MIXING OF SALT WATER AND FRESH WATER IN THE CHANGJIANG RIVER ESTUARY AND ITS EFFECTS ON SUSPENDED SEDIMENT

Using field hydrological data, the relationship between the mixing of salt water and fresh water and the tidal range/ high tidal level in the Changjiang (Yangtze) River estuary is discussed, and the transporting and concentrating of suspended sediment in the estuary were also analysed in respect to the circulation, flocculation and stratified interface resulting from mixing.The calculation results by two-dimentional box model have confirmed the effects of the circulation on the concentrating of suspended sediment in the estuary. The conclusions derived from this work have deepened the understanding on the mixing in the Changjiang River estuary and are of significance in bo’th theory and practice.

The environmental implication of diatom fossils in the surface sediment of the Changjiang River estuary(CRE) and its adjacent area

In order to depict the distribution of diatom fossils in surface sediments and to establish a reliable reference data for further paleoenvironmental study in the Changjiang (Yangtze) River estuary and its adjacent waters, the diatom fossils from 34 surface sediment samples and their relationship with environmental variables were analyzed by principal component analysis and redundancy correspondence analysis. The diversity and abundance of diatom fossils were analyzed. Some annual average parameters of the overlying water (salinity, temperature, turbidity, dissolved oxygen, depth, dissolved inorganic nitrogen, dissolved inorganic phosphate and dissolved inorganic silicate) were measured at each sampling site. A total of 113 diatom taxa and one silicoflagellate species were identified in the investigation area. Diatom fossils were better preserved in fine sediments. The absolute abundance of diatom fossils did not significantly diff er between inshore and off shore areas, the species diversity decreased from inshore to off shore. This may be because high nutrients and low salinity promoted the growth of more brackish species in coastal waters. The diatom taxa were divided into three groups, on the basis of their response and indication to environmental changes. For example, Actinocyclus ehrenbergii and Cyclotella stylorum were dominant in coastal waters (Group 1 and Group 3) with high nutrients and low salinity;the relative abundances of Paralia sulcata and Podosira stelliger were significantly higher in off shore sites (Group 2, average 39.5%), which were characterized by high salinity and deep water. Four environmental variables (salinity, dissolved inorganic nitrogen, temperature and water depth) explained the composition and distribution of diatom taxa independently ( P< 0.05), this finding can be applied in further paleoenvironmental reconstruction research in this area.

Terrestrial input and nutrient change reflected by sediment records of the Changjiang River Estuary in recent 80 years

A variety of environmental problems have been observed in the Changjiang River Estuary and adjacent coastal area, including eutrophication, harmful algal blooms（HABs）, and hypoxia in recent decades. Application of sedimentary biogenic element indicators on the study of paleoenvironment can reconstruct environmental evolution history of waters. Two 210Pb-dated cores were collected from the Changjiang River Estuary（S3） and adjacent coastal area（Z13）, and total organic carbon（TOC）, total nitrogen（TN）, biogenic silicon（BSi）, total phosphorus（TP） and phosphorus（P） species were analyzed. Three stages of environmental changes are deduced by the nutrient sedimentary records. First, nutrient concentration increased rapidly since the 1950 s, which attributed to agriculture development and overused chemical fertilizers. Second, nutrient concentration kept high and primary production began to promote during the 1960 s to 1980 s, while diatom abundance and proportion began to decline since the 1970 s, accompanied by reduced 23 Si O- concentration and flux from the river. Third, due to several dams and bridges constructed, river runoff and coastal hydrodynamic conditions reduced to a certain extent since the 1990 s, which aggravated the unbalance in nutrient structure. Multi-nutrient proxies in sediment can reflect the natural environm-ental changes as well as influence of human activities.

Records of bulk organic matter and plant pigments in sediment of the ＂red-tide zone＂ adjacent to the Changjiang River estuary

Cultural eutrophication caused by nutrient over-enrichment in coastal waters will lead to a cascading set of ecosystem changes and deleterious ecological consequences,such as harmful algal blooms(HABs) and hypoxia.During the past two decades since the late 1990s,recurrent large-scale HABs(red tides)and an extensive hypoxic zone have been reported in the coastal waters adjacent to the Changjiang River estuary.To retrieve the history of eutrophication and its associated ecosystem changes,a sediment core was collected from the "red-tide zone" adjacent to the Changjiang River estuary.The core was dated using the^(210)Pb radioisotope and examined for multiple proxies,including organic carbon(OC),total nitrogen(TN),stable isotopes of C and N,and plant pigments.An apparent up-core increase of OC content was observed after the 1970s,accompanied by a rapid increase of TN.The concurrent enrichment of δ^(13)C and increase of the C/N ratio suggested the accumulation of organic matter derived from marine primary production during this stage.The accumulation of OC after the 1970 s well reflected the significant increase of primary production in the red-tide zone and probably the intensification of hypoxia as well.Plant pigments,including chlorophyll a,p-carotene,and diatoxanthin,showed similar patterns of variation to OC throughout the core,which further confirmed the important contribution of microalgae,particularly diatoms,to the deposited organic matter.Based on the variant profiles of the pigments representative of different microalgal groups,the potential changes of the phytoplankton community since the 1970s were discussed.

Sediment Transport Capacity Under the River–Tide Interaction in the Changjiang Estuary

Sediment transport capacity is a fundamental parameter in sediment transport theory and its accurate calculation is important from both theoretical and engineering viewpoints. The capacity of sediment transport has been studied extensively by many researchers in the last decades. Nevertheless, the underlying mechanism behind sediment transport capacity in estuaries remains poorly understood. The current study aims to explore the impact of the river–tide interaction on sediment transport and establish a formula of sediment transport capacity under the river–tide interaction. The impact of the river–tide interaction on the hydrodynamics and sediment dynamics in the Changjiang Estuary was analyzed, a practical method for describing the variation in tide-runoff ratio was established,and a formula of sediment transport capacity considering the impact of river–tide interaction was proposed by introducing the tide-runoff ratio. The new method bridged the gap between two well-known sediment transport capacity methods by considering the variation in the index a for the gravitational term and overcomes the drawback of distinguishing flood/dry season or spring/ebb tide in the calculation of estuarine sediment transport. A large amount of flow and sediment data obtained from the Changjiang Estuary were collected to verify the proposed formula. The effect of salt-fresh water mixture and the morphological evolution on sediment transport capacity of the Changjiang Estuary were discussed.

Intratidal and neap-spring variations of suspended sediment concentrations and sediment transport processes in the North Branch of the Changjiang Estuary

Profles of tidal current and suspended sediment concentration （SSC） were measured in the North Branch of the Changjiang Estuary from neap tide to spring tide in April 2010. The measurement data were analyzed to determine the characteristics of intratidal and neap-spring variations of SSC and suspended sediment transport. Modulated by tidal range and current speed, the tidal mean SSC increased from 0.5 kg/m3 in neap tide to 3.5 kg/ma in spring tide. The intratidal variation of the depth-mean SSC can be summarized into three types： V-shape variation in neap tide, M-shape and mixed M-V shape variation in medium and spring tides. The occurrence of these variation types is controlled by the relative intensity and interaction of resuspension, settling and impact of water exchange from the rise and fall of tide. In neap tide the V-shape variation is mainly due to the dominant effect of the water exchange from the rise and fall of tide. During medium and spring tides, resuspension and settling processes become dominant. The interactions of these processes, together with the sustained high ebb current and shorter duration of low-tide slack, are respon- sible for the M-shape and M-V shape SSC variation. Weakly consolidated mud and high current speed cause significant resuspension and remarkable flood and ebb SSC peaks. Setting occurs at the slack water periods to cause SSC troughs and formation of a thin flufflayer on the bed. Fluxes of water and suspended sediment averaged over the neap-spring cycle are all seawards, but the magnitude and direction of tidal net sediment flux is highly variable.

Flood-ebb asymmetry in current velocity and suspended sediment transport in the Changjiang Estuary

Time series measurements were conducted on suspended sediment and current velocity from neap tide to spring tide in the South Branch of the upper Changjiang Estuary in the summer of 2011. Strong flood-ebb asymmetry in the current velocity was observed in the South Branch as a result of high river runoff and tide deformation, in which the magnitude and duration of ebb currents were significantly greater than those of flood currents. The suspended sediment concentration（SSC） and suspended median grain size also exhibited remarkable flood-ebb variation; these variables were considerably larger during the ebb than during the flood and increased from neap to spring tide. Affected by the strong asymmetry in the current velocity and SSC between the flood and ebb,suspended sediment flux during the ebb was notably larger than during the flood, and a seaward tidal net flux was observed in each tidal cycle. The balance of sediment flux illustrates that the seaward sediment transport was dominated by river flow and tidal trapping and the landward sediment transport was dominated by the Stokes drift and the shear effect. Notable resuspension occurred during the spring and moderate tides. The critical velocity for the resuspension of bed sediments was estimated based on the correlation between current velocity with SSC and suspended median grain size. The results show that the critical velocity was approximately 40 cm/s during the flood phases and approximately 80 cm/s during the ebb phases because the surficial flood bed sediments located in the lower reach are much finer than the surficial ebb bed sediments located in the upper reach. The flood-ebb variation in the critical erosion velocity has significant effect on the intratidal variation of SSC and sediment transport process, and it is a common phenomenon in many estuaries of the world due to the complicated spatial distribution of bed sediments.

Does reduced sediment load contribute to increased outbreaks of harmful algal blooms off the Changjiang Estuary?

Harmful algal blooms（HABs） have been increasingly frequent in coastal waters around the world over the last several decades. Accelerated coastal eutrophication, resulting from the increased anthropogenic loadings of nutrients, is commonly assumed to be the primary cause of this increase. However, although important,accelerated coastal eutrophication may not be the only explanation for the increasing blooms or toxic outbreaks in estuarine waters. Changes in riverine material fluxes other than nutrients, such as sediment load, may significantly affect biological activities and HAB incidence in estuarine and coastal waters. A case study off the Changjiang（Yangtze River） Estuary indicated that with the increasing riverine loadings of nutrients, the sediment load from the Changjiang River has been reduced by 70% over the past four decades. A comparison of long-term data revealed that the phytoplankton biomass maximum has expanded to a region of much lower salinity due to the drastic reduction in riverine sediment load and the subsequent improvement in light penetration in the Changjiang River plume. Furthermore, there was an apparent mirror-image relationship between the sediment load from the Changjiang River and the HAB incidence off the Changjiang Estuary over the past four decades, and the number of HAB incidents was significantly negatively correlated with the sediment load. Therefore, it is argued that the drastic decline in sediment load from the Changjiang River reduced turbidity in the Changjiang Estuary and thus contributed to the increased frequency of HABs in the buoyant discharge plumes.

Recent changing patterns of the Changjiang(Yangtze River) Estuary caused by human activities

To evaluate the controlling factors for coastline change of the Changjiang（Yangtze River） Estuary since 1974,we extracted the mean high tide line from multi-temporal remote sensing images that span from 1974 to 2014 at 2-year intervals.We chose 42 scenes to constrain the changing pattern of the Changjiang Estuary coastline,and implemented GIS technology to analyze the area change of the Changjiang（Yangtze） Subaerial Delta.Runoff,sediment discharge and coastal engineering were withal considered in the analysis of the coastline changes.The coastline has transgressed seaward since 1974,and a part of it presents inter-annual variations.The area of the Changjiang Subaerial Delta increased by 871 km2,with a net accretion rate of 21.8 km2/a.Based on the change of sediment discharge due to the major projects in the Changjiang River Basin,we divided the changing pattern of the coastline into three stages：the slow accretion stage（1974–1986）,the moderate accretion stage（1987–2002）,and the rapid accretion stage（2003–2014）.Liner regression analysis illustrated that there is a significantly positive correlation between the area changes and sediment discharge in the Chongming Eastern Shoal and Jiuduansha.This suggested that sediment load has a fundamental effect on the evolution of the Changjiang Estuary.Construction of Deep Waterway in the North Passage of the Changjiang River（1998–2010） led to a rapid accretion in the Hengsha Eastern Shoal and Jiuduansha by influencing the hydrodynamics in North Passage.Coastal engineering such as reclamation and harbor construction can also change the morphology of the Changjiang Estuary.We defined a contribution rate of area change to assess the impact of reclamation on the evolution of Changjiang Estuary.It turned out that more than 45.3% of area increment of the Changjiang Estuary was attributed to reclamation.

Identification and quantification of lead source in sediment in the northern East China Sea using stable lead isotopes

Stable Pb isotopes in surface and core sediments were determined to identify the sources of Pb contaminants in the northern East China Sea(ECS).The Bayesian stable isotope mixing model was used to quantify the contributions of Pb sources.The results show that since the late 1980 s,ratios of^(207)Pb/^(206)Pb and^(208)Pb/^(206)Pb increased in the top 34-cm sediment shown in the coastal core samples,reflecting elevated anthropogenic Pb input in coastal sea.Seaward increase of^(207)Pb/^(206)Pb and^(208)Pb/^(206)Pb ratios in surface sediments reveals that anthropogenic Pb came mainly via atmospheric transmission into the ECS.Anthropogenic sources accounted for 12.0%-21.1%of the total Pb in sediments after the 1990 s.Coal combustion was the largest anthropogenic contributor(47.5%±18.8%),and Pb mining and smelting,cement production,and vehicle exhaust/gasoline contributed 23.2%±7.1%,19.0%±13.0%,and 10.3%±6.9%,respectively.The proportions of the anthropogenic sources gradually increased while geogenic source(riverine sediment)decreased from the coast to the outer shelf.This study demonstrated that the significant influence of atmospheric input of Pb contaminants into the ECS,and also the urgent need to control coal combustion and Pb discharge from industrial dust and fume emission in China.It also highlights the promising application of the Simmr model to quantify the proportions of multiple sources of trace elements in an environment.

Spatiotemporal dynamics of ecological variation of waterbird habitats in Dongtan area of Chongming Island

Based on Landsat TM images, we explored the pattern of variation of suitable waterbird habitats from 1990 to 2008 in the Dongtan area of Chongming Island at the Changjiang (Yangtze) River mouth. By applying our highly accurate indicator model (R=0.999, P<0.01), we quantified the variations of fluctuation intensity for local waterbird habitats during 1990-2008, and for the main waterbird groups (Anatidae, Charadriidae, Ardeidae and Laridae) from 2006 to 2008, to evaluate the impact of habitat quantity change on the waterbird habitat status and the population dynamics of the different waterbird groups. The results show that the aquaculture ponds (AP) and the Scirpus mariqueter zone (SMZ) underwent drastic habitat changes during certain periods (AP: 1997-2000, 2000-2003, 2005-2008; SMZ: 1997-2000), and the fluctuation intensity differed among habitat types in the order AP>SMZ>TSH (total suitable habitat)>BSA (bare mud flat and shallow water area). The abandonment of tracts of aquaculture ponds in Dongtan in mid-2006 brought about an intensive population fluctuation, caused by rapidly changing habitat with the population expanding to adjacent areas. At present, Anatidae and Ardeidae are threatened in the Dongtan area with declining populations because of their very "picky" habitat requirements (i.e., high reliance on AP). The Charadriidae experienced enormous population declines in the late 1990s, however, they have since recovered to normal levels as habitat change has stabilized. Our findings suggest that the current challenges for habitat management are the protection and stabilization of AP and SMZ habitats.

长江口北槽最大浑浊带形态影响因素数值模拟研究

为了探明层化、絮凝沉降和可悬浮泥沙量对长江口北槽最大浑浊带形态的影响,基于长江口三维水流、盐度、泥沙耦合模型对三种影响因素开展数值模拟研究。研究结果表明:悬沙制紊可以在盐度制紊基础上进一步加强水体层化作用,促进形成近底高浓度泥沙分布形态。北槽上段受盐水入侵影响小于中下段,层化制紊作用相应较弱,这也是北槽上段悬沙垂向分布较为均匀的重要原因;泥沙絮凝沉速在北槽断面呈现显著的垂向和纵向变化,对最大浑浊带悬沙垂向分布形态影响也具有较明显的沿航道变化特征;悬浮泥沙量沿北槽不均匀分布是造成最大浑浊带含沙量与沿程流速没有良好对应关系的原因,可悬浮泥沙量增加不仅可以提高最大浑浊带整体含沙量,并且可以通过影响层化以及絮凝沉降加强泥沙垂向捕集作用,促进近底高浓度泥沙分布形态。

Preservation of black carbon in the shelf sediments of the East China Sea

Concentrations and carbon isotopic(14C,13C) compositions of black carbon(BC) were measured for three sediment cores collected from the Changjiang River estuary and the shelf of the East China Sea. BC concentrations ranged from 0.02 to 0.14 mg/g(dry weight) ,and accounted for 5% to 26% of the sedimentary total organic carbon(TOC) pool. Among the three sediment cores collected at each site,sediment from the Changjiang River estuary had relatively high BC contents compared with the sedi-ments from the East China Sea shelf,suggesting that the Changjiang River discharge played an im-portant role in the delivery of BC to the coastal region. Radiocarbon measurements indicate that the ages of BC are in the range of 6910 to 12250 years old B.P.(before present) ,that is in general,3700 to 9000 years older than the 14C ages of TOC in the sediments. These variable radiocarbon ages suggest that the BC preserved in the sediments was derived from the products of both biomass fire and fossil fuel combustion,as well as from ancient rock weathering. Based on an isotopic mass balance model,we calculated that fossil fuel combustion contributed most(60%―80%) of the BC preserved in these sediments and varied with depth and locations. The deposition and burial of this "slow-cycling" BC in the sediments of the East China Sea shelf represent a significant pool of carbon sink and could greatly influence carbon cycling in the region.

长江口疏浚弃土悬沙对2种浮游植物生长的影响

长江口疏浚弃土悬沙对微绿球藻(Nannochloropsis oculata)和牟氏角毛藻(Chaetoceros muelleri)的生长有一定的抑制作用。试验结果表明,当水体中含沙量持续96 h 达到3 g/L 时,藻类生长速率降低20%～30%。但当疏浚作业停止,悬沙将迅速沉淀,水体变清,藻类生长可恢复正常。

长江口冲积岛岸滩剖面形态和冲淤规律

根据海岸带和海岛两次调查的实测资料和水下地形图，用动力地貌学方法对长江口三岛岸滩剖面的形态和冲淤变化进行研究。结果表明，分汊河口冲积岛岸滩有“江岸型”、“洲头型”和“潮滩（tidalbeach）型”三种基本类型；河槽变迁导致“江岸型”岸滩发生数年周期的变化，岸滩的季节性变化受风、海面、潮差和含沙量等多种因子影响，台风和寒潮则引起周期更短的“风暴旋回”，受植被保护的沼泽滩面的冲淤过程与光滩截然不同。

ADCP在长江口悬沙输运观测中的应用

声学多普勒流速剖面仪 (ADCP)是近年来发展起来的一种用于测量流速的声学仪器 ,同时还可以通过建立回声强度和现场取得水样的回归关系式而获得悬沙浓度的数据。本文利用在长江口两个站位的高频观测数据 ,对现场取得的悬沙作粒度分析 ,在此基础上对枯季长江口地区悬沙输送机制和悬沙粒度对水动力的响应进行了分析和探讨。结果表明 :平均流输运在整个悬沙输送中占主导地位 ,同时潮扩散和垂向扩散作用也是引起两站悬沙输运的重要因子 ;不同层次和不同时刻的悬沙粒度参数的变化 ,既和海 陆转换有关 ,也和潮相变化密切相关。

长江河口底沙运动规律

于1981－1988年以浅地层剖面仪和回声测深仪取得沙波观测资料，运用这些资料以及河床表层沉积物和历年河口水文、地形资料，采用水文学、沉积学与泥沙运动力学相结合的研究方法，分析长江河口底沙的运移规律。结果表明：长江河口底沙运动非常频繁，一般有单颗粒滚动、跳跃，沙波及沙体推移等形式；在沙质床面上沙波发育良好，其形成、发展和消失与潮差和落潮流速有一定的相关性，因受涨、落潮流改造，沙波难于得到充分发展；沙体推移为长江河口底沙运动的主要形式，推移量很大，有时甚至能使滩槽移位，迫使通海航道改线。研究底沙运动规律为长江河口治理提供科学依据。

稀土元素在长江口及邻近陆架表层沉积物中的分布及物源示踪研究

长江口邻近陆架表层沉积物中稀土元素含量及组成表明其主要来源为中国大陆陆源物质。轻重稀土元素比值对于区分长江和黄河流域来源物质具有一定的示踪意义,古黄河三角洲以较低的轻重稀土元素比值为特征,而长江来源物质以相对较高的轻重稀土比值为特征,沉积物粒度和Cl含量不是控制沉积区内稀土元素分馏的主要因素。通过聚类分析将研究区域划分为长江口近岸区和东海北部陆架区两个分区,黄海沿岸流、长江冲淡水以及台湾暖流是造成分区的关键水动力因素。分区内常量和微量元素以及同位素组成皆表现为不同程度差异,长江口近岸区化学组成和长江流域物质基本一致,东海北部陆架区与黄河流域物质比较接近。

长江口和东海海域沉积物粒径对有机质分布的影响

利用2007年11月采集于长江口和东海陆架区域的78个站位的表层沉积物样品,以及2007年11月、2008年4月采集于长江口东南和浙江近岸泥质区的悬浮物样品,对长江口和东海海域表层沉积物以及部分站位的悬浮物进行了粒度、有机碳和总氮的分析和研究,探讨了沉积物粒径对有机质运移和沉积过程的影响。结果表明:(1)水体中悬浮物的平均粒径从长江口沿浙江近岸向南逐渐减小,与悬浮物含量和颗粒有机碳含量都有较好的相关性,表明悬浮物粒径是影响有机质运移的重要因素。(2)表层沉积物粒径分布呈现出明显的区域性变化特征,有机碳、总氮的分布趋势与表层沉积物的粒径分布密切相关,表层沉积物的粒径越小,有机碳和总氮的含量则越高;长江口和浙江沿岸泥质区表层沉积物中含砂量低,而中外陆架和长江口水下三角洲东北隅表层沉积物中的含砂量较高,含砂量呈现出由近岸向外海逐渐增加的趋势。由此可见,随着含砂量的增加、沉积物粒径的增大,有机质含量由陆向海方向逐渐降低,而有机质含量与粘土粉砂含量有着明显的正相关性,细粒沉积物是有机质运移的主要载体,同时也控制着沉积物中有机质的分布。

长江口动水絮凝沉降与拦门沙淤积的关系

根据1982年4月－1983年3月长江口南槽3个站位全年逐月大潮的水化学成分、悬沙粒度与粘土矿物资料，1984－1992年在环形水槽中获得的近百个组次的动水絮凝沉降试验研究资料，以及长江口细颗粒泥沙、浮泥与底质研究成果，对拦门沙形成过程及其机理进行综合研究。结果表明，长江口拦门沙为最年轻的活动三角洲，汛期集中输水输沙与台湾暖流同期相遇，为动水絮凝沉降最强和浮泥最为发育时期；涨潮期当盐度为（9－13）x10－3时，伊利石、高岭石在盐水楔顶部发生动水絮凝沉降，而落潮期当盐度在（19—24）X10－3时，蒙脱石在盐水楔尾部发生动水絮凝沉降，揭示了拦门沙水平淤积的基本规律；半月周期的浮泥透镜体所残积的中、粗粉沙形成钻孔中常见的透镜体与薄层理，则揭示了拦门沙的垂直加积的基本规律，为拦门沙综合治理与深水航道的开发利用提供科学依据。