Seasonal variations of several main water masses in the southern Yellow Sea and East China Sea in 2011

The seasonal variations of several main water masses in the southern Yellow Sea (SYS) and East China Sea (ECS) in 2011 were analyzed using the in-situ data collected on four cruises. There was something special in the observations for the Yellow Sea Warm Current (YSWC), the Yellow Sea Cold Water Mass (YSCWM) and the Changjiang Diluted Water (CDW) during that year. The YSWC was confirmed to be a seasonal current and its source was closely associated with the Kuroshio onshore intrusion and the northerly wind. It was also found that the YSCWM in the summer of 2011 occupied a more extensive area in comparison with the climatologically-mean case due to the abnormally powerful wind prevailing in the winter of 2010 and decaying gradually thereafter. Resulting from the reduced Changjiang River discharge, the CDW spreading toward the Cheju Island in the summer of 2011 was weaker than the long-term mean and was confined to flow southward in the other seasons. The other water masses seemed normal without noticeable anomalies in 2011. The Yellow Sea Coastal Current (YSCC) water, driven by the northerly wind, flowed southeastward as a whole except for its northeastward surface layer in summer. The Taiwan Warm Current (TWC) was the strongest in summer and the weakest in winter in its northward movement. The Kuroshio water with an enhanced onshore intrusion in autumn was stable in hydrographic features apart from the seasonal variation of its surface layer.     

Seasonal evolution of circulation and thermal structure in the Yellow Sea

In this paper, the authors used the Princeton Ocean Model (POM) to simulate the seasonal evolutions of circulation and thermal structure in the Yellow Sea. The simulated circulation showed that the Yellow Sea Warm Current (YSWC) was a compensation current of monsoon-driven current, and that in winter, the YSWC became stronger with depth, and could flow across the Bohai Strait in the north. Sensitivity and controlling tests led to the following conclusions, In winter, the direction of the Yellow Sea Coastal Current in the surface layer was controlled partly by tide instead of wind, In summer, a cyclonic horizontal gyre existed in the middle and eastern parts of the Yellow Sea below 10 m. The downwelling in upper layer and upwelling in lower layer were somehow similar to Hu et al. (1991) conceptual model. The calculated thermal structure showed an obvious northward extending YSWC tongue in winter, its position and coverage of the Yellow Sea Cold Water Mass in summer.     

Changes in fish diversity and community structure in the central and southern Yellow Sea from 2003 to 2015

The central and southern Yellow Sea is an important overwintering ground for many fish species in the Bohai Sea and Yellow Sea. For better understanding the status of the fish community after years of heavy exploitation, variations in fish community structure and diversity were analyzed using data from bottom trawls during 2003–2015. Five fish assemblage indices all showed fluctuations without clear trends from 2003 to 2015, yet there were strong positive and significant correlations(P 0.05) among them. The top-five dominant species accounted for a high weight percentage(49.7%–82.1%) in the annual fish catch. Multivariate analysis showed that two year groups could be pooled for the fish community: Group Ⅰ consisted of the years 2006, 2007, 2008 and 2015, while Group Ⅱ consisted of the years 2003, 2004, 2005, 2009, 2010 and 2014; the groups aggregated with 63.71% similarity, indicating a high level of similarity among all years. The multivariate dispersion values were 1.455 and 0.818 for Groups Ⅰ and Ⅱ, respectively, indicating greater variances in fish assemblage structure in Group Ⅰ than that in Group Ⅱ. Similarity of percentage analysis demonstrated that the average similarities for Group Ⅰ and Group Ⅱ were 71.58% and 67.51%, respectively. Size-spectra analysis revealed no consistent trend in the intercept and slope( P 0.05); there were also no significant differences between the slope of the size-spectra and fishing ef fort. The catch per unit ef fort and mean individual weight analyses of the whole fish assemblage both showed a significantly decreasing trend over time. Overall, the results showed that the fish community structure in the central and southern Yellow Sea was relatively stable from 2003 to 2015 and the study could be used as a reference for supporting ecosystem-based fishery management.     

Statistic characteristics of thermal structure in the southern Yellow Sea in summer

Based on the temperature data along 34°N, 35°N and 36°N sections in August from 1977 to 2003, the structure and formation of the Southern Yellow Sea Cold Water Mass (SYSCWM) and its responses to El Nino events are analyzed. Results show that: (1) There exist double cold cores under the main thermocline along the 35°N and 36°N sections. Also, double warm cores exist above the main thermocline along the 36°N section. (2) Thermocline dome by upwelling separates the upper warm water into two parts, the eastern and western warm waters. Additionally, the circulation structure caused by upwelling along the cold front and northeastward current along the coast in summer is the main reasons of double warm cores along the 36°N section. The intermediate cold water is formed in early spring and moves eastward slowly, which results in the formation of the western one of double cold cores. (3) Position of the thermocline dome and its intensity vary interannually, which is related to El Nino events. However, the     

Trophic interactions, ecosystem structure and function in the southern Yellow Sea

The southern Yellow Sea is an important fishing ground, providing abundant fishery resources. However, overfishing and climate change have caused a decline in the resource and damaged the ecosystem. We developed an ecosystem model to analyze the trophic interactions and ecosystem structure and function to guide sustainable development of the ecosystem. A trophic mass-balance model of the southern Yellow Sea during 2000–2001 was constructed using Ecopath with Ecosim software. We defined 22 important functional groups and studied their diet composition. The trophic levels of fish, shrimp, crabs, and cephalopods were between 2.78 and 4.39, and the mean trophic level of the fisheries was 3.24. The trophic flows within the food web occurred primarily in the lower trophic levels. The mean trophic transfer efficiency was 8.1%, of which 7.1% was from primary producers and 9.3% was from detritus within the ecosystem. The transfer efficiency between trophic levels II to III to IV to V to >V was 5.0%, 5.7%, 18.5%, and 19.7%–20.4%, respectively. Of the total flow, phytoplankton contributed 61% and detritus contributed 39%. Fishing is defined as a top predator within the ecosystem, and has a negative impact on most commercial species. Moreover, the ecosystem had a high gross efficiency of the fishery and a high value of primary production required to sustain the fishery. Together, our data suggest there is high fishing pressure in the southern Yellow Sea. Based on analysis of Odum’s ecological parameters, this ecosystem was at an immature stage. Our results provide some insights into the structure and development of this ecosystem.     

Seasonal variability of thermocline in the Yellow Sea

Based on the MASNUM wave-tide-circulation coupled numerical model, seasonal variability of thermocline in the Yellow Sea was simulated and compared with in-situ observations. Both simulated mixed layer depth (MLD) and thermocline intensity have similar spatial patterns to the observations. The simulated maximum MLD are 8 m and 22 m, while the corresponding observed values are 13 m and 27 m in July and October, respectively. The simulated thermocline intensity are 1.2℃/m and 0.5℃/m in July and October, respectively, which are 0.6℃/m less than those of the observations. It may be the main reason why the simulated thermocline is weaker than the observations that the model vertical resolution is less precise than that of the CTD data which is 1 m. Contours of both simulated and observed thermocline intensity present a circle in general. The wave-induced mixing plays a key role in the formation of the upper mixed layer in spring and summer. Tidal mixing enhances the thermocline intensity. Buoyancy-driven m     

Seasonal community structure of mesozooplankton in the Daya Bay, South China Sea

Mesozooplankton are key components of coastal ecosystems, linking the microbial food web to the classic food chain. In this study, species composition and abundance of mesozooplankton is studied for the Daya Bay in April (spring) and October (fall), 2006. A total of 27 species of mesozooplankton were identified in spring and 58 species in fall. Dominant species were Oithona tenuis, Flaccisagitta enflata, Penilia avirostris and Centropages tenuiremis in spring, shifting to Microsetella norvegica, Oithona tenuis and Parvocalanus crassirostris in fall. Higher mesozooplankton abundance was found at Aotou Cove and Dapeng’ao Cove compared to other stations, indicating the influence of eutrophication on mesozooplankton community in the Daya Bay. The outbreak of Noctiluca scintillans bloom in spring reduced the species diversity and abundance of mesozooplankton.     

Seasonal and spatial variations of macro-and megabenthic community characteristics in two sections of the East China Sea

In spring and summer 2011,the macro-and megabenthic fauna in two sections of the East China Sea were investigated using an Agassiz net trawl to detect the seasonal and spatial variations of benthic community characteristics and the relation to environmental variables.The total number of species increased slightly from spring(131 species)to summer(133)whereas the percentage of Mollusca decreased signifi cantly.The index of relative importance(IRI)indicated that the top fi ve important species changed completely from spring to summer.Species number,abundance and biomass in summer were signifi cantly higher than in spring,but no signifi cant dif ference was observed among areas(coastal,transitional and oceanic areas,divided basically from inshore to of fshore).Species richness(d),diversity(H′)and evenness(J′)showed no signifi cant seasonal or spatial variations.Cluster analysis and n MDS ordination identifi ed three benthic communities from inshore to of fshore,corresponding to the three areas.Analysis of Similarity(ANOSIM)indicated the overall signifi cant dif ference in community structure between seasons and among areas.K-dominance curves revealed the high intrinsic diversity in the off shore area.Canonical correspondence analysis showed that the coastal community was positively correlated to total nitrogen and total organic carbon in spring,but negatively in summer;oceanic community was positively correlated to total nitrogen and total organic carbon in both seasons.Species such as Coelorhynchus multispinulosus,Neobythites sivicola,Lepidotrigla alata,Solenocera melantho,Parapenaeus fi ssuroides,Oratosquilla gonypetes and Spiropagurus spiriger occurred exclusively in the of fshore oceanic area and their presence may refl ect the infl uence of the off shore Kuroshio Current.     

Current observations in the southern Yellow Sea in summer

Current data from three moored Acoustic Doppler Profilers (ADPs) deployed in the southern Yellow Sea at sites A (1-24.17°E, 34.82°N), B (122.82°E, 35.65°N) in summer 2001 and site C (120.85°E, 34.99°N) in summer 2003 were analyzed in this paper. Features of the tidal and residual currents were studied with rotary spectral and cross-spectral methods. Main achievements were as follows: 1) Tides dominated the currents. At sites A and B, the semidiurnal tidal current was basically homogeneous in the whole depth, taking a clockwise rotation at site A, and near-rectilinear counterclockwise rotation at site B; while the diurnal tidal current was strong and clockwise near the surface, but decreased and turned counterclockwise with depth; at site C, semidiurnal tidal current dominated and diurnal current took the second, both of which were counterclockwise and vertically homogeneous. Inertial motion contributed to the clockwise component of diurnal fluctuations; 2) The 3-5d fluctuation of residual current w     

Macrobenthic community structure and species composition in the Yellow Sea and East China Sea in jellyfish bloom

To understand the characteristics of macrobenthic structures and the relationship between environment and benthic assemblages in jellyfish bloom, we studied the macrobenthos and related environmental factors in the coastal waters of the Yellow Sea and East China Sea. Data were collected during two seasonal cruises in April and August of 2011, and analyzed with multivariate statistical methods. Up to 306 macrobenthic species were registered from the research areas, including 115 species of Polychaeta, 78 of Crustacea, 61 of Mollusca, 30 of Echinodermata, and 22 of other groups. Nine polychaete species occurred at frequencies higher than 25% from the sampling stations： Lumbrineris longifolia, Notomastus latericeus, Nin6e palmata, Ophelina acuminata, Nephtys oligobranchia, Onuphis geophiliformis, Glycera chirori, Terebellides stroemii, and Aricidea fragilis. Both the average biomass and abundance of macrobenthos are higher in August （23.8 g/m^2 and 237.7 ind./m^2） than those in April （11.3 g/m^2 and 128 ind./m^2）; the dissimilarity ofmacrobenthic structures among stations is as high as 70%. In terms of the dissimilarity values, we divided the stations into four clusters in spring and eight in summer. The ABC curve shows that the macrofauna communities in high jellyfish abundance were not changed. Canonical correspondence analysis showed that depth, temperature, median grain size, total organic carbon of sediment and total nitrogen in sediment were important factors affecting the macrozoobenthic community in the study area.     

Rotary spectrum analysis of tidal current in the southern Yellow Sea

A complete set of one-month Acoustic Doppler Profiler (ADP) current data at a station in the southern Yellow Sea (SYS) is analyzed using the rotary spectrum method. The results revealed different rotary properties between barotropic and baroclinic tidal currents. The barotropic and baroclinic tidal currents rotate elliptically counter-clockwise and clockwise, respectively. Meanwhile, baroclinic bottom tidal currents are almost along-isobath. The baroclinic cross-isobath velocities attenuate quickly at the bottom, implying important effects of bottom topography on the cross-isobath motions.     

Interannual variability of the southern Yellow Sea Cold Water Mass

Temperature data collected in the sections of 34°N, 35°N and 36°N in August from 1975 through 2003 were analyzed using Empirical Orthogonal Function (EOF) to investigate interannual variability of the southern Yellow Sea Cold Water Mass (YSCWM). The first mode (EOF1) reveals variations of basin-wide thermocline depth, which is mainly caused by surface heating. The second mode (EOF2) presents fluctuations of vertical circulation, resulting mainly from interannual variability of cold front intensity. In addition, it is found that the upward extent of upwelling in the cold front is basically determined by wind stress curl and the zonal position of the warm water center in the southern Yellow Sea is correlated with spatial difference of net heat flux.     

Biodiversity of free-living marine nematodes in the southern Yellow Sea,China

Biodiversity patterns of free-living marine nematodes were studied using specific,taxonomic and phylogenetic diversity measures in the southern Yellow Sea,China.The results showed that the average of Shannon-Wiener diversity index(H′) in the study area was 3.17.The higher values were distributed in the east part of Shandong coastal waters and north part of Jiangsu coastal waters,while the lower values were distributed in the southern Yellow Sea Cold Water Mass(YSCWM).The average of taxonomic diversity(Δ) was 62.09 in the study region.The higher values were distributed in the transitional areas between the coastal areas and the southern YSCWM,while the lower values were distributed near the north part of Jiangsu coastal waters and the YSCWM.Results of correlation analysis of species diversity and taxonomic diversity showed that some of the two kinds of diversity index were independent,which suggested that combining the two kinds of diversity indices can reflect the ecological characteristics better.A test for 95% probability funnels of average taxonomic distinctness and variation in taxonomic distinctness suggested that Station 8794(in the YSCWM) was outside of the 95% probability funnels,which may be due to the environmental stress.Results of correlation analysis between marine nematodes biodiversity and environmental variables showed that the sediment characteristics(Md? and Silt-clay fraction) and phaeophorbide a(Pha-a) were the most important factors to determine the biodiversity patterns of marine nematodes.     

Wintertime meso-scale horizontal distribution of large tintinnids in the southern Yellow Sea

Spatial distribution of some large tintinnid species (nominally > 76 μm) is investigated on samples vertically towed in the southern Yellow Sea in winters of 2001 to 2004. Nine tintinnid species are recorded: Codonellopsis morchella, Stenosemella pacifica, S. steini, Tintinnopsis schotti, T. radix, T. karajacensis, Eutintinnus tenuis, Parafavella sp., Leprotintinnus neriticus, of which C. morchella and T. radix dominated in the warm tongue-shaped zone of the Yellow Sea Warm Current (YSWC), and S. pacifica i...     

Spatio-temporal variations in the siphonophore community of the northern South China Sea

To understand how hydrological and biological factors affect near-to off-shore variations in the siphonophore community,we sampled zooplankton at 82 stations in the northern South China Sea during summer,winter,and spring.Forty-one species of siphonophore were collected by vertical trawling.The species richness of siphonophores increased from the nearshore to offshore regions in all three seasons of investigation,with maximum richness in summer and minimum richness in winter.The abundance of siphonophores was also higher in summer than in spring and winter,concentrated in the nearshore region in the warm season and scattered in the offshore region in the cold season.Four siphonophore groups were classified according to the frequency of occurrence:nearshore,near-offshore,offshore,and tropical pelagic.Among them,the nearshore group had higher abundance nearshore compared with the offshore.The tropical pelagic group had higher species number offshore than nearshore.Spatial and temporal fluctuations in taxonomic composition and abundance of siphonophores were due to the influence of the coastal upwelling and surface ocean currents of the South China Sea,driven by the East Asia monsoonal system.     

Grain-size related nitrogen distribution in southern Yellow Sea surface sediments

Forty-eight surface sediments of the southern Yellow Sea are separated into three grain-size fractions. Four forms of extractable nitrogen (nitrogen in ion-exchangeable form (Nie), nitrogen in weak-acid extractable form (Nwa), nitrogen in strong-alkali extractable form (Nsa) and nitrogen in strong-oxidant form (N50))are obtained by the sequential extraction. The results show that the contents and the distributions of the extractable nitrogen in the southern Yellow Sea surface sediments are closely related to sediment grain size. The distributions ofNie, Nso and total nitrogen (TN) present positive correlations with fine particles content, while Nwa and Nsa does not have such correlation. The net contents of all the forms of nitrogen increase with sediment grain size finer.     

Current status of small yellow croaker resources in the southern Yellow Sea and the East China Sea

We used data from bottom trawl surveys to study the factors influencing the abundance of small yellow croaker, Larimichthys polyactis, in the southern Yellow Sea (SYS) and the East China Sea (ECS). The resource density index (RDI) was generally higher in summer and autumn than in spring and winter. RDIs were also significantly greater in the SYS than in the ECS in summer and autumn. The bottom water salinity and depth of spatial distribution of small yellow croaker was similar between the two areas in summer, but different in other seasons. Regression analysis suggested that environmental factors such as bottom water temperature, salinity, and depth influenced the RDIs in summer in these areas. Growth condition factor (GCF) in the two areas varied monthly and the croaker in the SYS grew more slowly than those in the ECS. This was likely due to the low bottom temperature of the Yellow Sea Cold Water Mass in summer and autumn or to higher human fishing pressure in the ECS. To ensure sustainable utilization of the croaker stocks in these regions, we recommend reducing the fishing intensity, increasing the cod-end mesh size, and improving the protection of juveniles.     

Community structure changes of macrobenthos in the South Yellow Sea

The ecological environment in the Yellow Sea has changed greatly from the 1950s to 1990s and this has had significant impact on marine organisms. In this study, data on soft-sediment macrobenthos occurring in depths from 25 m to 81 m in the South Yellow Sea were used to compare changes in community structure. The agglomerative classification (CLUSTER) and multidimensional scaling (MDS) methods were applied. Five communities were recognized by cluster analysis: 1. The Yellow Sea Cold Water Mass community dominated by cold water species, which changed slightly in species composition since the 1950s; 2. The mixed community with the coexistence of cold water species and warm water species, as had been reported previously; 3. The polychaete-dominated eurythermal community in which the composition changed considerably as some dominant species disappeared or decreased; 4. The Changjiang (Yangtze) River Estuarine community, with some typical estuarine species; 5. The community affected by the Yellow Sea Warm Current. The greatest change occurred in the coastal area, which indicated that the change may be caused by human activities. Macrobenthos in the central region remained almost unchanged, particularly the cold water species shielded by the Yellow Sea Cold Water Mass. The depth, temperature and median grain size of sediments were important factors affecting the distributions of macrobenthos in the South Yellow Sea.     

Seasonal variability of phytoplankton community structure in relation to different nitrogen-phosphorus ratios in the southern coastal waters of Zhejiang, China

With the rapid development of economy and increase of population in the drainage areas, the nutrient loading has increased dramatically in the Changjiang estuary and adjacent coastal waters. To properly assess the impact of nutrient enrichment on phytoplankton community, seasonal microcosm experiments were conducted during August 2010–July 2011 in the coastal waters of Zhejiang Province. The results of the present study indicated that the chl a concentration, cell abundance, diversity indices, species composition and community succession of the phytoplankton varied significantly with different N/P ratios and seasons. Higher growth was observed in the 64:1 (spring), 32:1 (summer), 16:1 (autumn) and 128:1, 256:1 (winter) treatments, respectively. The values of Shannon-Wiener index (H′) and Pielou evenness index (J) were lower in the 8:1 and 16:1 treatments in autumn test, while H′ value was higher in the 128:1 and 8:1 treatments in winter test. A definite community succession order from diatoms to dinoflagellates was observed in the autumn and winter tests, while the diatoms dominated the community throughout the culture in the spring and summer tests.