Mechanism of the Impact of Floods and Typhoons on the Morphological Evolution of the Yangtze Estuary

In the last two decades,the Yangtze Estuary has undergone significant changes under the influence of reduced sediment inflow and estuary engineering.This study investigates the influence of floods and typhoons on sediment concentration and the morphological evolution of shoals and channels in the Yangtze Estuary.The analysis is conducted through the utilization of topographic data measured pre-and post-flood events and observations of hydro-sedimentary changes during typhoons.By using a generalized estuary mathematical model,this study examines the interplay between varying tidal ranges,tidal divisions,runoff volumes,and regulation projects on the erosion and deposition of shoals and channels in bifurcated estuaries.The results show that due to the implementation of river and waterway regulation projects,the impact of the 2020 flood on the main channel and shoal was significantly less than that of the1998 flood.The swing amplitude of the South Branch main channel decreased.However,local river sections such as the Southern Waterway of Baimao Shoal exhibited erosion.During typhoons,sediment concentration in the 20 cm above the bottom increased significantly and was closely related to wave processes,with a weakened correlation to tidal dynamics.After typhoons,high shoals in South Passage above 0 m were silted up,while the terrain on one side of the tail of Jiuduan Shoal in the downstream deep-water area was generally scoured due to strong wave action.The generalized mathematical model of the bifurcated estuary revealed that M2 tidal component contributed most to the ero sion and deposition evolution of estuary shoals and channels,with floods exhibiting characteristics of sedime ntation on shoals and erosion on channels.With the implementation of a branch rectification project,branch resistance increased,diversion decreased,and the riverbed changed from pre-project erosion to post-project sedimentation,with an increase in erosion in non-project branches.

Biogenic silica in intertidal marsh plants and associated sediments of the Yangtze Estuary

Biogenic silica （BSi） contents in the marsh plants （Phragmites australis, Scirpus mariqueter and Spartina alterniflora） and associated sediments in Chongming Island eastern intertidal flat of the Yangtze Estuary were determined. The BSi contents in P. australis, S. mariqueter and S. alterniflora varied from 25.78–42.74 mg/g, 5.71–19.53 mg/g and 6.71–8.92 mg/g, respectively. Over the entire growth season, P. australis and S. mariqueter were characterized by linear accumulation patterns of BSi. The aboveground biomass （leaves and culms） of the marsh plants generally contained more BSi than underground biomass （roots）. BSi contents were relatively higher in dead plant tissues than in live tissues which was probably due to the decomposition and the leaching of labile components of plant tissues such as organic carbon and nitrogen. Comparing with the habitats of S. mariqueter and S. alterniflora, the highest BSi content was recorded in sediments inhabited by P. australis, with an annual average of 15.69 mg/g. Overall, the intertidal marshes in the Yangtze Estuary may act as a net sink of BSi via plant uptake and sedimentary burial.

Seasonal bioconcentration of heavy metals in Onchidium struma(Gastropoda:Pulmonata)from Chongming Island,the Yangtze Estuary,China

The seasonal concentration changes of selected heavy metal Cd, Cr, Cu, Fe, Mn, Pb, and Zn in five tissues of marine gastropod Onchidium struma were studied in the Chongming Island, the Yangtze Estuary in April 2007, July 2006, September 2006, and November 2006, respectively. The results demonstrated that the bioconcentration factor of Cu （biomass/water） in all selected tissues was about 104 magnitudes, Fe and Cd were 103, Zn was 102, and Mn, Pb, and Cr were 101. Hepatopancreas was proven to be the dominant storage tissue of Cr, Cu, Mn, and Zn, whereas Fe and Pb were mainly stored in muscle and digenetic gland, and Cd was stored in vitelline gland and albumen gland. Additionally, it was found that Cu, Fe, Mn, and Zn were concentrated significantly by O. struma （whole-body） in summer or autumn, and Cd, Cr, and Pb increased slightly in spring and winter. Furthermore, the bioconcentration of Cr was nearly 2-fold higher and Zn was 1.6-fold higher in the water compared with the Water Quality Standard for Fisheries. With view of excessive amount of Pb, Cd, and Cu according to seafood standard, the consumption of O. struma might have the risk of health hazard.

Distribution and accumulation of biogenic silica in the intertidal sediments of the Yangtze Estuary

Sedimentary biogenic silica is known to be an important parameter to understand biogeochemical processes and paleoenviromental records in estuarine and coastal ecosystems. Consequently, it is of great significance to investigate accumulation and distribution of biogenic silica in sediments. The two-step mild acid-mild alkaline extraction procedure was used to leach biogenic silica and its early diagenetic products in intertidal sediments of the Yangtze Estuary. The results showed that total biogenic silica （t-BSi） in the intertidal sediments varied from 237.7-419.4 μmol Si/g, while the mild acid leachable silica （Si-HCl） and the mild alkaline leachable silica （Si- Alk） were in the range of 25.1-72.9 μmol Si/g and 208.1-350.4 μmol Si/g, respectively. Significant correlations were observed for the grain size distributions of sediments and different biogenic silica pools in intertidal sediments. This confirms that grain size distribution can significantly affect biogenic silica contents in sediments. Close relationships of biogenic silica with organic carbon and nitrogen were also found, reflecting that there is a strong coupling between biogenic silica and organic matter biogeochemical cycles in the intertidal system of the Yangtze Estuary. Additionally, the early diagenetic changes of biogenic silica in sediments are discussed in the present study.

Trends of soil organic matter turnover in the salt marsh of the Yangtze River estuary

Characteristics and tidal flat trends of soil organic matter （SOM） turnover were studied for the Chongmingdongtan Salt Marsh in the Yangtze River estuary, based on analyses of stable carbon isotope composition （δ^13C）, grain sizes and contents of particulate organic carbon （POC）, total nitrogen （TN） and inorganic carbon （TIC） for three cores excavated from high tidal flat, middle tidal flat and bare flat. Results demonstrate that correlations between soil POC contents and δ^13C values of the salt marsh cores were similar to those between soil organic carbon （SOC） contents and δ^13C values of the upper soil layers of mountainous soil profiles with different altitudes. SOM of salt marsh was generally younger than 100 years, and originated mainly from topsoil erosions in catchments of the Yangtze River. Correlations of TN content with C/N ratio, POC content with TIC content and POC content with δ^13C values for the cores suggest that turnover degrees of SOM from the salt marsh are overall low, and trends of SOM turnover are clear from the bare flat to the high tidal flat. Bare flat samples show characteristics of original sediments, with minor SOM turnover. Turnover processes of SOM have occurred and are discernable in the high and middle tidal flats, and the mixing degrees of SOM compartments with different turnover rates increase with evolution of the muddy tidal flat. The exclusive strata structure of alternate muddy laminae and silty laminae originated from dynamic depositional processes on muddy tidal flat was a great obstacle to vertical migration of dissolved materials, and SOM turnover was then constrained. The muddy tidal flat processes exerted direct influences on sequestration and turnover of SOM in the salt marsh, and had great constraints on the spatial and temporal characteristics of SOM turnover of the Chongmingdongtan Salt Marsh in the Yangtze River estuary.

Sediment Deposition and Resuspension in Mouth Bar Area of the Yangtze Estuary

A comprehensive analysis is conducted based on observations on topography, tidal current, salinity, suspended sediment and bed load during the years of 1982, 1983, 1988, 1989. 1996, and 1997 in the Yangtze Estuary. Results show that the deformation of tidal waves is distinct and the sand carrying capacity is large within the mouth bar due to strong tidal currents and large volume of incoming water and sediments. Owing to both temporal and spatial variation of tidal current, deposition and erosion ore extremely active. In general a change of up to 0.1 m of bottom sediments takes place during a tidal period. The maximum siltation and erosion are around 0.2 m in a spring to neap tides cycle. The riverbed is silted during flood when there is heavy sediment load, eroded during dry season when sediment lo:ld is low. The annual average depth of erosion anti siltation on the riverbed is around 0.6 m. In particular cases, it may increase to 1.4 m to 2.4 m at some locations.

Numrical Model of Total Sediment Transport in the Yangtze Estuary

Based on the non-equilibrium suspended load transport equation, bed load transport equation and sediment transport capacity formulas derived by Don et al. , a 2-D numerical model of total sediment transport in the Yangtze Estuary is presented. In the model, the actions of tidal currents and wind waves and the effect of salinity on sediment transport are considered. An automatically generated boundary-fitted grid is used to fit the boundaries of the estuary and the boundaries of engineering projects. The verification of calculations shows that the sediment concentration, the deformation of riverbed and siltation in the channels caused by typhoons can be successfully simulated.

Spatiotemporal Dynamics of Coastal Wetlands and Reclamation in the Yangtze Estuary During Past 50 Years(1960s–2015)

Reclamation is one of the fastest-growing land use type developed in coastal areas and has caused degradation and loss of coastal wetlands as well as serious environmental problems. This paper was aimed at monitoring the spatiotemporal patterns of coastal wetlands and reclamation in the Yangtze Estuary during the 1960s and 2015. Satellite images obtained from 1980 to 2015 and topography maps of the 1960 s were employed to extract changes of reclamation and coastal wetlands. Area-weight centroids were calculated to identify the movement trend of reclamation and coastal wetlands. The results show that from the 1960 s to 2015, the net area of natural wetlands declined by 574.3 km^2, while man-made wetlands and reclamation increased by 553.6 and 543.9 km^2, respectively. During the five study phases, the fastest areal change rate natural wetlands was –13.3 km^2/yr in the period of 1990–2000, and that of man-made areas was 24.7 km^2/yr in the same period, and the areal change rate of reclamation was 27.6 km^2/yr in the period of 2000–2010. Conversion of coastal wetlands mainly occurred in the Chongming Island, Changshu City and the east coast of Shanghai Municipality. Reclamation was common across coastal areas, and was mainly attributed to settlement and man-made wetlands in the Chongming Island, Lianyungang City and the east coast of Shanghai Municipality. Natural wetlands turned into farmlands and settlement, and man-made wetlands gained from reclamation of farmlands. The centroid of natural wetlands generally moved towards the sea, man-made wetlands expanded equally in all directions and inland, and the centroid of reclamation migrated toward Shanghai Municipality. Sea level rise, erosion-deposition changes, and reclamation activities together determine the dynamics of the Yangtze Estuary wetlands. However, reclamation activities for construction of ports, industries and aquaculture are the key causes for the dynamics. The results from this study on the dynamics of coastal wetlands and reclamation are valuable for local government to put forward sustainable land use and land development plans.

Study on Nutrient Limitation of Phytoplankton in the Field Experiment of the Yangtze River Estuary in Summer

From July 23rd to August 15th, 2001, a field cultivation experiment was carried out to determine the limitation factors of phytoplankton in the Yangtze River estuary and the adjacent areas. The results indicated that the potential limiting nutrient was phosphorus in the Yangtze River diluted water area, nitrogen in the offshore of the Yangtze River estuary and the conversion of phosphorus to nitrogen in the middle area. Iron and silicon were not the potential limiting factors. If there were some kinds of limiting factors in the water, the growth of phytoplankton would be limited obviously. In case of disappearance of the limiting factor, the phytoplankton would grow fast. When the Noctiluca scintiuans bloom occurred, the phytoplankton biomass level was very low in a short time due to the grazing pressure. When the grazing pressure disappeared, the phytoplankton would grow quickly in abundant nutrients condition.

Identification of the sources of polycyclic aromatic hydrocarbons based on molecular and isotopic characterization from the Yangtze estuarine and nearby coastal areas

Samples of suspended particulate matters （SPMs）, surface sediment and road dust were collected from the Yangtze estuarine and nearby coastal areas, coastal rivers, and central Shanghai. The samples were analyzed for the presence of 16 polycyclic aromatic hydrocarbons （PAHs） in the USEPA priority-controlled list by GC-MS. The compound-specific stable carbon isotopes of the individual PAHs were also analyzed by GC-C-IRMS. The sources of PAHs in the SPMs and surface sediments in the Yangtze estuarine and nearby coastal areas were then identified using multiple source identification techniques that integrated molecular mass indices with organic compound-specific stable isotopes. The results revealed that 3-ring and 4-ring PAH compounds were dominant in the SPMs and surface sediments, which are similar to the PAH compounds found in samples from the Wusong sewage discharge outlet, Shidongkou sewage disposal plant, Huangpu River, coastal rivers and central Shanghai. Principal component analysis （PCA） integrated with molecular mass indices indicated that gasoline, diesel, coal and wood combustion and petroleum-derived residues were the main sources of PAHs in the Yangtze Estuary. The use of PAH compound-specific stable isotopes also enabled identification of the PAHs input pathways. PAHs derived from wood and coal combustion and petroleum-derived residues were input into the Yangtze Estuary and nearby coastal areas by coastal rivers, sewage discharge outlets during the dry season and urban storm water runoff during the flood season. PAHs derived from vehicle emissions primarily accumulated in road dust from urban traffic lines and the commercial district and then entered the coastal area via the northwest prevailing winds in the dry season and storm water runoff during flood season.

INFLUENCES OF THE FISH-MOUTH PROJECT AND THE GROINS ON THE FLOW AND SEDIMENT RATIO OF THE YANGTZE RIVER WATERWAY

A depth_integrated two_dimensional numerical model of current, salinity and sediment transport was proposed and calibrated by the observation data in the Yangtze River Estuary. It was then applied to investigate the flow and sediment ratio of the navigation channel, i.e. the North Channel of the Yangtze River Estuary,before and after the first phase waterway project is implemented. Particularly, the influences of groin length and the orientation of the submerged dam on the flow ratio and sediment load discharging into the North Channel were discussed. The numerical results demonstrate that less sediment load discharges into the navigation channel, which unburdens the waterway dredging, but in the meantime the flow ratio is also decreased. The flow and sediment ratio can be adjusted by changing layout and dimensions of the hydro_structures, such as the groin length, the top height, etc. The effect of the orientation of the submerged dam is more obvious than the groin lengh.

A Comparative Study of Fish Community in Four Main Estuaries of China Southeastern Coastal Areas and Their Adjacent Waters

Based on literature and survery data of fish composition collected by bottom trawl investigation from 2006 to 2007 infour main estuaries of China southeastern coastal areas and their adjacent waters,changes of the taxonomic diversity across spatialand temporal scales of fish community were analyzed by taxonomic diversity indices.The results are as follows:a total number of1397 fish species(including some freshwater species),belonging to 2 classes,42 orders,186 families and 593 genera,were collected inthe studied sea areas.The species richness increased with lower latitudes,particularly so with Perciformes.There were 339 fish spe-cies in the Yangtze River Estuary and adjacent waters,belonging to 2 classes,31 orders,101 families and 231 genera.There were 535fish species in the Minjiang River Estuary and adjacent waters,belonging to 2 classes,33 orders,133 families and 323 genera.A totalnumber of 803 fish species were collected in the Jiulongjiang River Estuary and adjacent waters,which belonged to 2 classes,35 orders,155 families and 419 genera.And 1021 fish species which belonged to 2 classes,32 orders,153 families,and 466 genera were collectedin the Pearl River Estuary and adjacent waters.The numbers of orders and families of fish species from the northern to the southern wa-ters first increased and then decreased.The average variation in taxonomic distinctness(Λ+)gradually decreased with lower latitudes inthe four estuaries and adjacent waters.There were no significant differences in the average taxonomic distinctness(△+)among the fourestuaries and adjacent waters,and fish fauna were closely related with each other,and all of them belonged to the same zoogeographicalfauna(Indian-Malaysia fauna).Fish composition in the Minjiang River Estuary and adjacent waters was more similar to that in the Jiu-longjiang River Estuary and adjacent waters among the four estuaries.Compared with the historical data,the average taxonomic dis-tinctness of fish community showed a great decrease in the Minjiang River Estuary and the Jiulongjiang River Estuary and their adjacentwaters.

Variations of mercury distribution in the water column during the course of a tidal cycle in the Yangtze Estuarine intertidal zone,China

Tidally induced resuspension processes play an important role in the release of mercury (Hg) into the water column, which increases the risk of Hg exposure to estuarine eco-systems. In order to further understand the geochemical activities of Hg in the intertidal area, the temporal variations of dissolved Hg (Hg D ) and particulate Hg (Hg P ) in the water column during the course of a tidal cycle and its geochemical processes were studied in the southern intertidal zone of the Yangtze Estuary, China. The concentrations of Hg D and Hg P varied between 37-612 ng/L and 51-638 ng/L respectively during the tidal cycle. The increase of Hg D was distinguished at the early flood tide and late ebb tide when the water flow rates were higher. The Hg D concentrations were negatively correlated with Hg P (r = 0.523, p < 0.05) and positively correlated with dissolved organic carbon (DOC) (r = 0.605, p < 0.05) in the bottom water, indicating that the Hg D released from the sediments into the overlying water was associated with the simultaneously released colloidal material in the bottom water. The main pathways for the translocation of Hg from the sediments to the overlying water include the processes of desorption from resuspended particles, advection or diffusion from sediments, and the oxidation of resuspended sulfide. The results of principal components analysis (PCA) and Pearson correlation analysis showed that the combined effects of the total suspended substrate (TSS), DOC, pH and dissolved oxygen (DO) influenced the geochemical activities of Hg in the water column during the course of a tidal cycle.