A numerical study on water diversion ratio of the Changjiang(Yangtze)estuary in dry season

We studied the flood, ebb and tidal averaged along (net) water diversion ratio (WDR) during dry season in the Changjiang (Yangtze) estuary, China, along with the effects of northerly wind, river discharge, tide and their interactions on WDR using the improved version of three-dimensional numerical model ECOM. Using data for annual mean wind speed and river discharge during January, we determined that the flood, ebb, net WDR values in the North Branch of the estuary were 3.48%, 1.68%,-4.06% during spring tide, and 4.82%, 2.34%,-2.79% during neap tide, respectively. Negative net WDR values denote the transport of water from the North Branch into the South Branch. Using the same data, the corresponding ratios were 50.09%, 50.92%, 54.97%, and 52.33%, 50.15%, 43.86% in the North Channel and 38.56%, 44.78%, 103.96%, and 36.92%, 43.17%, 60.97% in the North Passage, respectively. When northerly wind speed increased, landward Ekman transport was enhanced in the North Branch, increasing the flood WDR, while the ebb WDR declined and the net WDR exhibited a significant decrease. Similarly, in the North Channel, the flood WDR is increased, the ebb WDR reduced, and the net WDR showed a marked decrease. In the North Passage, the flood WDR also increased while the ebb and net WDR declined. As the river discharge increased, the flood and ebb WDR of the North Branch increased slightly and the net WDR increased markedly. In the North Channel the flood and ebb WDR changed very slightly, while the net WDR declined during spring tides and increased during neap tides. The WDR in the North Passage changed slightly during flood and ebb tides while the net WDR showed a marked increase. The WDR values of different bifurcations and the responses to northerly wind, river discharge, and tide are discussed in comparison with variations in river topography, horizontal wind-induced circulation, and tidal-induced residual current.

Size-fractionated phytoplankton biomass in autumn of the Changjiang(Yangtze) River Estuary and its adjacent waters after the Three Gorges Dam construction

A cruise was undertaken from 3rd to 8th November 2004 in Changjiang(Yangtze) River Estuary and its adjacent waters to investigate the spatial biomass distribution and size composition of phytoplankton.Chlorophyll-a(Chl-a) concentration ranged 0.42-1.17 μg L-1 and 0.41-10.43 μg L-1 inside and outside the river mouth,with the mean value 0.73 μg L-1 and 1.86 μg L-1,respectively.Compared with the Chl-a concentration in summer of 2004,the mean value was much lower inside,and a little higher outside the river mouth.The maximal Chl-a was 10.43 μg L-1 at station 18(122.67°E,31.25°N),and the region of high Chl-a concentration was observed in the central survey area between 122.5°E and 123.0°E.In the stations located east of 122.5°E,Chl-a concentration was generally high in the upper layers above 5 m due to water stratification.In the survey area,the average Chl-a in sizes of >20 μm and <20 μm was 0.28 μg L-1 and 1.40 μg L-1,respectively.High Chl-a concentration of <20 μm size-fraction indicated that the nanophytoplankton and picophytoplankton contributed the most to the biomass of phytoplankton.Skeletonema costatum,Prorocentrum micans and Scrippsiella trochoidea were the dominant species in surface water.The spatial distribution of cell abundance of phytoplankton was patchy and did not agree well with that of Chl-a,as the cell abundance could not distinguish the differences in shape and size of phytoplankton cells.Nitrate and silicate behaved conservatively,but the former could probably be the limitation factor to algal biomass at offshore stations.The distribution of phosphate scattered considerably,and its relation to the phytoplankton biomass was complicated.

Decreasing Reference Evapotranspiration in a Warming Climate-A Case of Changjiang (Yangtze) River Catchment During 1970-2000

This study deals with temporal trends in the Penman-Monteith reference evapotranspiration estimated from standard meteorological observations, observed pan evaporation, and four related meteorological variables during 1970-2000 in the Yangtze River catchment. Relative contributions of the four meteorological variables to changes in the reference evapotranspiration are quantified. The results show that both the reference evapotranspiration and the pan evaporation have significant decreasing trends in the upper, the middle as well as in the whole Changjiang （Yangtze） River catchment at the 5% significance level, while the air temperature shows a significant increasing trend. The decreasing trend detected in the reference evapotranspiration can be attributed to the significant decreasing trends in the net radiation and the wind speed.

Spatial distribution and diurnal variation of chemical oxygen demand at the beginning of the rainy season in the Changjiang (Yangtze) River Estuary

A field observation was carried out in the Changjiang (Yangtze) River Estuary from May 19 to 26, 2003. A total of 29 stations, including 2 anchored stations, were occupied through almost the whole salinity gradient. Based on the observation data, biogeochemistry of chemical oxygen demand (COD) was examined. Spatial distribution pattern of COD shows that it decreased downstream. The COD concentration varied generally within a narrow range of 1.24–1.60 mg/L in the zone around the river mouth, beyond which it decreased rapidly to 0.20 mg/L. In the mixed water zone, the fluctuation in COD was smaller at 2 m above the bottom layer than at the surface layer in 48 h. In the seawater zone, the 48-h fluctuation at the surface was the largest, followed by that of 5 m below the surface and 2 m above the bottom layers in a range of from 2.50 to 0.55 mg/L. Freshwater discharge was the dominant source of COD in the estuary. The average COD beyond the river mouth was 2.7 mg/L, which accorded with the Chinese seawater quality Grade I. Relationships between dissolved oxygen and biogeochemical parameters such as suspended particulate matter, dissolved organic matter and chlorophyll-a were also discussed.

Numerical simulation of the transport and diffusion of dissolved pollutants in the Changjiang (Yangtze) River estuary

Based on a coupled hydrodynamic–ecological model for regional and shelf seas (COHERENS), a three-dimensional baroclinic model for the Changjiang (Yangtze) River estuary and the adjacent sea area was established using the sigma-coordinate in the vertical direction and spherical coordinate in the horizontal direction. In the study, changing-grid technology and the "dry-wet" method were designed to deal with the moving boundary. The minimum water depth limit condition was introduced for numerical simulation stability and to avoid producing negative depths in the shallow water areas. Using the Eulerian transport approaches included in COHERENS for the advection and dispersion of dissolved pollutants, numerical simulation of dissolved pollutant transport and diffusion in the Changjiang River estuary were carried out. The mass centre track of dissolved pollutants released from outlets in the south branch of the Changjiang River estuary water course has the characteristic of reverse current motion in the inner water course and clockwise motion offshore. In the transition area, water transport is a combination of the two types of motion. In a sewage-discharge numerical experiment, it is found that there are mainly two kinds of pollution distribution forms: one is a single nuclear structure and the other is a double nuclear (dinuclear) structure in the turbid zone of the Changjiang River estuary. The rate of expansion of the dissolved pollutant distribution decreased gradually. The results of the numerical experiment indicate that the maximum turbid zone of the Changjiang River estuary is also the zone enriched with pollutants. Backward pollutant flow occurs in the north branch of the estuary, which is similar to the backward salt water flow, and the backward flow of pollutants released upstream is more obvious.

Water discharge variability of Changjiang(Yangtze) and Huanghe(Yellow) Rivers and its response to climatic changes

Influences of large-scale climatic phenomena, such as the E1Nifio/La Nifia-Southem Oscillation （ENSO） and the Pacific Decadal Oscillation （PDO）, on the temporal variations of the annual water discharge at the Lijin station in the Huanghe （Yellow） River and at the Datong station in the Changjiang （Yangtze） River were examined. Using the empirical mode decomposition-maximum entropy spectral analysis （EMD- MESA） method, the 2- to 3-year, 8- to 14-year, and 23-year cyclical variations of the annual water discharge at the two stations were discovered. Based on the analysis results, the hydrological time series on the inter- annual to interdecadal scales were constructed. The results indicate that from 1950 to 2011, a significant downward trend occurred in the natural annual water discharge in Huanghe River. However, the changes in water discharge in Changjiang River basin exhibited a slightly upward trend. It indicated that the changes in the river discharge in the Huanghe basin were driven primarily by precipitation. Other factors, such as the precipitation over the Changjiang River tributaries, ice melt and evaporation contributed much more to the increase in the Changjiang River basin. Especially, the impacts of the inter-annual and inter-decadal climate oscillations such as ENSO and PDO could change the long-term patterns of precipitation over the basins of the two major rivers. Generally, low amounts of basin-wide precipitation on interannual to interdecadal scales over the two rivers corresponded to most of the warm ENSO events and the warm phases of the PDO, and vice versa. The positive phases of the PDO and ENSO could lead to reduced precipitation and consequently affect the long-term scale water discharges at the two rivers.

Impacts of anthropogenic activities on the Changjiang(Yangtze)estuarine ecosystem(1998–2012)

Estuarine ecosystem has greatly changed in the recent decades due to anthropogenic perturbations in the Changjiang Estuary. Change patterns and impact factors were analyzed based on the continuous data in relation to the Changjiang estuarine ecosystem from 1998 to 2012. The results showed significant decreases in plankton species and annual output of Coilia nasus, Coilia mystus. Furthermore, species and biomass of benthos showed abrupt change in 2003, downward before that and upward after that. It was noted that, Eriocheir sinensis, a high value commercial fish, had an annual production increase of 97%. Reduction of riverine nutrients, especially dissolved silicate （DSI） loads into the estuary could contribute to the decline in BaciUariophyta species. Dredging and dumping works of the North Passage led to the decreases in regional plankton species. However, the species reproduction and releasing projects could restore the estuarine ecosystem through increasing the output of E. sinensis, as well as species and biomass of benthos.

A new method for the estimation of fine-sediment resuspension ratios in estuaries-taking the turbidity maximum zone of the Changjiang(Yangtze) estuary as an example

Based on the principle of conservative matter removal in estuary,a new method is proposed for estimating the ratio of sediment resuspension in estuaries with fine suspended sediments in the turbidity maximum zone(TMZ) of the Changjiang(Yangtze) estuary during 2005.Results show that there was a range of 18.7%±27.9% to 73.9%±22.5% per annum of total suspended particulate matter(SPM),with an average of 49.2%.Nearly half of the particulate matter in the TMZ originates from sediment resuspension.This indicates that sediment resuspension is one of the major mechanisms involved in formation of the TMZ.Compared with traditional method for calculating these ratios in the estuary,this new method evaluates the dynamic variation of SPM content carried by river runoff from the river mouth to the ocean.The new method produced more reliable results than the traditional one and could produce a better estimation of resuspension flux for particulate matter in estuaries.

Numerical simulation on the process of saltwater intrusion and its impact on the suspended sediment concentration in the Changjiang(Yangtze)estuary

To study the relationship between sediment transportation and saltwater intrusion in the Changjiang (Yangtze) estuary, a three-dimensional numerical model for temperature, salinity, velocity field, and suspended sediment concentration was established based on the ECOMSED model. Using this model, sediment transportation in the flood season of 2005 was simulated for the Changjiang estuary. A comparison between simulated results and observation data for the tidal level, flow velocity and direction, salinity and suspended sediment concentration indicated that they were consistent in overall. Based on model verification, the simulation of saltwater intrusion and its effect on sediment in the Changjiang estuary was analyzed in detail. The saltwater intrusion in the estuary including the formation, evolution, and disappearance of saltwater wedge and the induced vertical circulation were reproduced, and the crucial impact of the wedge on cohesive and non-cohesive suspended sediment distribution and transportation were successfully simulated. The result shows that near the salinity front, the simulated concentrations of both cohesive and non-cohesive suspended sediment at the surface layer had a strong relationship with the simulated velocity, especially when considering a 1-hour lag. However, in the bottom layer, there was no obvious correlation between them, because the saltwater wedge and its inducing vertical circulation may have resuspended loose sediment on the bed, thus forming a high-concentration area near the bottom even if the velocity near the bottom was very low during the transition phase from flood to ebb.

Heavy metals in suspended matters during a tidal cycle in the turbidity maximum around the Changjiang(Yangtze) Estuary

In order to discuss the content distributions and fluxes of heavy metals in suspended matters during a tidal cycle in the turbidity maximum around the Changjiang （Yangtze） Estuary, the contents of heavy metals （Zn, Pb, Cd, Co and Ni） have been analyzed. During a tidal cycle, the average contents of heavy metals are in the order of Zn〉Ni〉Pb〉Co〉〉Cd. The average contents in ebb tide are generally higher than that in flood tide. However, at the inshore Sta. 11, influenced by the contamination from the nearby waste treatment plant, the average contents of Zn and Ni in flood tide are higher than those in ebb fide and at the offshore Sta. 10, the content of Cd in flood tide higher than that in ebb tide due to marine-derived materials. The five heavy metals, mainly terrigenous, are transported towards east-northeast, and settle down with suspended matters in the area between Sta. 11 and Sta. 10. Influenced by marine-derived materials, the flux value of Cd does not alter significantly with obviously changing in flux direction towards northwest. The source of heavy metals, the salinity of water and the concentration of suspended matters are the main factors controlling the content distributions of heavy metals during a tidal cycle. There is a positive correlation between the contents of heavy metals （Zn, Pb, Co and Ni） and the salinity of water, while the opposite correlation between the contents and the concentrations of suspended matters. Because of marine-derived materials, the content of Cd is not correlated with the concentration of suspended matters and the salinity of water.

Approach of moving boundary and its application in 3D tidal current simulation of Changjiang (Yangtze) River Estuary based on ECOMSED model

Considering three-dimensional model ECOMSED can not simulate wetting-drying of shoal with its fixed boundary, an approach to represent moving boundary in the model is introduced here. This approach smoothly joints the internal and external mode by making use of wetting and drying technique and is verified by a numerical test which presents a good agreement with the previous test results obtained by other researchers. A three dimensional numerical model is established to simulate the hydrodynamics in spring tide in the Changjiang （Yangtze） River estuary by this modified ECOMSED model which is also validated through the observed field data, the simulation presents a good periodic tidal change. It also successfully simulates the tidal current of computational areas and reproduces the tidal flat intermittent appearance.

Geochemical processes controlling dissolved selenium in the Changjiang(Yangtze) Estuary and its adjacent waters

Dissolved selenium in the Changjiang（Yangtze） Estuary and its adjacent waters was determined by hydride generation atomic fluorescence spectrometry to elucidate the source, behavior in estuary, adsorption-desorption process and biological role. In surface water, Se（IV） concentration ranged 0.05–1.14 nmol/L and Se（VI） concentration varied 0.01–1.20 nmol/L, with the means of 0.76 and 0.49 nmol/L, respectively. In bottom water, Se（IV） content varied 0.03–0.27 nmol/L and Se（VI） content ranged 0.04–0.85 nmol/L, with the averages of 0.10 and 0.40 nmol/L, respectively. High level of Se（IV） was observed near the shore with a significant decrease towards the open sea, suggesting the continental input from the adjacent rivers. Large value of Se（VI） was found in bottom water, reflecting the release from suspended sediment. Besides, high value appeared in the same latitude of the Changjiang Estuary and Hangzhou Bay illustrated the effect of lateral mixing and the long-distance transport of selenium. Se（VI）, more soluble, occupied higher percentage in aqueous environment. The presence of Se（IV） resulted from the degradation of residue and the reduction of Se（VI） under anaerobic condition. The positive relationship to suspended particulate material（SPM） and negative correlation to depth indicated that Se（IV） tended to be released from the high density particulate matter. Instead, Se（VI） content did not significantly relate to SPM since it generally formed inner-sphere complex to iron hydroxide. Se（IV） content negatively varied to salinity and largely depended on the freshwater dilution and physical mixing. While, Se（VI） level deviated from the dilution line due to the in situ biogeochemical process such as removal via phytoplankton uptake and inputs via organic matter regeneration. As the essential element, Se（IV） was confirmed more bioavailable to phytoplankton growth than Se（VI）, and moreover, seemed to be more related to phosphorus than to nitrogen.

Carbon biomass,production rates and export flux of copepods fecal pellets in the Changjiang(Yangtze) River estuary

Copepod fecal pellets are ubiquitous throughout the oceans. Their production and export can represent a highly efficient pathway of carbon export. However, the role these fecal pellets play in carbon export in the Changjiang(Yangtze) River estuary is not well known. Two cruises were carried out in the Changjiang estuary in the spring and summer of 2013, during which time carbon biomass, production, and export of copepod fecal pellets were studied. Spring and summer fecal pellet carbon biomass ranged 0.30–1.01 mg C/m^3(mean=0.56±0.20 mg C/m^3) and 0.31–1.18 mg C/m^3(mean=0.64±0.24 mg C/m^3), respectively, significantly lower than phytoplankton. At most stations, fecal pellet carbon biomass was higher in surface or subsurface layers than deeper layers. Production rates ranged 0.65–1.49 pellets/(ind.?h)(mean=1.02±0.27 pellets/(ind.?h)) in spring and 0.62–1.34 pellets/(ind.?h)(mean=0.98±0.22 pellets/(ind.?h)) in summer, within the range reported in previous studies. Higher production rates of fecal pellets occurred at stations with higher chlorophyll a concentrations, and production rates of copepods of size 500–1 000 μm greater than copepods >1 000 μm during both cruises. The potential export flux of fecal pellets was slightly higher in summer(mean=68.95±14.37 mg C/(m^2 ?d)) than spring(mean=52.08±11.33 mg C/(m^2 ?d)) owing to higher summer copepod abundances. To our knowledge, this study is the first of its kind in the Changjiang estuary, and it confirms the significant role of copepod fecal pellets in local carbon export.

The environment effect on fish assemblage structure in waters adjacent to the Changjiang(Yangtze) River estuary(1998-2001)

We collected fish abundance data in the Changjiang(Yangtze River) estuary and adjacent waters in November 1998,May 1999,November 2000,and May 2001.Using the data,we evaluated the characteristics of the fish assemblages at each site and investigated the effect of several environmental factors.We used a multivariate analysis,including community ordination methods such as detrended correspondence analysis(DCA) and canonical correspondence analysis(CCA),and two-way indicator species analysis(TWINSPAN).We analyzed the biological community structure and environmental factors to determine their spatial distributions,temporal dynamics,and seasonal variations.Among the fish species,five exceeded 5% of the total abundance:Harpodon nehereus(42.82%),Benthosema pterotum(13.85%),Setipinna taty(11.64%),Thryssa kammalensis(9.17%) and Apogonichthys lineatus(6.49%).These were separated into four ecological assemblages:hypsithermal-saline,hypsithermal-brackish,hypothermal-brackish,and hypothermal-saline.We evaluated the degree of influence of environmental factors on the fish community.Our analyses suggested that environmental factors including water depth,salinity,turbidity,transparency,nutrient,and suspended matter formed a synthetic spatial gradient between the coastal and pelagic areas.Ecological and environmental factors changed temporally from 1998 to 2001,and drove the fish community succession.The environmental factors driving the fish community structure included bottom temperature,water depth,bottom and surface pH,surface total phosphorous,and bottom dissolved oxygen.This investigation was completed before completion of the Three Gorges Dam;therefore the results of this study provide an important foundation for evaluating the influence of the human activities.

Peaked abundance of Calanus sinicus earlier shifted in the Changjiang River (Yangtze River) Estuary:a comparable study between 1959,2002 and 2005

The sea surface temperature （SST） of the East China Sea （ECS） increased in the past decades, which may have a great impact on the ecosystem of the ECS, including the changes in planktonpopulation structure. In this paper, the changes in peaked abundance of Calanus sinicus in the Changjiang River （Yangtze River） Estuary were compared between 1959 and 2002, based on the data collected from the seasonally oceanographic cruises and those performed in spring of 2005. It was much higher in spring compared with that in other seasons both in 1959 and 2002. Furthermore, in spring 2005, the time for occurrence and decrease of the peaked C. sinicus abundance advanced about one month, accompanying the increase in the sea surface water temperature （SST）. It peaked in June and decreased in July in 1959, however, in 2005, it peaked in May and attenuated sharply in early June. The earlier decrease of peaked C. sinicus abundance may further deteriorate the ecosystem in the Changjiang River Estuary and north nearshore of the ECS.

Spatial distribution and deposition history of nonylphenol and bisphenol A in sediments from the Changjiang River(Yangtze River) Estuary and its adjacent East China Sea

Nonylphenol（NP） and bisphenol A（BPA） are endocrine disruptors causing harmful effects including feminization and carcinogenesis to various organisms,and consequently,their contamination in natural environment has received wide concerns.This study reports the distribution characteristics of NP and BPA in surface sediments and their deposition history based on a dated sediment core in the Changjiang River（Yangtze River） Estuary and its adjacent East China Sea.The contents of NP and BPA in surface sediments ranged from 1.56-35.8 and 0.72-13.2 ng/g（dry mass）,respectively,with high values recorded in the two mud zones,the Changjiang River Estuarine Mud Zone and the Zhejiang Coastal Mud Zone.High values in the Zhejiang Coastal Mud Zone suggest the possibility of long distance transport of both contaminants through the Changjiang riverine plume.The contents were not correlated with the distance from the pollution source,indicating other factors including particle deposition rate and sediment grain size obviously affecting the distribution pattern.NP was also detected in a sediment core at layers deposited from the year of 1971 to 2001 with contents of up to 20.9 ng/g（dry mass）.The deposition fluxes of NP varied from 0.68 to 17.9 ng/（cm^2 · a） with peaks and valleys reflecting the traces of economic development history in China during the previous three decades.BPA was detected at sediment layers deposited from 1973 to 2001 with contents of up to 3.66 ng/g.The fluxes of BPA varied from 0.62 to 3.13 ng/（cm^2 · a） showing a similar pattern as NP.The contents of NP and BPA also indicated potential risks on benthic organisms in the study area.

Effect of regional warming on the abundance of P seudeuphausia sinica Wang et Chen(Euphausiacea)off the Changjiang River(Yangtze River)Estuary

Distribution and abundance of Pseudeuphausia sinica off the Changjiang River Estuary （30 ° 00′ – 32 ° 00′ N, 122 ° 00′ –123 ° 30 ′E）, the East China Sea were studied in relation to environmental features associated with the regional warming. P. sinica is a subtropical species. Off the Changjiang River Estuary, its abundance reached maximum in summer. To examine spatial and temporal changes of P. sinica off the Changjiang River Estuary, the authors have combined all available sampling data in 1979, 1981, and 2000–2007. This database shows that a significant increase in abundances of P. sinica was observed in spring of 2000–2007 as compared with 1979, 1981. The abundance of P. sinica increased from 0.18–0.21 ind./m 3 in 1979 and 1981 to 0.68–4.00 ind./m 3 in 2000–2007. Accordingly, the sea temperature increased obviously from spring of 1979, 1981 to the 2000s. The authors further found a positive relationship between average surface temperature and average abundance of P. sinica. Regional warming, together with the release of predator induced stress due to a sharp decline in the abundance of its predator （e.g., fishes）, were thought to be responsible for the increase in abundance of P. sinica in water off the Changjiang River Estuary.

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 （δ13C）. Based on this analysis, about 36 surface sediment samples were selected from various environments and separated into sand （〉0.250 ram, 0.125-0.250 ram, 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 613C 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 δ13C was -21.64‰, -22.03‰, -22.52‰, -22.46‰, -22.36‰, and -22.28%0, 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 ram） 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 δ13C 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.

Seasonal variations of particulate silicon in the Changjiang (Yangtze River) Estuary and its adjacent area

Temporal and spatial distribution of biogenic （BSi） and lithogenic （LSi） silica were studied in the Changjiang （Yangtze River） Estuary and its adjacent area. The annual average BSi and LSi concentrations were （1.714-1.79） #mol/L and （0.564-1.41） mmol/L, respectively. Both BSi and LSi were high ii~. tbe inshore ar- eas, where they received terrigenous discharge from the Changjiang, and decreased towards the offshore region. BSi and LSi were most abundant at the near bottom layer due to the high sedimentation rates and resuspension of sediment. Diatom blooms occurred in summer with high Chl a concentration in the sur- face layer, which induced that BSi in the surface layer during summer was obviously higher than that in the surface layer of other seasons. LSi concentration was maximal in autumn and spring and minimum in summer, associated with the seasonal variation of SPM values. Drifting investigation and mesocosm exper- iments were conducted during dinoflagellate bloom, aiming to understand the effect of nutrients on BSi by changing the phytoplankton composition. The results show that the low dissolved inorganic phosphorus concentration and high molar ratio of N/P （dissolved inorganic nitrogen vs. dissolved inorganic phospho- rus）, were the important factors for decreasing diatom biomass in the study area, and it would subsequently decrease the BSi concentration in aquatic ecosystem.

Analysis of the characteristics of offshore currents in the Changjiang(Yangtze River) estuarine waters based on buoy observations

A buoy of 10 m in diameter was used to record the current speed and direction in a vertical profile in the offshore area of the Changjiang（Yangtze River） Estuary（with an average water depth of 46.0 m） for one year.The results include：（1） the currents rotate clockwise and the current direction is consistent in a vertical profile without clear seasonal variations.（2） The horizontal current speeds are generally high,with a maximum of 128.5 cm/s occurring in summer and 105.5 cm/s appearing in winter commonly close to the surface.The average current speeds in the vertical profile fall in the same range（the differences are less than 8.0 cm/s）,with the maximum of47.0 cm/s occurring in summer and 40.8 cm/s in winter.The average current speed during spring tides is twice that during neap tides（26.5 cm/s）.（3） Significant differences of speeds are observed in the vertical profile.The maximum current speed occurs at either surface（spring and winter） or sub-surface（summer and autumn）,with the minimum current speed appearing at the bottom.The maximum average current speed of all layers is 57.9cm/s,which occurs in the 18-m layer during summer.（4） The average speed of the residual currents ranges from7.5 cm/s to 11.3 cm/s,with the strongest occurring in spring and weakest in winter.The residual currents of all layers are eastward during spring and winter,whereas northeastward or northward during summer and autumn.（5） The currents in the offshore of Changjiang Estuary are impacted collectively by diluted Changjiang River discharge,the Taiwan Warm Current,monsoon and tides.