Changjiang River Mouth Research Articles

Mapping the Changjiang Diluted Water in the East China Sea during summer over a 10-year period using GOCI satellite sensor data

The Changjiang River loads freshwater into the East China Sea (ECS) and the Yellow Sea (YS). A low-salinity plume known as Changjiang Diluted Water (CDW) is formed by mixing with saline ambient waters; it disperses toward Jeju Island and the East/Japan Sea due to the dominant southerly wind and tide during summer. To map the offshore surface CDW in the ECS, a proxy was developed using surface water beam attenuation (c) and salinity from 18 annual summer cruises matched with synchronous Geostationary Ocean Color Imager (GOCI) satellite data. We followed a two-step empirical approach. First, a relationship between in situ salinity and c was obtained from hydrographic cruises (1998, 2003–2018, and 2021). Second, in situ c was matched with GOCI remote-sensing reflectance (Rrs) band ratios of all available blue-to-green wavelengths (2011–2018). Finally, satellite-derived surface salinity was determined directly by combining the two empirical relationships, providing a robust estimate over a range of salinities (22–34 psu). Our algorithm was then compared and validated with five previous satellite-derived salinity algorithms based on colored dissolved organic matter and particle concentrations. In the first step, surface salinity was matched and well correlated with c using an 18-year full conductivity–temperature–depth (CTD) dataset (N = 1,345, R2 = 0.93). In the second step, c was synchronously matched and well correlated with GOCI Rrs band ratios using an 8-year summer CTD dataset (N = 391, R2 = 0.93). The satellite-derived surface salinity based on the GOCI was compared with five other approaches and validated using an 8-year summer CTD dataset (2011–2018, N = 707, RMSE = 0.43, bias = 0.053, mean ratio = 1.002). We mapped satellite-derived surface salinity using monthly GOCI images during the summer from 2011 to 2020. The spatial patterns of the CDW were connected to the Changjiang River mouth and extended to the east–northeast during summer. Saline water was confined to the warm current and the upper slope in the eastern part of the study area. Anomalous dispersion of the CDW occurred in August 2016, 2017, and 2020, and limited dispersion occurred in August 2014 and 2019.

Temporal Variation of Summer Hypoxia off Changjiang Estuary During 1997–2014 and Its Association With ENSO

Temporal variation of summer hypoxia, along a repeated hydrographic section from the Changjiang River mouth to Cheju Island, is investigated using cruise data during 1997–2014. The climatological mean of dissolved oxygen (DO) presents a “dual-core structure” below the oxycline, associated with two hypoxic centers near the river mouth and offshore of the Changjiang Bank, respectively. The DO value is highly variable from year to year, with dramatic interannual variation but an insignificant linear trend during 1997–2014. Composite analysis indicates that the interannual variation of DO and associated hypoxia events are strongly affected by the El Niño–Southern Oscillation (ENSO) cycle. In the El Niño decaying summer, large river discharge and relatively strong stratification, combined with calm climatic and hydrographic conditions, enhance DO depletion off the Changjiang Estuary. It is likely that El Niño is not a prerequisite for hypoxia occurrences. However, El Niño did enhance bottom hypoxia probability off the Changjiang Estuary during 1997–2014.

Impacts of River Discharge on the Sea Temperature in Changjiang Estuary and Its Adjacent Sea

Freshwater plume at the Changjiang River (CR) mouth are essential to the coastal water quality and ecosystem because they can cause estuary stratification and hypoxia, potentially deteriorating the water environment. Furthermore, the advection heat transport is modulated by increasing anthropogenic effects. A comprehensive understanding of the influence of river discharge on the three-dimensional sea temperature, fronts and thermal stratification in the CR estuary remains lacking. A well-calibrated numerical model using Regional Ocean Modeling Systems (ROMS) is used to investigate the impacts of CR discharge on the sea temperature in coastal zones. Model results show that the amplitude and spatial distribution of the heating or cooling rate can be influenced by CR freshwater, especially in frontal areas. Specifically, the large runoff flow will reduce the heating or cooling rate in shallow waters (<20 m) near the CR estuary, whereas it has an opposite effect on the Zhoushan islands region (>20 m). Generally, the effect of the freshwater discharge on the upper layer is greater than on the bottom layer, and the runoff has a positive correlation to the intensity of the frontal zones in the CR estuary, though this relationship is weakened in autumn because of the weak intensity of the frontal zone. Note that seawater thermal stratification and its seasonal variation can be regulated by runoff; thermal stratification will be strengthened in abundant runoff conditions and weakened in scarce runoff conditions.

Modeling contemporary and future dimethylsulfide flux in the eastern China marginal seas

A biogeochemical model is used to simulate the sea-to-air flux of dimethylsulfide (DMS) in the Eastern China Marginal Seas (ECMS) for the period 2011 to 2015. Seasonal and inter-annual variability of sea surface temperature (SST), cloud cover (CLD), wind speed (WIND) and mixed layer depth (MLD) are analyzed. SST peaked in August and displayed a north to south gradient. CLD is around 60% and WIND is within the range 5–11 m/s. MLD ranges from 17m in summer to more than 149m in March. CHL and DMS surface concentration peaked near the Changjiang River mouth during summer.DMS field data was collected by the Ocean University of China (OUC). These data are used together with a genetic algorithm (GA) technique to calibrate both CHL and DMS related key model parameters. Simulated DMS concentrations and DMS flux reach a maximum of 6.1 nM in May and 8.8 μmol m−2d−1 in the middle of July respectively.CMIP5 experiment archive is used to provide physical forcings for the simulation under 4 × CO2 climate conditions. Compared with contemporary climate conditions, under 4 × CO2, annual mean SST increases 19%; WIND decreases 24%; MLD decreases 7.8%. DMS decreases by 25%, and DMS flux decreases by 38%. The change in DMS flux is most likely due to the significant increase of SST and decrease of wind speed. The projected decrease in MLD is likely caused by projected more precipitation in the region leading to increased fluvial discharge and a decrease in salinity under warming.

Organic geochemical evidence of past changes in hydro- and sediment-dynamic processes at river mouths: A case study of Holocene sedimentary records in the Changjiang River delta, China

Estuarine fronts are the boundaries between different water masses at river mouths and play important roles in trapping suspended particles. Investigating the response of estuarine fronts to climate and sea-level changes during the Holocene could elucidate the evolution of complex hydro- and sediment-dynamic processes at river mouths. To assess the estuarine frontal dynamics of the Changjiang (Yangtze) River mouth, we examined organic geochemical proxies in surficial sediments to investigate the spatial distribution of terrestrial- and marine-sourced organic carbon in recent decades. To explore the Holocene migration of estuarine fronts, we analysed organic carbon compositions in seven age-constrained sediment cores collected across the onshore and offshore delta. The suspended sediment front was identified as a major boundary for trapping terrestrial organic carbon, and the landward limit of the salt wedge was found to constrain the landward spread of marine-sourced organic carbon. The temporal and spatial distribution of organic carbon compositions in the sediment cores indicated a rapid landward retreat of the estuarine fronts between ~10.0 and 6.0 ka (cal. kyr BP) in response to sea-level rise; this coincided with the retreat of the entire river mouth system. Despite significant deltaic progradation, the estuarine fronts slowly migrated seaward after ~6.0 ka in response to a weakened summer monsoon and a resultant decline in freshwater discharge or the lateral spreading of the delta. Accelerated deltaic progradation induced by both intensified human impacts and infilling of the palaeo-incised Changjiang valley resulted in the accelerated seaward migration of the estuarine fronts over the past ~1500 years. This interdisciplinary study strengthens our understanding of the links between hydro-sediment dynamic processes and sedimentary records to further elucidate the complex systemic behaviour at tide-dominated river mouths.

Phytoplankton Blooms off a High Turbidity Estuary: A Case Study in the Changjiang River Estuary

AbstractThe occurrence of phytoplankton blooms in estuarine and coastal waters is codetermined by the export of fluvial nutrients and other water properties such as turbidity. In this study, the Changjiang River Estuary was used as an example to investigate the intrinsic link between river plume dynamics, riverine nutrient loading, suspended sediment concentration, and phytoplankton blooms with a coupled hydrodynamic‐sediment‐ecosystem numerical model. Suspended sediment concentration from the sediment module was used to parameterize light attenuation in waters, which, in association with river plume dynamics, was found to impact phytoplankton distribution. Data that was collected in situ, in addition to satellite data for chlorophyll, nutrients, and suspended sediments, were used to validate the model, which performed well. The surface high chlorophyll centers exhibited a patch‐like pattern from the northeast to the south off the Changjiang River mouth. The high chlorophyll concentration within the river plume was mainly determined by riverine nutrient exports, and potential phosphate limitation occurred away from the river mouth. The chlorophyll concentration decreased dramatically shoreward of the sediment front, which was controlled by the bottom river plume front. The bottom plume front was an intrinsic feature of the Changjiang River plume and similar river plume systems, which set a boundary separating the well‐mixed (turbid) nearshore waters and stratified (clear) offshore waters. Without considering the sediment shading effect, the model yielded an unrealistic chlorophyll distribution with artificially high values in the turbid area. This indicated that light attenuation was a key factor limiting phytoplankton blooms in turbid river estuaries.

Winter storms induced high suspended sediment concentration along the north offshore seabed of the Changjiang estuary

Fine-grained sediments suspended in coastal waters play an important role in submarine topography evolution and associated environment changes. The convergence of suspended sediments concentrated near the seabed results in high sediment concentration, and contributes significantly to sediment transport. In order to investigate the mechanism triggering high suspended sediment concentration (SSC), we deployed a tripod to the seabed to obtain in situ bottom boundary layer measurements of sediment dynamics and a buoy to the sea surface to collect meteorological and wave data at the northern Changjiang River mouth from December 20, 2015 to January 20, 2016. The high SSC (e.g. >3 g/L) events were observed together with fluid mud (thicknesses of 4–16 cm) near the seabed during neap tides when cold air intrusion generated winter storms and strong waves. Further, we found that the high SSC event was mainly resulted from wind waves and sediment resuspension supported by local benthic fluid mud, which was associated with three stages. At the setting up stage, the winter storm brought long duration of strong-waves (e.g. significant wave height >1.5 m) more than 15 h, resulting in a maximum wave-current combined bottom shear stress of 3.2 Pa and the increase of SSC to >1 g/L. At the reinforcement stage, the strong waves and bottom shear stress lasted for several hours, and further increased the SSC to >3 g/L. At the final decay stage, wind waves and the maximum wave-current shear stresses decreased significantly, with the disappearance of high SSC. Thus, there was no high SSC event observed even with strong wind waves during spring tides because strong wave duration was too short for the reinforcement process. The sediment source for the high SSC events was mainly from bottom fluid mud resuspension, as well as the advection transport from the adjacent subaqueous Changjiang River delta.

East China Sea ecosystem under multiple stressors: Heterogeneous responses in the sea surface chlorophyll-a

The East China Sea and southern Yellow Sea ecosystems have undergone drastic changes over the past decades. The changes in the ecosystems are attributable to both natural and anthropogenic stressors. We analyzed the seasonal and interannual variability of the sea surface chlorophyll-a in the East China Sea and the southern Yellow Sea using a suite of remotely sensed data (1998–2012). When seen on a Large Marine Ecosystem level, there seems no trend in the region. However, when seen on a sub-regional scale, heterogeneous responses can be recognized among the subregions. There was an increasing trend of chlorophyll-a in the vicinity of the Changjiang (Yangtze) River mouth, while there was a decreasing trend in the southeastern slope area which can be attributed to anthropogenic nutrient enrichment and warming, respectively. Contrary to some previous studies, our analysis clearly showed that the summer-autumn averaged chlorophyll-a decreased by about 14% in a large area (circa 178,000 km2) in the northeastern East China Sea after 2003 coinciding with the initial impoundment of the Three Gorges Dam. Our analysis demonstrates that our ability to detect the trends in response to multiple stressors largely depends on choosing an appropriate spatiotemporal scale.

Spatial Distribution and Physical Controls of the Spring Algal Blooming Off the Changjiang River Estuary

The Changjiang River discharges huge amounts of nutrients, which cause frequent harmful algal blooms off its estuary. By analyzing historical and in situ observational data, this study aimed to determine the distribution and physical controls of these blooms. The results indicated that phytoplankton bloomed within the 30–50 m isobaths south of the Changjiang River mouth, with several high-value centers. South of the river mouth, the river plume front propagated along the coast as bottom-trapped river plume, which reached the bottom along the 25–30 m isobaths. The algae bloomed between the locations of the bottom and surface fronts. The surface front determined the seaward limit of the nutrient-rich river-influenced water and set the outer boundary of algal blooming; the latter separated the stratified (hence less turbid) offshore surface water from the well-mixed thus turbid nearshore surface water, forming the shoreward limit of the algal blooming due to high turbidity. The surface plume front was relatively unstable compared with the bottom. The surface plume front relaxes around the bathymetric irregularities, which enlarges the distance between the plume’s surface and bottom fronts, providing a larger area favorable to algal blooming. Consequently, several high chlorophyll-a centers developed around the bathymetric irregularities. East and northeast of the river mouth, the river plume floated above ambient seawater as a surface-trapped river plume. A subsurface chlorophyll-a maximum was observed above the pycnocline in the far-field plume area, which descended gradually to ~ 10 m in the offshore direction. The associated dynamical mechanism was also discussed.

Organic matter degradation in surface sediments of the Changjiang estuary: Evidence from amino acids

Organic matter degradation is a key component of the processes of carbon preservation and burial in seafloor sediments. The aim of this study was to explore organic matter degradation state within the open-shelf Changjiang Estuary of the East China Sea, using an amino acids–based degradation index (DI) in conjunction with information about organic matter source (marine versus terrestrial), bottom water oxygenation state, and sediment grain size. The relative molar percentages of 17 individual amino acids (characterized using principal component analysis) in surface sediments indicate that organic matter is degraded to varying extents across the estuary seabed. Sediments with DI >0 (relatively labile) were found mostly within a coastal hypoxic area. Sediments of DI less than −1 (relatively refractory) were found near the Changjiang River mouth and the northern and southern parts of the central shelf. We consider DI to be a more reliable indicator of degradation than simple ratios of AAs. DI was inversely correlated with the proportion of terrestrial organic material (Ft) in the sediments, indicating that relatively fresh/labile organic matter was generally associated with marine sources. DI was significantly correlated with Ft and bottom water apparent oxygen utilization (AOUbot) together. The parameter DI and the (labile) amino acid tyrosine were highest in hypoxic areas, suggesting the presence of relatively fresh organic matter, probably due to a combination of marine-source inputs and better preservation of organic matter in the silt and clay sediments of these areas (as compared to sandy sediments). Less degraded organic matter with high amino acids was also favorable to benthic animals. Overall, sedimentary estuarine organic matter was least degraded in areas characterized by marine sources of organic matter, low-oxygen conditions, and fine-grained sediments.

Significant production of humic fluorescent dissolved organic matter in the continental shelf waters of the northwestern Pacific Ocean

Fluorescent dissolved organic matter (FDOM) is important for marine organisms and the global carbon cycle contributing to the optical properties of surface seawater and organic carbon budgets. Rivers are known to be the main source of FDOM in coastal oceans and marginal seas. In this study, however, we show that the contribution of FDOM produced from organic sediments of the northwestern Pacific continental shelf is similar to that from the Changjiang River. FDOM showed relatively higher concentrations at stations off the Changjiang River mouth and in the central Yellow Sea. Based on temperature-salinity diagrams, the major source of surface FDOM in summer surface waters was found to be from the Changjiang River while that observed in the winter water column was produced mainly in the continental shelf. A good correlation between 228Ra and the humic-like FDOM (FDOMH) during the winter suggests that the FDOMH is produced mainly from marine sediments and enriched in water over the water residence times. A simple mass balance calculation shows that the excess FDOMH fluxes produced from marine sediments account for 30–40% of the riverine source. This result suggests that the continental shelf is an important hidden source of FDOM in the upper ocean.

The influence of seasonal climate on the morphology of the mouth-bar in the Yangtze Estuary, China

The geomorphology of the Yangtze Estuary in the Changjiang River Delta in Eastern China has been the subject of extensive research. This study extends previous work to examine the influence of wind-waves on the mouth-bar, where about half of the river-borne material settles to the bed. The site is located just outside of Changjiang River mouth, which is meso-tidal and subject to seasonally varying river flows and wind-wave conditions. Modeling was performed with a coupled wave-current hydrodynamic model using TELEMAC and TOMAWAC and validated against observed data. Bottom Shear Stress (BSS) from river, tide and waves based on the numerical model output was used to infer the respective contribution to the evolution of the subaqueous delta. Our examination did not however extend to modeling the sediment transport or the morphological bed changes. The results suggest that (i) the dominance of river discharge is limited to an area inside the mouth, while outside, the mouth-bar is tide-wave dominant; (ii) considering just the tide, the currents on the shallow shoals are flood dominant and deep channels are ebb dominant, which induces continued accretion over the shallows and erodes the deeper parts of the mouth-bar until the tidal currents become too weak to transport sediment; (iii) whereas waves are very efficient at reshaping the shallow shoals, with the effect being subtly dependent on the depth distribution over the mouth-bar; (iv) the stability of shallow shoal morphology is highly dependent on the presence of seasonal wind-waves and characterized as “summer storing and winter erosion”, while deep channels perform like corridors of water and sediment, exporting sediment all year round. The nature of the mouth-bar response has important implications for coastal management, such as the ongoing deep water channel maintenance, reclamations and coastal defense measures.

Observations of Tidal Straining Within Two Different Ocean Environments in the East China Sea: Stratification and Near‐Bottom Turbulence

AbstractTidal straining describes the straining effect induced by the vertical shear of oscillatory tidal currents that act on horizontal density gradients. It tends to create tidal periodic stratification and modulate the turbulence in the bottom boundary layer (BBL). Here, we present observations of current, hydrology and turbulence obtained at two mooring stations that are characterized by two typical hydrological environments in the East China Sea (ECS). One is located adjacent to the Changjiang River's mouth, and the other is located over a sloping shelf which is far from the freshwater sources. Tidal straining induces a semidiurnal switching between stable and unstable stratification at both stations. Near‐bottom high‐frequency velocity measurements further reveal that the dissipation rate of turbulent kinetic energy (TKE) is highly elevated during periods when unstable stratification occurs. A comparison between the TKE dissipation rate ( ) and the shear production ( ) further reveals that the near‐bottom mixing is locally shear‐induced most of the time except during the unstable stratification period. Within this period, the magnitude of dissipation exceeds the expected value based on the law of the wall by an order of magnitude. The buoyancy flux that calculated by the balance method is too small to compensate for the existing discrepancy between the dissipation and shear production. Another plausible candidate is the advection of TKE, which may play an important role in the TKE budget during the unstable stratification period.

Heavy metals in surface sediments of the continental shelf of the South Yellow Sea and East China Sea: Sources, distribution and contamination

The heavy metals in surface sediments of the continental shelf of the South Yellow Sea (SYS) and the East China Sea (ECS) collected in 2016 were analyzed to assess the heavy metal distributions, contamination status, and likely sources. The metal concentrations generally met the China Marine Sediment Quality criteria. However, the enrichment factor and geoaccumulation index values clearly showed elevated Cd and Pb concentrations in the region. Cd and Pb contamination occurred mostly to the southwest of the Changjiang River mouth and to the east of the Shandong Peninsula because of rapid economic and industrial development. The results are likely to be useful for authorities responsible for sustainable coastal zone management.

The variations of sediment transport patterns in the outerChangjiangEstuary andHangzhouBay over the last 30 years

AbstractThe research objective is to investigate the variations of sediment transport in outer Changjiang Estuary and adjacent Hangzhou Bay, induced by the decline of Changjiang River sediment discharge and massive land reclamation in the last three decades. A synchronous hydrographic survey was conducted along two transects (at the bay‐mouth and outer Changjiang Estuary, respectively) during the spring‐neap tides of January and July 2014. The results show that the suspended sediment grain size, current velocity, suspended sediment concentration (SSC), and the water and sediment fluxes varied with the tidal cycles. Quantitative correlations with the tidal range were found for SSC and fluxes. These data have been compared with those at the same hydrographic stations in the summer and winter of the early 1980s. Along the outer Changjiang Estuary transect, the SSCs and sediment fluxes decreased in the winter, but no apparent changes occurred in the summer. The SSCs in the northern Hangzhou Bay decreased in both summer and winter, while the southern bay mouth has evolved from a low SSC region to a high SSC region. The findings clarify that the SSC and sediment flux changes in this area have only an indirect connection to the dramatic riverine sediment decline, because the sediment resuspension by the strong tidal currents provided a major source. At the present stage, the impact of the riverine sediment decline is insignificant for the SSC variation off the Changjiang River mouth. Finally, a sediment flux model is proposed to explain and predict the morphological evolution trends.

Modeling the largest inflow of Changjiang freshwater into the Yellow Sea in 2012 with particle-tracking experiment

Abnormally low-salinity water originating from the Changjiang River (CR) was observed at the central Yellow Sea (YS) in 2012, which was quite unique compared to other years. In this study, the intrusion process of the Changjiang Diluted Water (CDW) into the YS interior was examined using a hindcast simulation (2003–2012) with particle-tracking experiments. The particles representing the behavior of the CDW were released at the CR mouth from May to August, and then tracked. The simulated salinity patterns coincide fairly well with those derived from observations, particularly showing a large low-salinity structure around the central YS in 2012. A substantial intrusion of freshwater into the YS occurred in 2012, and this accounted for approximately 16% of all the released particles in 2012 which is twice as high as the mean average covering the 10 years. According to the trajectories in 2012, the particles took less than 50 days to travel from the mouth to the YS interior and followed mainly two paths toward the YS. One pathway traveled northward to the central entrance of the YS and then reached the western coast of Korea. This pathway was attributed to the strong easterly winds in late June and early August when three consecutive typhoons passed through the YS, which was a unique pattern that is rarely found in other years. The other pathway involved particles trapped along the Jiangsu coast drifting farther to the north up to the Shandon Peninsula against the anticyclonic tidal residual circulations during the passage of typhoons.

Chemical speciation of iron in sediments from the Changjiang Estuary and East China Sea: Iron cycle and paleoenvironmental implications

Chemical speciation of iron (Fe) in sediment offers a better understanding on iron cycle, early diagenesis, paleoredox condition and paleoclimatic changes. This study reports the Fe chemical speciation (total concentration FeT, highly reactive FeHR, poorly reactive FePR and unreactive FeU) in the surface sediments from the Changjiang (Yangtze River) Estuary and the East China Sea shelf. The FeHR concentrations in the estuarine and shelf sediments are much lower than those from the Changjiang and other rivers in the world, confirming the previous observations on geochemical fractionation effect in estuarine and shelf sediments. The high correlations of FeHR with FeT and Al concentrations and sediment surface area, suggest the dominance of clay minerals on FeHR composition in the sediments. The apparent enrichments of FeHR, total organic carbon and surface area in the hypoxia zone off the Changjiang Estuary are primarily caused by the local hydrodynamics and sedimentary processes. Despite the wide application of FeHR/FeT ratio for redox condition, its spatial distribution from the Changjiang river mouth to open shelf reveals no obvious enrichment in the hypoxia zone. This suggests that the episodic occurrence of hypoxia off the Changjiang Estuary (mostly in August) is hardly imprinted in particulate iron speciation. The distributions of Fe chemical speciation along the “river-estuary-open shelf” transect suggest the complex controls of particulate iron “shuttle transport” and deposition in the continental margin where fluvial input dominates and intense anthropogenic impact exists. Our study sheds new light on the source-to-sink process of particulate iron in the estuary and open shelf, and also provides important constraints of Fe speciation in modern and ancient archives from various depositional environments.

Spatial and seasonal pattern of macrozoobenthic assemblages and the congruence in water quality bioassessment using different taxa in artificial Mingzhu Lake in Shanghai

The spatial and seasonal pattern of macrozoobenthic structure and its relationship with environmental factors were studied from July 2006 to April 2008 in Mingzhu Lake, Chongming Island, Shanghai at the Changjiang River mouth. The congruences in water quality bioassessment based on diversity and biotic indices and using different taxonomic categories were also explored to find the best assessment method of water quality for the lake. All major structural characteristics of macrozoobenthic community, including species composition, abundance, biomass and four biomass-based diversity indices (Shannon’s diversity, Simpson’s diversity, Pielou’s evenness and Simpson’s evenness index) fluctuated significantly in season but in space. The above four abundance-based diversity indices plus abundance-based Margalef’s richness index did not display significant spatial variations; and significant seasonal differences were found in three indices only. Water temperature was the key environmental factor responsible for macrozoobenthic spatio-temporal distribution patterns. Water quality assessed by Shannon’s index (Ha′) and biological pollution index (BPI) rather than the other four biotic indices were consistent with those by trophic state index (TSI). Results from chironomids and oligochaetes did not always agree to those from the whole community when Ha′ or Hilsenhoff biotic index was applied to bioassessment. Therefore, combining multiple indices and avoiding a single taxonomic category to assess water quality are strongly recommended and in Mingzhu Lake using a mixture of Ha′ and BPI will ensure the most effective investigation of water quality. Our results also show that the main structural characteristics of macrozoobenthic communities in the small lake may display consistent spatial patterns.

Distribution of diatoms and silicoflagellates in surface sediments of the Yellow Sea and offshore from the Changjiang River, China

The spatial distribution of siliceous microfossils (diatoms and silicoflagellates) in the surface sediments was mapped at 113 sites in the Yellow Sea and sea areas adjacent to the Changjiang (Yangtze) River, China. In total, 267 diatom taxa and two silicoflagellate species were identified from the sediments. The spatial variations in abundance and diversity were classified into three distinct geographic patterns using Q mode clustering: a south-north geographic pattern, a coastal-offshore pattern and a unique pattern in the Changjiang River mouth. The south-north geographic pattern was related to the spatial variations in sea temperature. Coscinodiscus oculatus, a warm-water species, indicated these variations by a gradual decrease in abundance from the south to the north. The coastal-offshore pattern was in response to the spatial variations in salinity. Cyclotella stylorum, Actinocyclus ehrenbergii and Dictyocha messanensis, the dominant brackish species in coastal waters, significantly decreased at the isobaths of approximately 30 m, where the salinity was higher than 31. Paralia sulcata and Podosira stelliger indicated the impact of the Yellow Sea Warm Current in the central Yellow Sea. The unique pattern in the Changjiang River mouth showed the highest species diversity but lower abundance, apparently because: freshwater input can significantly increase the proportion of brackish species; nutrients can supply the growth of phytoplankton; and high sedimentation rates can dilute the microfossil abundance in the sediments. Our results show that an integration of environmental factors (e.g., nutrient levels, sedimentation rate, sea temperature, salinity and water depth) determined the spatial characteristics of the siliceous microfossils in the surface sediments.

Distribution and assessment of heavy metals off the Changjiang River mouth and adjacent area during the past century and the relationship of the heavy metals with anthropogenic activity

Forty-three surface sediment samples and one gravity core obtained from the offshore area of the Changjiang River were analyzed for selected heavy metals (Cu, Pb, Zn, Cd, As, Hg) to evaluate the spatial distribution and potential ecological risk during the last century. The results indicated that the sediments are composed of silty sand, sandy silt and silt and were deposited in a relatively stable environment over the last century. The studied marine sediments are fine and easily adsorb heavy metals from aquatic systems. The heavy metal concentrations were found to be enriched in the sediments and were generally closely related to anthropogenic activities. However, the data analysis demonstrated that the levels of heavy metal contamination were below background values during the last century, indicating low ecological risk. Spatially, a higher concentration was found at the entrance to the Changjiang River, while it decreased to the northeast. The vertical distribution of contamination levels and ecological risk can be divided into four periods based on the downcore variation in heavy metals: pre-1940s, 1940s–1970s, 1970s–1990s and the late 1990s to the present. These conclusions form the basis for implementing appropriate policies to protect marine sediment quality.