the quiescent state of over-summering C5 copepodites, detected the effects of environmental changes on over-summering termination and subsequent development, and provided a foundation for future investigations of the mechanisms involved in over-summering in YSCWM.

Seasonal evolution of the Yellow Sea Cold Water Mass and its interactions with ambient hydrodynamic system

AbstractThe Yellow Sea Cold Water Mass (YSCWM) is an important component of the hydrodynamic system in the South Yellow Sea (SYS). However, its intricate interactions with the ambient flows over long time scales are not fully understood. This paper presents the analysis of the data set obtained from a seabed‐mounted Acoustic Doppler Current Profiler (ADCP) deployed for nearly 1 year in the western SYS. It allowed us to study the evolution of YSCWM, including the seasonal changes of tidal currents, near‐inertial oscillations (NIOs), and the wind‐driven currents due to typhoons and winter storms. Strong NIOs were found near the bottom of mixed layer and in the pycnocline with nearly opposite current directions, with maximum velocity of nearly 20 cm·s−1 in summer. The YSCWM can also inhibit the direct downward energy transport in the water column due to typhoons. Conversely, the hydrodynamic system also feeds back to influence the change of YSCWM. A large current shear (S) of 20 cm·s−1·m−1 is generated near the top of pycnocline. Generally, the intensity and depth of the pycnocline determine S's magnitude and vertical location, respectively. Based on the monthly averaged density profile data, the Richardson number and wavelet analysis, the NIOs are considered to be capable of inducing predominant shear instability around the pycnocline. However, the NIOs are not strong enough to influence the lower YSCWM. In addition, in autumn, each fortnightly spring tide corresponds with a bottom temperature increase of nearly 2°C, indicating that tidal currents are the leading hydrodynamic driving force to decline the YSCWM.

Distribution, partitioning and sources of dissolved and particulate nitrogen and phosphorus in the north Yellow Sea

Little is known about characteristics of dissolved and particulate N and P forms in the north Yellow Sea (NYS). In this study, water and particulate samples were collected from the NYS to understand the biogeochemical behaviors, interactions and sources of dissolved and particulate N and P. Among the various N and P forms, dissolved organic N (DON) and P (DOP) were the predominant forms, accounting for 64% and 65% of total N (TN) and P (TP). Dissolved and particulate inorganic N and P displayed a decreasing trend from northwest region to the middle region, which was mainly influenced by riverine input along the Liaodong Peninsula coast. However, dissolved and particulate organic N and P showed higher values at northwest region and southern region, which were dominantly affected by biological activities and the Bohai Sea input. Vertical distribution patterns of dissolved and particulate N and P generally displayed the higher values at surface and bottom waters, which was the combined result of the influences by thermocline, the Yellow Sea Cold Water Mass (YC), biological activities and sediment resuspension. There were significant correlations between dissolved and particulate pools and between inorganic and organic forms, indicating their transformations through phytoplankton and bacteria activities and adsorption/desorption processes. Budgets suggested that net sink of dissolved inorganic N and P in the NYS could be mainly removed from water column. Particulate N and P were mainly from phytoplankton productivity, contributing to 84% and 74% of total particulate N (TPN) and P (TPP) influx.

Effect of diurnal temperature difference on lipid accumulation and development in Calanus sinicus (Copepoda: Calanoida)

Calanus sinicus, the dominant copepod in the Yellow Sea, develops a large oil sac in late spring to prepare for over-summering in the Yellow Sea Cold Water Mass (YSCWM). The lipid accumulation mechanism for the initiation of over-summering is unknown. Here, we cultured C3 copepodites at four constant temperatures (10, 13, 16, and 19°C) and at three temperature regimes that mimicked the temperature variations experienced during diurnal vertical migration (10–13°C, 10–16°C, and 10–19°C) for 18 days to explore the effects of temperature differences on copepod development and lipid accumulation. C. sinicus stored more lipid at low than at high temperatures. A diurnal temperature difference (10–16°C and 10–19°C) promoted greater lipid accumulation (1.9–2.1 times) than a constant temperature of either 16°C or 19°C, by reducing the energy cost at colder temperatures and lengthening copepodite development. Thereafter, the lipid reserve supported gonad development after final molting. Only one male developed in these experiments. This highly female-skewed sex ratio may have been the result of the monotonous microalgae diet fed to the copepodites. This study provides the first evidence that diurnal temperature differences may promote lipid accumulation in C. sinicus, and provides a foundation for future investigations into the mechanisms involved in over-summering in the YSCWM.

In situ study on photosynthetic characteristics of phytoplankton in the Yellow Sea and East China Sea in summer 2013

In situ studies on photosynthetic characteristics of phytoplankton were important for the analysis of changes in community structure and for the prediction and control of algal blooms, but such studies of phytoplankton in offshore China were few. In this study, the detailed distribution of photosynthetic characteristics of phytoplankton in the summer of 2013 in the Yellow Sea and East China Sea was measured using Phyto-PAM (Pulse Amplitude Modulation). The phytoplankton community structure and the environmental parameters were also investigated to estimate the relationship between the distribution of the photochemical competence of phytoplankton and ecological factors. The total average Fv/Fm (the potential maximum quantum yield) value of phytoplankton in the Yellow Sea and East China Sea in summer 2013 was less than 0.5, reflecting that the photosynthetic activity of phytoplankton was relatively low. Fv/Fm of phytoplankton in summer was significantly positively associated with nitrate content (NO2−), which reflects relationship between metabolism and photosynthesis of phytoplankton: accompanied by NO2− metabolism, photosynthesis and photosynthetic capacity may be enhanced simultaneously, so the Fv/Fm value would increase with the NO2− released by phytoplankton. Through the in situ study on photosynthetic characteristics of phytoplankton in the Yellow Sea and East China Sea, we come to the conclusion that photosynthetic characteristics and activity of phytoplankton are influenced by its biological characteristics and surrounding ecological factors, such as irradiance, nutrients and phytoplankton community. Meanwhile, the thermally stratified structure and the movement of water masses, such as the Yangtze River diluted water, the Yellow Sea cold water mass and other different water system, also have an important impact on phytoplankton photosynthetic activity and characteristics. Greater understanding of the detailed photosynthetic characteristics of phytoplankton coupled with their community structure and the environmental parameters will encourage new researches and developments in studies of marine ecosystems.

Response of free-living marine nematodes to the southern Yellow Sea Cold Water Mass

The Yellow Sea Cold Water Mass is a remarkable seasonal hydrographic event in the bottom water of the Yellow Sea. In order to reveal the response of free-living marine nematodes to this event, community structure and biodiversity indices of nematodes were studied in June and November 2013. The dominant species were Dorylaimopsis rabalaisi, Spilophorella sp., Daptonema sp., Sabatieria sp. and Parasphaerolaimus sp. In terms of trophic structure, epigrowth feeders were the most dominant group. Correlation analysis showed that Shannon–Wiener diversity index had significantly negative correlation with sediment silt–clay percentage, organic matter content and water content. Results of BIOENV indicated that sediment phaeophorbide content, water content, bottom water salinity and temperature were the most important factors related to nematode community. In conclusion, community structure and biodiversity indices of nematodes were consistent in the two sampling seasons.

Differences in the physiological processes ofCalanus sinicusinside and outside the Yellow Sea Cold Water Mass

Calanus sinicus, the key zooplankton species in the Yellow Sea, China, over-summers in the Yellow Sea Cold Water Mass (YSCWM). Here, we compared the metabolic rates, morphological characteristics and relative expressions of seven genes associated with molting, gonad development, lipid catabolism and stress tolerance of C. sinicus captured both inside and outside the YSCWM in summer. With a large oil sac, low metabolic rate and suppressed molting development, the C5-stage copepods inside the YSCWM were probably quiescent. The gene expressions revealed differences in physiology between quiescent and active copepods in the two regions. When quiescent, the gene associated with molting [ecdysteroid receptor (EcR)] was down-regulated, while genes related to lipid catabolism (Hydroxyacyl CoA dehydrogenase) and stress tolerance (ferritin) were up-regulated. C5s at the margin of the YSCWM up-regulated EcR expression and this could be in response to the elevated Chl a concentration, suggesting that elevated food condition may serve as a trigger that terminated the over-summering of C. sinicus in the Yellow Sea. In conclusion, this study revealed the physiological processes of quiescent and active C. sinicus via morphological, physiological and molecular methods simultaneously, providing a foundation for future investigations of the mechanisms involved in over-summering in YSCWM.

Seasonally chemical hydrology and ecological responses in frontal zone of the central southern Yellow Sea

Based on annual-cycle survey data collected in 2006–2007 in the southern Yellow Sea (SYS) and analyses on the seasonally chemical hydrologic characteristics of the boundary front of the Yellow Sea Cold Water Mass (YSCWM) and Yellow Sea Warm Current (YSWC), the seasonal variations in upwelling along the frontal zone were determined, and the ecological impacts of the front were investigated. During the generation and dissipation of the YSCWM, the implied upwelling along its western front exhibited seasonal variation. The upwelling first shifted westward from the deep-water region to its westernmost point in summer then returned eastward. The intensity of the upwelling gradually increased from spring to summer and decreased in autumn. In spring, the existence of cold water west of the YSWC was not conducive to the reproduction of phytoplankton. Additionally, the front to the east of this cold water mass also made the western boundary of the phytoplankton bloom region in the central SYS more obvious, forming a prominent chlorophyll a (Chl-a) front. During the entire stratified season (summer and autumn), the upwelling in the frontal zone of the YSCWM played an essential role in maintaining the relatively high concentrations of Chl-a. In winter, the front that formed at the intersection of the YSWC and coastal cold water was also favorable for the formation of the high-Chl-a region. The distribution of anchovy biomass was closely related to the seasonal variations in the position of the frontal zone. In winter and spring, the tongue-shaped warm water and front associated with the intrusion of the YSWC into the SYS had a significant impact on anchovy. During the stratified season in summer and autumn, the development of a front near the boundary of the YSCWM was an important physical driving mechanism for the dense distribution of anchovy. This work enhanced the study of the seasonal relationships between the physical, chemical and biological processes in the frontal zone of the central SYS and deepened our understanding of the ecological significance of this front.