Nitrogen and phosphorus budgets of the Changjiang River estuary

Eutrophication has emerged as a key environmental problem in Chinese coastal waters, especially in the Changjiang (Yangtze) River estuary. In this area, large nutrient inputs result in frequent harmful algal blooms and serious hypoxia in bottom waters. Four cruises were made in the estuary in 2006 to assess the concentration and distribution of dissolved inorganic nitrogen (DIN) and phosphorus (DIP). The concentration of DIN decreased gradually in a linear relationship with salinity from the river mouth to ...     

THE DISTRIBUTION PATTERNS OF BACTERIA,ATP AND THE RELATIONS WITH RESPIRATION RATES AND PHYTOPLANKTON IN THE CHANGJIANG ESTUARY AND ITS ADJACENT EAST CHINA SEA

Two cruises were conducted in January and July 1986 in the Changjiang (Yangtse River) Estuary and its adjacent East China Sea (30°45′ -32°00′N,121°00′-124°00′E). Direct epifluorescence counts of planktonic bacteria and determinations of ATP concentrations were made. Subsamples were taken for measurement of oxygen consumption rates and chlorophyll concentrations.Bacteria and ATP concentrations were higher in summer than in winter, highest in the river and the river mouth, and gradually lower offshore. The bacteria number was correlated positively with suspended matter, nitrates and oxygen consumption rates, and negatively with salinity.In winter bacteria were the main contributors of ATP and the main consumers of dissolved oxygen in the whole studied area. In summer two maxima of ATP were found along the salinity gradient. The first one which coincided with the peak of turbidity near the river mouth was attributed to bacte -ria, and the second which occurred in the waters with a salinity range be     

Seasonal variation of tidal prism and energy in the Changjiang River estuary:a numerical study

Tidal rivers are intrinsically complex because tidal propagation is influenced by river discharge. This study aims to examine the seasonal variation of tidal prism and energy variance in the tidal river of the Changjiang(Yangtze) River estuary in China. In order to quantify the behaviour of river and tide,we use numerical modelling that has been validated using measured data. We conduct our analysis by quantifying the discharge and energy variance in separate components for both the river and the tide,during wet and dry seasons. We note various definitions of tidal prism and explore the difference between tidal discharge on the flood and ebb and tidal storage volume. The results show that the river discharge attenuates the tidal motion and reduces the tidal flood discharge but the tidal storage volume is approximately constant with different riverine discharge since part of the fresh water discharge is intercepted and captured in the estuary due to the backwater effect. It appears that the tidal discharge adjusts according to the variation of river discharge to keep a constant tidal storage volume. An analysis of the hydraulics shows that the transition from tidal dominance(at the mouth) to river dominance(upstream) depends on the location of tidal current reversal which varies from wet season to dry season. Duringthe wet season,the Changjiang River estuary is totally dominated by energy from fresh water discharge.     

Spatiotemporal distribution of fish eggs and larvae in the Huanghe(Yellow) River estuary,China in 2005–2016

Decreasing fish resources in estuaries is a subject of anthropogenic activities.Studies of the spatiotemporal distribution of fish eggs and larvae can help identify the status and processes underlying recruitment in a fishery.As the fifth largest river estuary in the world,the Huanghe(Yellow)River estuary(HRE)is a typical estuary that has been seriously affected by human activities.Annual surveys on ichthyoplankton and environmental factors were conducted in the months of May of 2005 and 2009-2016 in the HRE to investigate the spatiotemporal distribution of fish eggs and larvae and the associated influencing factors.A total of 23 and 20 species of eggs and larvae,respectively,were collected.The dominant orders were Perciformes(51.2%)and Clupeiformes(25.6%).The average number of fish species eggs and larvae were 6.0 and 4.1 in average abundance of 0.91 and 0.13 ind./m~3,respectively.The dominant species were mainly low-commercial-value small-sized fishes,such as Clupanodon punctatus,Hare,ngula zutnasi,and Acanthogobius,whereas certain traditional commercial fishes,such as Trichiurus lepturus,and Clupea pallasii,were not seen.Analysis of the fish egg and larval community revealed four temporal assemblages and two spatial assemblages.Salinity was the main factor on the spatial distribution of ichthyoplankton communities,the species number and Shannon-Weiner diversity index(H')of the fish egg and larval community near the river mouth with lower salinity were significantly lower than the community far away from the river mouth with higher salinity.In addition,increases of water temperature promoted the number and abundance of fish species eggs,and the areas of abundant prey tended to have a more diversified and abundant of ichthyoplankton species.In overall,overfishing,dam construction,and other human activities were the main drivers that led to the substantial decline in fishery resources in the HRE.     

Morphodynamic evolution of the Xiaoqing River mouth: a Huanghe River-derived mixed energy estuary

In an estuary,tidal,wave and other marine powers interact with the coast in different ways and affect estuary morphology as well as its evolution.In the Huanghe(Yellow) River estuaries and nearby delta,there are many small sediment-affected estuaries with a unique morphology,such as the Xiaoqing River estuary.In this study,we investigated the special evolution and genetic mechanism of the Xiaoqing River estuary by analyzing graphic and image data with a numerical simulation method.The results show that NE and NE-E tide waves are the main driving force for sandbar formation.Sediment shoals have originated from huge amounts of sediment from the Huanghe River,with consequent deposition at the Xiaoqing River mouth.The lateral suspended sediments beyond the river mouth move landward.Siltation takes place on the northern shoreline near the river mouth whereas erosion occurs in the south.The deposits come mainly from scouring of the shallow seabed on the northern side of the estuary.Storm surges speed up deposition in the estuary.Development of the sediment shoals has occurred in two steps involving the processes of growth and further southward extension.Although the southward shift increases the river curvature and length,the general eastward orientation of the estuary is unlikely to change.Processes on the adjacent shorelines do not affect the development of the sediment shoals.The study presents a morphodynamic evolutionary model for the Xiaoqing River estuary,with a long-term series cycle,within which a relatively short cycle occurs.     

Modeling the Water-Flushing Properties of the Yangtze Estuary and Adjacent Waters

As a multi-branch estuary system, the Yangtze Estuary presents distinctive characteristics of hydrodynamic processes through co-action among river runoff, tides, wind-waves, and gravitational circulation. To study the pathways of flushing water along all of the estuary's branches and analyze their differences, especially those due to the influence of seawater intrusion and discharge variations, a free surface flow modeling suite TELEMAC-MASCARET involving passive tracers was applied to the Yangtze Estuary and the adjacent waters. The open boundary conditions were provided by the Nao.99 b model(Matsumoto et al., 2000), which was calibrated using observed velocity and salinity data obtained in March 2002. The water age, which was used as the diagnostic tool to study the flushing efficiency of the water body across the estuary, was solved by additional advection-diffusion-reaction equations implemented in the TELEMAC modeling system. The transport properties were investigated under different river discharge scenarios, which represented seasonal impacts; aspects relating to the influence of tide, surface wind stress, and density-induced circulation on age were also investigated. Model results showed that river runoff is one of the dominant factors influencing the spatial distribution of the mean age, while tidal force is another important factor. The horizontal freshwater age distribution demonstrated similarity compared with the salinity distribution; the vertical age distribution resembled the stratification pattern of salinity in all branches where stratification persists. An experimental numerical simulation of tracing saltwater age from the lower reaches of the estuary was conducted, and implicated the connectivity with transport processes of freshwater from upstream. Additionally, a particle tracking algorithm was used to analyze the dynamic characteristics of the four passages. The South Passage and South Channel were found to be significant as main water flow passages, while salinity intrusion in the North Branch was found to cause a return flow that partially joins the South Branch flushing water.     

Dynamic genetic features of eukaryotic plankton diversity in the Nakdong River estuary of Korea

Estuaries are environments where freshwater and seawater mix and they display various salinity profiles.The construction of river barrages and dams has rapidly changed these environments and has had a wide range of impacts on plankton communities.To understand the dynamics of such communities,researchers need accurate and rapid techniques for detecting plankton species.We evaluated the diversity of eukaryotic plankton over a salinity gradient by applying a metagenomics tool at the Nakdong River estuary in Korea.Environmental samples were collected on three dates during summer and autumn of 2011 at the Eulsukdo Bridge at the mouth of that river.Amplifying the 18 S rDNA allowed us to analyze 456 clones and 122 phylotypes.Metagenomic sequences revealed various taxonomic groups and cryptic genetic variations at the intra-and inter-specific levels.By analyzing the same station at each sampling date,we observed that the phylotypes presented a salinity-related pattern of diversity in assemblages.The variety of species within freshwater samples reflected the rapid environmental changes caused by freshwater inputs.Dinophyceae phylotypes accounted for the highest proportion of overall diversity in the seawater samples.Euryhaline diatoms and dinoflagellates were observed in the freshwater,brackish and seawater samples.The biological data for species composition demonstrate the transitional state between freshwater and seawater.Therefore,this metagenomics information can serve as a biological indicator for tracking changes in aquatic environments.     

Benthic Habitat Quality Assessment Based on Biological Indices in Xiaoqing River Estuary and Its Adjacent Sea of Laizhou Bay,China

Based on the macrofauna data(2008-2011) in Xiaoqing River estuary and its adjacent sea, Laizhou Bay of Bohai Sea, China, the AZTI Marine Biotic Index(AMBI) and Multivariate AMBI(M-AMBI) were used for benthic habitat quality(BHQ) assessment. Results showed that BHQ presented an obvious trend of improvement along the direction of stream channel and river mouth, and in the coastal areas. AMBI and M-AMBI were significantly related to environmental pressure gradient data. Therefore, the two indices can well indicate BHQ in the studied area. However, there were significant differences between results of the two indices. In the cases of low taxa number and high abundance of single species, AMBI might overestimate BHQ. We thus adjusted its thresholds to solve this problem. And M-AMBI might overestimate BHQ when benthic assemblage was dominated by the opportunistic species. Then we could raise the weight of AMBI in the calculation of M-AMBI to handle the problem.     

How can saline and hypersaline lakes contribute to aquaculture development? A review

A considerable part of the world's population is currently experiencing a severe scarcity of freshwater and nutrition. Inland aquaculture has the fastest growth in fresh waters, and this contributes to the eutrophication of freshwater bodies. The increase in freshwater aquaculture impacts on the increasing demand for fresh water. A way to overcome this is to develop aquaculture in saline lakes. This article discusses how saline and hypersaline lakes may contribute to overcome this problem and gives a list of fish and shrimp species that can be cultivated in saline lakes. Successful development of aquaculture depends on a healthy cultured stock of commercial fish and shrimps. A sustainable healthy stock of fish and shrimps can be only maintained using live food for the cultured fish larvae, fry and fingerlings. As well as Artemia spp. there are many other crustacean species with the potential for growing in hypersaline waters. At least 26 copepod species around the world can live at a salinity above 100 g/L with 12 species at a salinity higher than 200 g/L, and these all have excellent nutritional value. There is a high potential to use eukaryotic organisms of diff erent taxa in saline/hypersaline aquaculture for food, agri-aquaculture, diff erent industries and as food supplements.     

Evolution of Yellow River Delta Coastline Based on Remote Sensing from 1976 to 2014, China

Coastal regions are threatened by natural processes, such as erosion driven by storm surges and the effect of jetties, as well as by human behavior. The coastline of the Yellow River Delta(YRD) was monitored using the general high-tide line method, which combines Remote sensing(RS) and geographic information system(GIS) technology, using multi-spectral scanner(MSS), thematic mapper(TM), and enhanced thematic mapper plus(ETM+) images of the YRD from 1976 to 2014 as a data source. The results demonstrated that the shape and length of the YRD coastline has changed dramatically since 1976. The course of the Diaokouhe channel has resulted in mainly inland erosion in the north, and is primarily marine erosion; therefore, it was termed an erosion-type estuary. However, the coastline of the Qingshuigou course has moved seaward, demonstrating an accretion stage, and was therefore termed an accretion-type estuary. The coastline advanced forward before 1997 and shrank after 2003 in the southern part of the river mouth, which was due to the shift in the river mouth in 1996. It has continually extended outward in the northern part of the river mouth from 2003 onward. The coastline in the southern part of the river mouth has moved randomly, with the occurrence of both erosion and sedimentation caused by land reclamation and sea wave intrusion. In most cases, the coastline has extended offshore, especially in the northern part of the river mouth. The YRD coastline has changed frequently and rapidly from 1992 to 2014. The river mouth channel, river water and sediments, and precipitation were the major factors affecting the YRD. The YRD coastline was mainly in an accretion stage during flow periods. The erosion rate decreased and tended to be stable during a dry period. The coastline was basically stable when dry periods occurred over a long period. The location of Yellow River ports and sea erosion were the main factors driving coastline changes. The coastline was mainly influenced by the flow path of the Yellow River, with recent human activity also becoming a factor.     

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 dif fer between inshore and of fshore areas, the species diversity decreased from inshore to of fshore. 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 of fshore 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.     

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.     

Behavior of different phosphorus species in suspended particulate matter in the Changjiang estuary

Suspended particulate matter (SPM) collected in the Changjiang (Yangtze River) estuary in June 2006 was separated into five fractions via water elutriation: clay-very fine silt (<8 μm), fine silt (8–16 μm), medium silt (16–32 μm), coarse silt (32-63 μm) and sand (>63 μm). The SPM and fractionated particles were sequentially analyzed by a modified SEDEX sequential extraction method to obtain six species of phosphorus: exchangeable or loosely-sorbed P, organic P, Fe-bound P, authigenic P, detrital P and refractory P. The results indicated that all particulate phosphorus species except for detrital P were negatively correlated to particle size; a high detrital P content was found in coarse silt and very coarse silt. From the inside of the river mouth to the gate of the river mouth, organic P, Fe-bound P and refractory P in the suspended particles decreased and a higher amount of exchangeable P appeared around the gate of the river mouth. From the gate of the river mouth to the sea, exchangeable P and organic P in suspended particles increased distinctly. The total particulate P flux into the estuary from the Changjiang River was about 45.45×10⁸ μmol/s during sampling. Of this, about 8.27×10⁸ μmol/s was associated with the “truly suspended” fraction. The bio-available particulate P flux was about 13.58×10⁸ μmol/s. Of this, about 4.24×10⁸ μmol/s was transported by “truly suspended” particles.     

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.     

The effects of estuarine processes on the fluxes of inorganic and organic carbon in the Yellow River estuary

Riverine carbon flux is an important component of the global carbon cycle. The spatial and temporal variations of organic and inorganic carbon were examined during both dry and wet seasons in the Yellow River estuary. Concentrations of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in the Yellow River during dry seasons were higher than those during wet seasons. The effective concentrations of DOC (C_DOC*) were higher than the observed DOC at zero salinity. This input of DOC in the Yellow River estuary was due to sediment desorption processes in low salinity regions. In contrast to DOC, the effective concentrations of DIC were 10% lower than the DIC measured at freshwater end, and the loss of DIC was caused by CaCO₃ precipitation in low salinity region. Particulate organic carbon (POC) and particulate inorganic carbon (PIC) contents of the particles stabilized to constant values (0.5%±0.05% and 1.8%±0.2%, respectively) within the turbidity maximum zone (TMZ) and showed no noticeable seasonal variations. A rapid drop of PIC and rise of POC occurred simultaneously outside the TMZ due to an intense dilution of riverine inorganic-rich particles being transported into a pool of aquatic organic-poor particles outside the TMZ. Annually, the Yellow River transported 6.95×10⁵ t of DIC, 0.64×10⁵ t of DOC, 78.58×10⁵ t of PIC and 2.29×10⁵ t of POC to the sea.     

Phytoplankton assemblage structure shaped by key environmental variables in the Pearl River Estuary,South China

Spatial distribution and temporal dynamics of phytoplankton community and their relationships with environmental factors were studied in the Pearl River Estuary (PRE), South China, in three seasons. Salinity was considered as the key environmental variable controlling horizontal distributions of phytoplankton community composition. A transition from dominance of freshwater diatoms (Aulacoseira granulata and A. granulata v. angustissima) to estuarine species (Skeletonema costatum and Pseudonitzschia delicatissima) was observed in the high flow season (summer) along the estuary gradient; in the low flow season (spring), the inner estuary was relatively homogeneous and some typical estuarine species could be found near the river mouth. In the normal flow season (autumn), a potentially toxic bluegreen species, Microcystis spp. was predominant in the middle reaches of the estuary, which should be seeded from upstream and transported downstream by river discharges. Phytoplankton abundance was negatively correlated with suspended solid content and nutrient concentration in the PRE, suggesting that turbidity and nutrient availability were the crucial factors regulating the algal biomass. Phytoplankton abundance in the outer estuary was enhanced by increasing irradiance and continued to be enhanced until phosphorus-limitation.     

Changes in Huanghe （Yellow） River estuary since artificial re-routing in 1996

With the combination of historical data, field observations and satellite remotely sensed images(Landsat TM/ETM and CBERS), changes in Huanghe (Yellow) River estuary since 1996 when artificial Chahe distributary was built up were studied, mainly including water and sediment discharge from the river, tides, tidal currents, suspended sediment diffusion, coastline changes and seabed development. During following six and half years (up to the end of 2002), runoff and sediment loads into the river mouth declined dramatically. At the beginning of the re-routing, abundant sediment loads from the river filled up nearshore shallow water areas so that the newborn delta prograded quickly. With rapid decrease of sediment loads transported to the estuary, the delta retrograded. In 1997, subaerial tip of the abandoned delta receded 1.5km; its annual mean recession rate was about 150 m in following years. In addition, marine dynamic condition near the artificial outlet had also changed. Under the interaction of ocean and river flow, most of incoming sediment loads deposited in the vicinity of the outlet. Seabed erosion occurred at the subaqueous delta front. Between 1999 and 2002, erosion thickness averaged at 0.3 m in the subaqueous delta of 585.5 km2.     

Long-term changes in sedimentary diatom assemblages and their environmental implications in the Changjiang (Yangtze) River estuary,China

Long-term data on diatom assemblages in a sediment core （60 cm） obtained from the Changjiang （Yangtze） River estuary were analyzed in order to assess the environmental changes that took place in the approximately 38 years （as determined by 210pb measurements）, i.e., between 1974 and 2012, of sediment accumulation. From the sediment core, 62 diatom taxa and genera were identified. The diatom biomass in the core generally increased beginning in the mid-1990s （core depth： 35 cm）, accompanied by a shift in the dominant species from Podosira stelliger and two species of Cyclotella （C. stylorum and C. striata） to Paralia sulcata, three species of Thalassiosira （ T. eccentria, I＂. oestrupii, and T. excentrica）, Actinoptychus undulates, and Thalassionema nitzschioides. The changes in both species diversity and abundance suggested that since the 1980s the estuary has undergone extensive eutrophication. This conclusion was supported by the increased proportion of planktonic species, another indicator of high nutrients inputs, in the Changjiang River estuary.     

Impact of estuarine reclamation projects on saltwater intrusion and freshwater resource

Estuarine projects can change local topography and influence water transport and saltwater intrusion. The Changjiang (Yangtze) River estuary is a multichannel estuary, and four major reclamation projects have been implemented in the Changjiang River estuary in recent years: the Xincun Shoal reclamation project (RP-XCS), the Qingcao Shoal reclamation project (RP-QCS), the Eastern Hengsha Shoal reclamation project (RP-EHS), and the Nanhui Shoal reclamation project (RP-NHS). The effects of the four reclamation projects and each project on the saltwater intrusion and water resources in the Changjiang River estuary were simulated in a 3D numerical model. Results show that for a multichannel estuary, local reclamation projects change the local topography and water diversion ratio (WDR) between channels and influence water and salt transport and freshwater utilization in the estuary. During spring tide, under the cumulative effect of the four reclamation projects, the salinity decreases by approximately 0.5 in the upper reaches of the North Branch and increases by 0.5–1.0 in the middle and lower reaches of the North Branch. In the North Channel, the salinity decreases by approximately 0.5. In the North Passage, the salinity increases by 0.5–1.0. In the South Passage, the salinity increases by approximately 0.5 in the upper reaches and decreases by 0.2–0.5 on the north side of the middle and lower reaches. During neap tide, the cumulative effects of the four reclamation projects and the individual projects are similar to those during spring tide, but there are some differences. The effects of an individual reclamation project on WDR and saltwater intrusion during spring and neap tides are simulated and analyzed in detail. The cumulative effect of the four reclamation projects favors freshwater usage in the Changjiang River estuary.

     

冰后期长江河口段古河谷地层层序特征

末次盛冰期,由于基面大幅度下降,长江河口段形成了深切古河谷。河流携带的部分粗砂砾在河床上形成滞留沉积。冰后期海平面上升引发的海侵造成了长江古河谷的充填和河床相、河漫滩河口湾相和部分河口湾浅海相的形成。尔后的进积产生了部分河口湾浅海相及三角洲相。根据钻孔资料绘制的古河谷地质剖面图显示,冰后期海侵旋回出现在三角洲顶点到现在河口口门地区,溯河而上受海洋影响的程度逐渐减弱。古河谷的中部,靠近主泓线附近,沉积层序完整。古河谷的边缘地带,往往缺失底部的河床相砂砾层。底部的厚层河床相砂砾层是由河床滞留沉积和溯源加积堆积形成的,在古河谷沉积层序中存在着多种沉积间断。