Changjiang River Diluted Water Research Articles

Seasonal dynamics of dissolved organic matter bioavailability coupling with water mass circulation in the South Yellow Sea

As a typical shelf-marginal sea, the South Yellow Sea (SYS) is significantly influenced by various factors such as land-based inputs and water mass movements, leading the complex biogeochemical processes of dissolved organic matter (DOM) to become highly dynamic. However, the bioavailability of dissolved organic matter (DOM) coupled with water mass circulation has not been accurately assessed, despite being crucial for understanding the source-sink pattern of organic carbon in marginal sea. In this study, four cruises were conducted in the SYS to analyze the spatial and temporal distribution characteristics of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and total dissolved amino acids (TDAA). Combined with the bioassay experiments, TDAA carbon normalized yield [TDAA (%DOC)] and TDAA degradation index (DIAA) were used as indicators to explore the bioavailability of DOM across different water masses. Results show that the DOC of the SYS exhibits higher average value in late autumn and early winter, and lower value in spring and summer due to the seasonal alternation of water mass and biological activities. The collective results indicate that DOM bioavailability is higher in the Changjiang River diluted water (CDW) and lower in the Yellow Sea warm current (YSWC) and the Yellow Sea cold water mass (YSCWM). Approximately 20 % of DON can be degraded in the YSCWM during autumn. Notably, although the YSCWM constitutes merely constitutes 10 % of the SYS volume, it stores 18.1 % dissolved inorganic nitrogen (DIN) and 23.9 % PO43− of total nutrients, indicating that the YSCWM is a significant nutrient reservoir within the SYS.

Functional diversity and groups of crustacean zooplankton in the southern Yellow Sea

Functional trait and functional diversity indices reflect the variations of community structure and indicate their responses to environmental stress. In this study, based on four functional traits, including body length, feeding type, trophic group, and reproductive mode, the functional diversity and functional groups of crustacean zooplankton from the southern Yellow Sea (SYS) were explored over four seasons of 2019. A total of 53 species were identified and categorized into six functional groups. Copepods with ambush-feeding and omnivore–carnivore diet (Group Ⅱ) were dominant in spring (55.6%), while omnivore–herbivore copepods (Group Ⅰ) were the most abundant group in the other three seasons (>54.7%). The comparable distributions of functional diversity indices in different seasons were most likely due to hydrological variations. The Changjiang River Diluted Water promoted the coexistence of multiple traits carried by different functional groups (high functional richness (FRic) and functional dispersion (FDis)), but the stability of ecosystem function in this water mass was vulnerable to the loss of individual species (low functional evenness (FEve)) in summer. In winter, as a result of the Yellow Sea Warm Current, warm-water taxa increased FRic and FEve, suggesting that the community could buffer environmental fluctuations more effectively. Moreover, distribution of FRic values indicates the possible range and influence intensity of the Yellow Sea Cold Water Mass.

Interannual Variation in Phytoplankton Community Driven by Environmental Factors in the Northern East China Sea

The relative importance of interannual environmental changes in shaping phytoplankton community structure remains unclear in the East China Sea (ECS), which is the largest marginal sea in the western North Pacific Ocean. Based on high-performance liquid chromatography (HPLC) analysis of a variety of photosynthetic pigments, we investigated spatiotemporal variations in the phytoplankton community in the northern East China Sea (NECS) from 2018 to 2020 to understand biomass and compositional responses to environmental conditions in a complex current system. Correlation heatmaps and generalized additive models (GAMs) were used to explore the variations in major phytoplankton groups and their relationships to temperature, salinity, depth, and nutrients. The results indicated that the phytoplankton community structure was significantly different between 2018/2019 and 2020. Under the conditions of high dissolved inorganic nitrogen (DIN) and low dissolved inorganic phosphate (DIP) concentrations in winter 2020, cryptophytes replaced diatoms as the dominant species and were observed for the first time in this study area. Dinoflagellates predominated throughout the euphotic zone in spring 2020, and statistical analysis showed that dinoflagellates tended to prevail at low DIP and high DIN conditions compared with diatoms. Summer was the only season with strong water stratification that reoccurred every year, and diatoms were mainly concentrated at nutrient-enriched 1% light depths in the summers of 2018/2019. However, a large amount of Changjiang River Diluted Water (CDW) from the surface and western part of the study area affected all sampling sites eastward to 126°E in 2020, allowing diatoms to be dominant at 100% and 30% light depths in the westernmost stations in the NECS. In autumn 2020, diatoms decreased significantly because of DIP-limited environmental conditions and were replaced by small phytoplankton. Under expected warming ocean scenarios with human-induced nutrient inputs, small phytoplankton will become dominant, while spring dinoflagellate blooms will occur more frequently in the NECS. International monitoring programs for marine ecosystems are currently needed in the ECS.

High-Resolution Vertical Observations of Phytoplankton Groups Derived From an in-situ Fluorometer in the East China Sea and Tsushima Strait

Vertical distribution of phytoplankton composition in the East China Sea (ECS) and Tsushima Strait (TS) was highly variable in the region where the Changjiang River diluted water (CDW), Kuroshio water (KW), and Tsushima water (TW) intersected. An in-situ multiple excitation fluorometer was used to obtain the high-resolution phytoplankton groups data from every meter of the water column. Sharp differences were noted in the distribution of phytoplankton groups in the CDW, KW, and TW. In the CDW, brown algae were generally present ~60% of all depths with exception of subsurface chlorophyll-a maximum (SCM), whereas cyanobacteria (>40%) and green algae plus cryptophytes (>40%) were found above and below the SCM, respectively. In TW, where chlorophyll a (CHL) was lower than in the CDW, brown algae predominated the water column (>60%) and SCM (>80%), except the surface layer where cyanobacteria dominated. In KW, a high fraction of cyanobacteria (>40%) extended up to 40 m, while brown and green algae dominated (>60%) the deeper waters below 40 m at western and eastern stations, respectively. These results can be further related to water property and nutrient concentration of the water masses in each region. This new data show that the in-situ multiple excitation fluorometer can be a powerful tool to estimate high-resolution vertical profiles of phytoplankton groups on a large scale in marine environments.

The distribution of zooplankton and the influencing environmental factors in the South Yellow Sea in the summer

Zooplankton were sampled using large plankton net (LPN) (mesh size: 505 μm) and medium plankton net (MPN) (160 μm) in the summer of 2008. 96 and 87 zooplankton species were identified in LPN and MPN, respectively; Calanus sinicus, Aidanosagitta crassa, and Oithona plumifera were common dominant species between the two nets; other dominant species in MPN, such as O. similis, were rare in LPN. The average abundance was 228.99 ± 130.06 ind./m3 and 2835.05 ± 3356.37 ind./m3 in LPN and MPN, respectively. The average biomass was 194.56 ± 130.06 mg/m3 and 216.45 ± 131.89 mg/m3. The diversity indices were also higher in MPN. Correlation analysis indicated that the abundance was negatively correlated with sea bottom temperature, which meant that zooplankton species tended to be clustered in the Yellow Sea Cold Water Mass. The diversity indices were negatively correlated with salinity, which indicated that diversity was higher in sea areas that were affected by Changjiang River Diluted Water.

Temporal and spatial variations of alkaline phosphatase activity related to phosphorus status of phytoplankton in the East China Sea

Phosphorus (P) is a potential limiting nutrient for primary production in the East China Sea (ECS). Four cruises over four seasons were conducted during 2009–2011 to evaluate the dynamics of alkaline phosphatase (AP) activity (APA) and the P status of phytoplankton in the ECS. Sampling for bulk and single-cell APA assays was performed across the ECS, which included the Changjiang River diluted water (CDW), the mid-shelf surface water (MSW), and the Kuroshio surface water (KSW) masses. The results showed that the distribution patterns of APA varied between water masses and with season. Higher APA was normally observed in the CDW, which was influenced by the Changjiang River plume. In the CDW, phytoplankton were P-stressed during the late spring with an average bulk APA of 20.4 ± 19.5 nmol h−1 L−1, which strengthened during the late summer (average APA maximizing at 106.2 ± 133.3 nmol h−1 L−1) and remained severe during the late autumn (average APA of 48.7 ± 34.1 nmol h−1 L−1) until reducing during the winter (average APA of 9.1 ± 13.6 nmol h−1 L−1). The variation patterns of APA and the P status of phytoplankton in the MSW were similar but with slight variations. In the KSW, a certain amount of APA occurred during the late spring and late autumn (averages of 18.7 ± 19.8 and 23.1 ± 18.7 nmol h−1 L−1, respectively). Single-cell APA coincided with the bulk APA to exhibit the temporal and spatial P-stress status of the dominant micro-phytoplankton. Phytoplankton were the main AP producers in the CDW, especially during the late summer, while pico-plankton contributed most to AP in the MSW and KSW. Our results suggest that phytoplankton suffer P-stress in most seasons, and emphasize the importance of AP in the recycling of organic P to support primary production in the P-deficient ECS.

Interannual changes in summer phytoplankton community composition in relation to water mass variability in the East China Sea

The surface distribution of the phytoplankton community was investigated in July 2009, 2010, 2011, and 2013 on the mid-shelf of the East China Sea (ECS), which is under the influence of Changjiang River Diluted Water (CDW) and the Kuroshio Current. This study, based on a CHEMTAX analysis of phytoplankton pigments, revealed a predominance of cyanobacteria and prochlorophytes in the eastern ECS, which is perennially under the influence of oligotrophic Kuroshio Surface Water. Towards the west, on the mid-shelf of the ECS, the composition of the phytoplankton community varied from year to year. Diatoms dominated in 2009 and 2013, when dissolved inorganic phosphate (DIP) concentrations were higher than during 2010 and 2011. During the latter two years, characterized as high-nitrate years, a mixed population of cyanobacteria, chlorophytes, and other groups was observed. Cluster analysis based on the phytoplankton community composition together with a PCA of shipboard hydrographic and nutrient data for all four years helped to confirm that the summer phytoplankton community structure of the ECS was regulated by the mixing of water masses and the variability of nutrient ratios within the CDW as it moved offshore. Our results show that elevated DIP concentrations in the CDW favor the growth of diatoms and dinoflagellates. The primary pathway for DIP inputs appears to be the upwelling of high-phosphate subsurface waters along the coast of China.

Distributions and Relationships of CO2, O2, and Dimethylsulfide in the Changjiang (Yangtze) Estuary and Its Adjacent Waters in Summer

The distributions and relationships of O2, CO2, and dimethylsulfide (DMS) in the Changjiang (Yangtze) Estuary and its adjacent waters were investigated in June 2014. In surface water, mean O2 saturation level, partial pressure of CO2 (pCO2), and DMS concentrations (and ranges) were 110% (89%–167%), 374 μatm (91–640 μatm), and 8.53 nmol L−1 (1.10–27.50 nmol L−1), respectively. The sea-to-air fluxes (and ranges) of DMS and CO2 were 8.24 μmol m−2 d−1 (0.26–62.77 μmol m−2 d−1), and −4.7 mmol m−2 d−1 (−110.8-31.7 mmol m−2 d−1), respectively. Dissolved O2 was oversaturated, DMS concentrations were relatively high, and this region served as a sink of atmospheric CO2. The pCO2 was significantly and negatively correlated with the O2 saturation level, while the DMS concentration showed different positive relationships with the O2 saturation level in different water masses. In vertical profiles, a hypoxic zone existed below 20 m at a longitude of 123°E. The stratification of temperature and salinity caused by the Taiwan Warm Current suppressed seawater exchange between upper and lower layers, resulting in the formation of a hypoxic zone. Oxidative decomposition of organic detritus carried by the Changjiang River Diluted Water (CRDW) consumed abundant O2 and produced additional CO2. The DMS concentrations decreased because of low phytoplankton biomass in the hypoxic zone. Strong correlations appeared between the O2 saturation level, pCO2 and DMS concentrations in vertical profiles. Our results strongly suggested that CRDW played an important role in the distributions and relationships of O2, CO2, and DMS.

Characteristics of the $$\delta ^{15} N_{NO_3 } $$ δ 15 N N O 3 distribution and its drivers in the Changjiang River estuary and adjacent waters

In this study, we conducted investigations in the Changjiang (Yangtze) River estuary and adjacent waters (CREAW) in June and November of 2014. We collected water samples from different depths to analyze the nitrogen isotopic compositions of nitrate, nutrient concentrations (including inorganic N, P, and Si), and other physical and biological parameters, along with the vertical distribution and seasonal variations of these parameters. The compositions of nitrogen isotope in nitrate were measured with the denitrifier method. Results show that the Changjiang River diluted water (CDW) was the main factor affecting the shallow waters (above 10 m) of the CREAW, and CDW tended to influence the northern areas in June and the southern areas in November. \(\delta ^{15} N_{NO_3 } \) values in CDW ranged from 3.21‰–3.55‰. In contrast, the deep waters (below 30 m) were affected by the subsurface water of the Kuroshio Current, which intruded into the waters near 31°N in June. The \(\delta ^{15} N_{NO_3 } \) values of these waters were 6.03‰–7.6‰, slightly higher than the values of the Kuroshio Current. Nitrate assimilation by phytoplankton in the shallow waters of the study area varied seasonally. Because of the favorable temperature and nutrient conditions in June, abundant phytoplankton growth resulted in harmful algae blooms (HABs). Therefore, nitrate assimilation was strong in June and weak in November. The \(\delta ^{15} N_{NO_3 } \) fractionations caused by assimilation of phytoplankton were 4.57‰ and 4.41‰ in the shallow waters in June and November, respectively. These results are consistent with previous laboratory cultures and in situ investigations. Nitrification processes were observed in some deep waters of the study area, and they were more apparent in November than in June. The fractionation values of nitrification ranged from 24‰–25‰, which agrees with results for Nitrosospira tenuis reported by previous studies.

Origins of suspended particulate matter based on sterol distribution in low salinity water mass observed in the offshore East China Sea

The molecular composition and distribution of sterols were investigated in the East China Sea to identify the origins of suspended particulate matter (SPM) in offshore waters influenced by Changjiang River Diluted Water (CRDW). Total sterol concentrations ranged from 3200 to 31,900pgL−1 and 663 to 5690pgL−1 in the particulate and dissolved phases, respectively. Marine sterols dominated representing 71% and 66% in the particulate and dissolved phases, respectively. Typical sewage markers, such as coprostanol, were usually absent at ~250km offshore. However, sterols from allochthonous terrestrial plants were still detected at these sites. A negative relationship was observed between salinity and concentrations of terrestrial sterols in SPM, suggesting that significant amounts of terrestrial particulate matter traveled long distance offshore in the East China Sea, and the Changjiang River Diluted Water (CRDW) was an effective carrier of land-derived particulate organic matter to the offshore East China Sea.

Grain size composition and transport of sedimentary organic carbon in the Changjiang River (Yangtze River) Estuary and Hangzhou Bay and their adjacent waters

Surface sediments from the Changjiang River (Yangtze River) Estuary, Hangzhou Bay, and their adjacent waters were analyzed for their grain size distribution, organic carbon (OC) concentration, and stable carbon isotope composition (δ 13C). Based on this analysis, about 36 surface sediment samples were selected from various environments and separated into sand (>0.250 mm, 0.125–0.250 mm, 0.063–0.125 mm) and silt (0.025–0.063 mm) fractions by wet-sieving fractionation methods, and further into silt- (0.004–0.025 mm) and clay-sized (<0.004 mm) fractions by centrifugal fractionation. Sediments of six grain size categories were analyzed for their OC and δ 13C contents to explore the grain size composition and transport paths of sedimentary OC in the study area. From fine to coarse fractions, the OC content was 1.18%, 0.51%, 0.46%, 0.42%, 0.99%, and 0.48%, respectively, while the δ 13C was–21.64‰,–22.03‰,–22.52‰,–22.46‰,–22.36‰, and–22.28‰, respectively. In each size category, the OC contribution was 42.96%, 26.06%, 9.82%, 5.75%, 7.09%, and 8.33%, respectively. The OC content in clay and fine silt fractions (<0.025 mm) was about 69.02%. High OC concentrations were mainly found in offshore modern sediments in the northeast of the Changjiang River Estuary, in modern sediments in the lower estuary of the Changjiang River and Hangzhou Bay, and in Cyclonic Eddy modern sediments to the southwest of the Cheju Island. Integrating the distribution of terrestrial OC content of each grain size category with the δ 13C of the bulk sediment indicated that the terrestrial organic material in the Changjiang River Estuary was transported seaward and dispersed to the Cyclonic Eddy modern sediments to the southwest of the Cheju Island via two pathways: one was a result of the Changjiang River Diluted Water (CDW) northeastward extending branch driven by the North Jiangsu Coastal Current and the Yellow Sea Coastal Current, while the other one was the result of the CDW southward extending branch driven by the Taiwan Warm Current.

A numerical study of the seasonal variations of nutrients in the Changjiang River estuary and its adjacent sea area

A lower trophic level nutrient–phytoplankton–zooplankton model is coupled to the Regional Ocean Modeling System (ROMS) in the Changjiang River estuary and its adjacent sea area with the aim of simulating the seasonal variations of three main nutrients (nitrate, phosphate, and silicate). The coupled model successfully reproduces the long-term variations of nutrients during 2004–2007, and the simulated nutrient distributions are also well consistent with that obtained by cruises. Based on the climatological simulation, the seasonal variations of nutrients in the study area are analyzed. In winter, the spatial distributions of nitrate and silicate show that they are mainly from terrestrial input. As for phosphate, the concentration is relatively high, but it is difficult to distinguish the main source merely on its spatial distribution. In summer, as the Changjiang River diluted water (CDW) flows eastward or northeastward, a great amount of nutrients are transported to the sea area on the east of 123°E via a thin layer in the surface. Despite all this, the available phosphate is still insufficient to meet the consumption by phytoplankton, which shows a strong phosphorus limitation in the surface. By contrast, the phosphate concentrations in lower layers are much higher, and they are mainly from two physical sources: one is the intrusion of the Taiwan Warm Current (TWC) from the south, and the other is a shoreward transport from the east. The spatial variations of N/P and N/Si ratios show that phosphate is the main limiting nutrient in the Changjiang River estuary and its adjacent sea area, although slight nitrogen limitation appears in the sea areas on the east of 122.5°E in winter. In addition, the seasonal variations of surface phytoplankton and zooplankton are briefly discussed.

Spatio-temporal distribution of dissolved sulfide in China marginal seas

With sulfide increasingly recognized as an important parameter to assess the oxidation-reduction level in aqueous environment, research on its geochemical behavior is becoming important. Water samples collected in Bohai Sea (1–19 August, 2010), Yellow Sea (20–30 November, 2010) and East China Sea (3–17 June, 2010 and 1–10 November, 2010) were used to determine the occurrence and distribution of dissolved sulfide by methylene blue spectrophotometric method. Results show that: (1) horizontally, concentration of dissolved sulfide significantly varied from the coastal region to the open sea and profoundly influenced by physical processes. High values occurred in the river-sea boundary zone “marginal filter” due to rich riverine input, frequent upwelling and active exchange in shelf edge. Terrestrial input from adjacent rivers and the current cycling contributed to the high sulfide appeared in western Bohai Sea, eastern Shandong Peninsula, and northeast of Changjiang (Yangtze) River estuary. Especially, relative higher sulfide values occurred in Yellow Sea, which is consistent with the variation of salinity largely due to the hydrodynamic feature; (2) vertically, measurement of dissolved sulfide in bottom water was higher and more variable than that in surface water caused by the wind-induced resuspension and dissimilatory sulfate reduction. Moreover, nutrient-type profile clearly identified that oxidation plays a major role in the biogeochemistry cycle of sulfide in water; (3) seasonally, investigation for East China Sea in June and November reflected seasonal variation of Changjiang River Diluted Water, Kuroshio Current, and Taiwan Warm Current. Concentration in June was much higher than that sampled in November at most stations. Mean concentration of dissolved sulfide varied seasonally from 2.26 μg/L (June) to 1.16 μg/L (November) in surface and 3.00 μg/L (June) to 1.56 μg/L (November) in bottom. Progress in the field is slow and more effort is needed to ensure the accuracy and reliability of determination and estimate the natural or anthropogenic contribution of dissolved sulfide in ecosystems.

Influence of the Changjiang River Flood on Synechococcus Ecology in the Surface Waters of the East China Sea

Synechococcus spp. have been suggested as the primary component of picophytoplankton in the East China Sea (ECS). However, the influences of sudden environmental changes on Synechococcus assemblage composition have not yet been investigated. In the summer of 2010, a disastrous flood occurred in the Changjiang River basin. To improve our understanding of how this flood affected the Synechococcus ecology on the ECS surface, their assemblages and distributions have been described using two-laser flow cytometry and phylogenetic analysis of the phycocyanin operon. During the nonflooding summer of 2009, phycoerythrin-rich (PE-rich) Synechococcus thrived near the outer boundary of the Changjiang River diluted water (CDW) coverage, while phycocyanin-rich (PC-rich) Synechococcus predominated inside the turbid CDW with a transparency of <80%. During the 2010 summer, flooding expanded the CDW coverage area to over half of the ECS. PE-rich cells showed a homogeneous distribution and a decline in abundance, while the spatial pattern of the PC-rich Synechococcus resembled the pattern from 2009. Based on the phycocyanin operon phylogeny, the Synechococcus in the ECS were categorized into five groups, ECS-1 to ECS-4 and ECS-PE, comprising a total of 19 operational taxonomic units. In the summer of 2009, ECS-2 dominated in the coast, and the ECS-3 and ECS-PE clades prevailed in the offshore waters. However, during the summer of 2010, ECS-4 and ECS-PE became the dominant strains. The injection of abundant anthropogenic pollutants and the enhancement of transparency within the CDW expansion area appear to be the factors needed to transiently alter the ecology of Synechococcus after flooding.

Nitrate δ 15N and δ 18O evidence for active biological transformation in the Changjiang Estuary and the adjacent East China Sea

The Changjiang Estuary has been considered as one of the most polluted estuaries in the world due to high nitrate (NO 3 − ) input, especially in spring and summer. In this study, δ 15N and δ 18O of NO 3 − , along with other chemical parameters in this area, were measured in spring to evaluate NO 3 − biogeochemical processes. A simple two end-members mixing model was used to examine the relative contribution of the Changjiang River Diluted Water and marine water to NO 3 − sources in the Changjiang Estuary and the adjacent East China Sea. The isotopic signals show that NO 3 − behaved relatively and conservatively in Transect F and Transect P where assimilation was weak possibly due to vertical mixing, while active assimilation and weak nitrification occurred in Transect D. Spatial difference in assimilation was indicated by the ∼1:1 enrichment of δ 15N and δ 18O in the three transects, while spatial difference in nitrification was reflected by deviations of δ 15N and δ 18O from assimilation line. Our results suggest that the input of the Changjiang River Diluted Water promoted NO 3 − assimilation possibly by stratifying the water column which favored the phytoplankton growth.

An Observational Analysis of the Relationship Between Wind and the Expansion of the Changjiang River Diluted Water during Summer

This paper presents an analysis of the impact of wind on the transport of the Changjiang River Diluted Water (CRDW) in August by using the salinity data collected on two zonal sections near Cheju-d...

Coastal upwelling in the East China Sea in winter

The dynamic mechanisms of the upwelling off the East China Sea (ECS) coast in wintertime are studied. First, the upwelling signals off the ECS coast are identified by the observed temperature, salinity, nutrients, and dissolved oxygen data obtained during the cruises in January 1999. The MASNUM wave‐tide‐circulation coupled model is then employed to simulate the hydrography of the ECS. Comparisons between the simulations and observations show that the model performance is satisfactory. On the basis of successful simulation, four numerical experiments are conducted to investigate the upwelling mechanisms. The results suggest that the density (or salinity) front, which separates the inshore Low Salinity Coastal Water and the offshore Taiwan Warm Current (TWC), is the primary inducement for the upwelling. Owing to strong density gradient, the baroclinic pressure gradient force (PGF) is quite large near the frontal zone, and this PGF elicits an upwelling branch along the topography slope. Wind, TWC, and tide affect the density front in extension and intensity, thus exerting subsidiary influences on the upwelling. According to Ekman's theory, the northerly monsoon is downwelling favorable. However, the net effects of wind on the upwelling off the ECS coast in winter are positive because it drives the Changjiang River Diluted Water (CDW) flowing southward and forms the density front. Similarly, the resultant effects of TWC on the upwelling are negative for obstructing the pathway of CDW. Tide contributes to the upwelling because tidal mixing facilitates the expansion of CDW.

Seasonal variability of the sea surface salinity in the East China Sea during 1990–2002

The seasonal distribution of the sea surface salinity (SSS) in the East China Sea (ECS) during 1990–2002 is examined based on a historical data set from the World Ocean Database 2001 (WOD2001), the data available from the Japan Oceanographic Data Center (JODC), and the cruise observations conducted by China during 1997–2002. The SSS data set has been mapped into a regular 0.25° by 0.25° grid for the study. It is shown that the largest SSS variation is at the area where the Changjiang River Diluted Water (CRDW) changes its flow directions due to the southerly wind in summer and northerly wind in fall. The variation is further complemented by the Changjiang River discharge that reaches its maximum in summer and minimum in winter. It is also shown that the seasonal variation of the evaporation minus precipitation (E‐P) has a secondary effect on the SSS distribution.

Species composition and spatial distribution of euphausiids of the yellow sea and relationships with environmental factors

We investigated species composition and spatial distribution of the euphausiid community in the Yellow Sea and identified the relationship with environmental factors (temperature, salinity, chlorophyll a, nitrate, phosphate, and silicate) using bimonthly data from June, 1997 to April, 1998. The environment varied during the sampling period. In warm seasons, thermocline was well developed rendering lower temperature and higher salinity and nutrient concentrations in the bottom layer. During cold seasons the water column was well mixed and no such vertical stratification was noted. Horizontal distribution of temperature, however, differed slightly between near-coast and offshore areas because of the shallow depth of the Yellow Sea, and between southern and northern areas because of the intrusion of water masses such as Yellow Sea Warm Current and Changjiang River Diluted Water. Four euphausiid species were identified:Euphausia pacifica, E. sanzoi, Pseudeuphausia sp. andStylocheron affine. E. sanzoi andS. affine were collected, just one juvenile each, from the southern area in June and December, respectively.Pseudeuphausia sp. were collected in the eastern area all the year round except June.E. pacifica occurred at the whole study area and were the predominant species, representing at least 97.6% of the euphausiid abundance. Further, the distribution pattern of the species was varied in regards to developmental stages (adult, furcilia, calyptopis, egg). From spring to fall,E. pacifica adults were abundant in the central area where the Yellow Sea Bottom Cold Water prevailed. Furcilia and calyptopis extended their distribution into nearly all the study area during the same period. From late fall to winter, adults were found at the near-coastal area with similar pattern for furcilia and calyptopis. The distribution pattern ofE. pacifica was consistent regarding temperature, salinity, and three nutrients during the sampling period, whereas chlorophyll a showed a different pattern according to the developmental stages. The nutrients should indirectly affect via chlorophyll a and phytoplankton concentration. With respect to these results, we presented a scenario about how the environmental factors along with the water current affect the distribution ofE. pacifica in the Yellow Sea.

The Current System in the Yellow and East China Seas

During the 1990s, our knowledge and understanding of the current system in the Yellow and East China Seas have grown significantly due primarily to new technologies for measuring surface currents and making high-resolution three-dimensional numerical model calculations. One of the most important new findings in this decade is direct evidence of the northward current west of Kyushu provided by satellite-tracked surface drifters. In the East China Sea shelf region, these recent studies indicate that in winter the Tsushima Warm Current has a single source, the Kuroshio Branch Current in the west of Kyushu, which transports a mixture of Kuroshio Water and Changjiang River Diluted Water northward. In summer the surface Tsushima Warm Current has multiple sources, i.e., the Taiwan Warm Current, the Kuroshio Branch Current to the north of Taiwan, and the Kuroshio Branch Current west of Kyushu. The summer surface circulation pattern in the East China Sea shelf region changes year-to-year corresponding to interannual variations in Changjiang River discharge. Questions concerning the Yellow Sea Warm Current, the Chinese Coastal Current in the Yellow Sea, the current field southwest of Kyushu, and the deep circulation in the Okinawa Trough remain to be addressed in the next decade.