The process of change and factors influencing dissolved CH4 concentration and flux in the coastal wetlands of Jiuduansha (JDS) and Xisha (XS) in the Yangtze Estuary were explored. The concentration of dissolved CH4 varied significantly during the sampling period, with the highest in JDS wetland being (0.30±0.19) μmol·L–1 during autumn, while that in XS wetland being (1.16±1.52) μmol·L–1 during summer. The average dissolved CH4 concentration in XS wetland ((0.56±0.91) μmol·L–1) was slightly higher than that in JDS wetland ((0.18±0.17) μmol·L–1). Principal component analysis revealed that the temporal and spatial variations in CH4 were mainly related to seasonal variation and tidal cycling in coastal wetlands. The CH4 emission under low-temperature, high-salinity, and oxygen-rich water environments was limited. The fluxes of dissolved CH4 also showed seasonal and regional variations. The water-to-air diffusion of CH4 was the largest in autumn in JDS wetlands ((0.45±0.43) nmol·m–2·s–1) and in summer in XS wetlands ((3.34±5.21) nmol·m–2·s–1). The lateral fluxes of dissolved CH4 were maximum in autumn in JDS wetlands ((2.32±9.32) nmol·m–2·s–1) and in summer in XS wetlands ((1.66±5.06) nmol·m–2·s–1). Use of water quality parameters and dissolved CH4 concentration to fit a multiple regression equation produced a high-frequency and continuous CH4 concentration. The annual average lateral transport flux (JDS wetland: 1.46 mg·m–2·d–1; XS wetland: 0.34 mg·m–2·d–1) and annual average vertical diffusion flux (JDS wetland: 1.85 mg·m–2·d–1; XS wetland: 2.90 mg·m–2·d–1) of dissolved CH4 was calculated. The results show that dissolved CH4 in coastal wetlands is an important sources of CH4 in the atmosphere and coastal waters.
Heterosigma akashiwo is one of the species of harmful algal blooms (HABs) threatening marine ecosystems and the fishing industry across coastal waters worldwide. With increasing global levels of atmospheric CO2 and global warming, HABs have increased in prevalence, duration, and geographic span; this phenomenon has been further stressed by intensified anthropogenic influences, such as eutrophication. Through controlled experiments with different nutrients, CO2 concentrations, and temperatures, our study aimed to understand the response of H. akashiwo to different nutrients with changes in climate. In all simulated CO2 and temperature scenarios, both cell density and the specific growth rate of H. akashiwo in the low phosphorus groups were significantly lower than those in the high phosphorus groups. Furthermore, the maximum cell density and specific growth rate of H. akashiwo were significantly enhanced by increased CO2, while the specific growth rate was accelerated by the dual effect of increased CO2 levels and temperature. The growth response of H. akashiwo to CO2 and temperature was similar between different nutrient treatments. Taken together, the results indicate that phosphorus concentration could be the major factor controlling the growth of H. akashiwo, and the intensity and risk of H. akashiwo blooming in the future is increasing. Hence, controlling the increase of nutrients, particularly phosphate, could be a critical pathway to decrease the occurrence of H. akashiwo blooms. In summary, our case study provides scientific support for marine ecological management of HABs.
To understand the metabolic potential and environmental adaptation mechanisms of Nitrospira in tidal flat wetlands, 14 high-quality Nitrospira genomes were constructed from five tidal flat wetlands along the coast of China using metagenomic binning and assembly methods. Phylogenetic analysis showed that among these genomes, three belonged to Comammox (complete ammonia oxidizer), nine belonged to lineage II and IV of Nitrospira, and two belonged to lineage III, which has yet to be discovered; taken together, this data suggests that abundant and diverse Nitrospira are present in China’s tidal flat wetlands. Metabolic reconstruction revealed that these Comammox and Nitrospira contained cyanase, urease, and other enzymes involved in the degradation of nitrile compounds and amide compounds; hence, they may utilize the organic nitrogen as energy by coupling with ammonia-oxidizing microorganisms. In addition, Nitrospira possessed flexible strategies to resist environmental stresses such as viral attack and osmotic changes. These results provide insights on the diversity, ecological function, and environmental adaptation mechanisms of Nitrospira from tidal wetlands.
In February 2014, a persistent and strong northerly wind caused an extremely severe saltwater intrusion event in the Changjiang Estuary, which posed a threat to the safety of water intake from this source. Increasing river discharge from upstream reservoirs in the river basin is a method to combat severe saltwater intrusion. To simulate and analyze the effects of various river discharges on saltwater intrusion, we used the Unstructured quadrilateral grid, Finite-differencing, Estuarine and Coastal three-dimensional Ocean numerical Model (UnFECOM). By taking into account realistic river discharge and wind conditions, the model accurately reproduced the extremely severe saltwater intrusion process that occurred in February 2014. Our findings indicated that the net water flux (NWF) across the section at the upper reaches of the North Channel (NC) remained landward during the most critical period of saltwater intrusion from February 10 to 13, 2014, despite the increase in river discharge. However, the magnitude of NWF tended to decrease with increasing river discharge. The net water diversion ratio (WDR), NWF (Net Water Flux), and salt flux increased with the increase in river discharge. Under realistic river discharge conditions, WDR was –29% (the negative sign indicates that the NWF is landward), NWF was –2300 m 3/s, and the net salt flux (NSF) was –68 t/s, indicating that the NWF and NSF were landward due to the landward Ekman transport effect induced by the persistent severe northly wind. When the river discharge increased by 3000 m 3/s, WDR and NWF across the section were nearly zero, and NSF was –34 t/s. When the river discharge increased by 8000 m 3/s, WDR was 21.5% and NWF was seaward, at 3550 m3/s. NSF was –6 t/s and landward. At the water intake of Qingcaosha Reservoir, the longest continuous unsuitable water intake time decreased slightly when the river discharge increased by less than 4000 m 3/s. When the river discharge increased to 5000 m3/s, the longest continuous unsuitable water intake time decreased significantly to 10.5 days. It may be challenging to achieve the necessary value and duration of river discharge increase required for the actual operation of the Three Gorges Reservoir. To ensure the safety of water intake, implementing an early warning and forecasting system for saltwater intrusion and storing water into the reservoir at a high level before the intrusion occurs are recommended as effective methods.
This study analyzed the spatial and temporal variations of depth-averaged residual currents of Eulerian, Lagrangian, and Stokes during a tidal cycle in the eastern waters of Macao. Data were collected for 15 days during the summer from two field stations using a method specifically designed to deal with stratified residual flow. Our findings revealed similar dynamical characteristics during flood and ebb tides in the southeast and northeast waters of Macao, with currents being more pow-erful in the southeast. We found that the material carried by the tidal current tends to travel northwest at the station A6 and southeast at A7, with Lagrange residuals being around 2.2 cm/s and 5.1 cm/s, respectively, which is slightly smaller than Eulerian residuals. Influenced by the southwest monsoon, the directions of Euler, Lagrangian, and Stokes surface residual currents on the surface generally pointed northeast. Furthermore, our results showed that the depth-averaged residuals during a tidal cycle pointed towards the open sea during upstream flooding, towards nearshore in northeast waters, and towards the east in southeast waters under non-flooding conditions. The intensity of Stokes drift in southeast waters was primarily influenced by wind on the water surface, while the effect of wind on the northeast waters was limited and weakened by enhanced runoff. The intensity and direction of transportation in both southeast and northeast waters were related to wind speeds on the sea surface. During summer, analysis of net tidal flux indicated a relatively stable counterclockwise residual current circulation in the waters downstream to the outlet of the Macao waterway. This circulation was driven by the strong northeast littoral current outside the Pearl River Estuary, leading to the formation of an east-west flowing structure compensated in the east waters of Macao. During this season, part of the tidal sediment was intercepted by this unique dynamic structure discharged by east four mouths in the Lingdingyang Estuary upstream and re-entered the Macao waters with rising tidal water, which leaded to sedimentation of beaches and troughs and poor water exchange.
Yingwuzhou Wetland is an artificially restored coastal salt marsh wetland aimed at improving ecosystem services. Development of the wetland has restored the original damaged coastal ecosystem through comprehensive coastline ecological engineering measures. The birds in the study site have been investigated and researched using the route survey method since 2018, and changes in the bird population and species diversity have been analyzed to evaluate the effectiveness of coastal zone ecological restoration projects and the impact of different wetland habitat types on bird diversity. The results showed that 67 bird species were recorded in the wetland, belonging to 13 orders and 32 families, with the largest number of birds belonging to Passeriformes, including 42 species belonging to 18 families. There were 35 species of resident birds, 24 species of winter migratory birds, 10 species of summer migratory birds, and 8 species of migratory birds. Among these, one species of national class I and seven species of class II are in the List of Key Protected Wild Animals in China, respectively. Remiz consobrinus, Gallinula chloropus, Acridotheres cristatellus, Tachybaptus ruficollis, Spodiopsar cineraceus, Hirundo rustica, and Passer montanus were the dominant species. The number of wetland bird species increased annually. There were significant differences in the bird species, quantity, and Shannon–Wiener indexes among different seasons. The declining trends of bird species, quantity, and Shannon–Wiener index were in the orders of fall > winter > spring > summer, fall > winter > summer > spring, and fall > spring > winter > summer, respectively. The bird numbers and species were the highest in the natural wetland complex area. Declining trends of the Shannon–Wiener index in different habitat areas were observed for the natural wetland complex area, salt marsh wetland restoration area, clear water conservation area, lawn activity area, and wetland purification exhibition area. The ecological restoration of the coastline has enriched the bird diversity of the wetland. Habitats with rich patch types and high patch mosaic have a markedly positive impact on bird diversity. The results of this study can provide a scientific basis for the coastal ecological restoration and sustainable development of coastline wetlands.
Under the dual influence of human activities and natural factors, the coastal zone patterns are prone to rapid changes which can directly or indirectly affect the structure, function, and sustainable development of the coastal ecosystem. Using the coastal zone of Nanhui Dongtan in Shanghai as a typical research area, we used remote sensing interpretation, sea chart digitization, and field investigation to analyze changes in spatial patterns and changes in coastal zones over the last 20 years (from 2000 to 2020). In addition, the effects of coastal engineering (including reclamation engineering and siltation promotion engineering) and S. alterniflora invasion on coastal pattern dynamics were analyzed. The results showed that: ① Since 2000, under the influence of coastal engineering and biological invasion, the land use types of Nanhui Dongtan coastal zone changed from a simple pattern dominated by coastal wetlands to a complex pattern which included multiple land use types (i.e. coastal wetlands, inland wetlands, constructed wetlands, farmland, and construction land). ② Coastal reclamation engineering decreased 11894.7 hm2 of coastal wetlands in the Nanhui Dongtan coastal zone from 2000 to 2005. The reclaimed coastal wetlands were transferred into land use types such as rice fields, ponds, and farmland due to human activities; the reclamation engineering promoted deposition of sediment in the estuary and tidal mudflat (above 0 m) and the intertidal salt marsh developed with increased rates of 320.5 hm2/a and 110.9 hm2/a, respectively; meanwhile, the siltation rate decreased to 286.8 hm2/a and 15.7 hm2/a, respectively, after 2015. After 10 years (2005—2015) of natural recovery, the area of coastal wetlands did not reach the levels seen before reclamation in Year 2000. ③ Two types of siltation promotion engineering—hard siltation promotion engineering and biological siltation promotion engineering—have both significantly promoted the rapid development of coastal wetlands in Nanhui Dongtan. Hard silting promotion engineering with propagation rates of 516.9 hm2/a in tidal mudflats (above 0 m) and 915.7 hm2/a in intertidal salt marshes, respectively, was 5.4 times and 13.9 times higher than rates observed in non-siltation areas; hence, the effects were more significant than biological siltation promotion engineering which only resulted in pattern changes in a limited area between the seawall and the wave dissipation dike by planting S. alterniflora. ④ After introducing S. alterniflora in Nanhui Dongtan, it became the most dominant plant in the Nanhui Dongtan salt marsh, accounting for 56% of the total area; this significantly changed the ecological structure and function of coastal wetlands in 2020. Coastal engineering and biological invasion have a great impact on coastal zone patterns. Although the coastal ecosystem showed a certain resilience to coastal human activities, changes in the wetland type, area, and function were difficult or impossible to recover at a great cost in the future. How to integrate the ecological functions of coastal wetlands and inland wetlands through the implementation of coastal zone restoration, ecological protection, and other measures to achieve the sustainable and healthy development of coastal zones is an important problem for future land and sea planning.
In the context of rising sea-levels under global warming and fluvial sediment load decline, the risk of coastal erosion is increasing in global deltas. Evaluating the delta morphological change and its vulnerability to erosion in the future and analyzing land use exposure to coastal erosion, is of great significance for spatial planning, disaster prevention and mitigation, and the sustainable development of coastal regions. In this study, Pudong New Area of Shanghai, which is located in the Yangtze River Delta, was selected to compare its coastal erosion vulnerability in 2016 and 2035 using bathymetry of 2016 and modelled bathymetry of 2035 under a scenario of fluvial sediment load of 125 Mt/a and a cumulative sea-level rise of 16.5 cm. A comparison of land use exposure is made between the pattern in 2019 and the pattern projected for 2035 by the government. The results show that the spatial pattern of coastal erosion vulnerability is controlled by the morphological evolution of the delta over the last 1000 years. Coastal sections with high and very high categories of land use exposure and erosion vulnerability in the recent past (2016—2019) total 32.3 km in length, which accounts for 31.1% of the coast of the Pudong New Area. These figures are projected to be 47.5 km and 45.8%, respectively, in 2035, under the condition of reduced fluvial sediment load and planned land usage. In 2035, coastal sections along the Pudong Airport and Luchaogang will face an increase of erosion vulnerability category. According to the current plan, these two sections also show an increase in land use exposure due to airport expansion and new city development. The results suggest that these sections require erosion monitoring and erosion management during implementation of the plan. The methods used in this study can provide references for exposure assessment of coastal erosion and the formulation of territorial planning in coastal regions elsewhere.
With the present global warming scenario, the erosion of intertidal flats in estuarine zones often occurs due to rising sea levels and an increase in human activities. Intertidal flats have an important ecological function and economic value, including for carbon sequestration, preventing flooding, water purification, attenuating waves, and tourism development. Hence, it is of great theoretical and practical significance to study the stability of the wetland ecosystem for intertidal flats. Previous studies mainly focused on the stability of intertidal bare flats, while the stability of salt marsh ecosystems has attracted relatively less attention. The mechanisms of their respective influencing factors are, as of yet, poorly understood. In this study, we took a typical muddy intertidal zone of Chongming Dongtan in the Yangtze River Estuary as an example and made a comparative analysis on sediment stability for both the salt marsh zone and the adjacent bare flat using in-situ sampling and laboratory tests. The results indicate that: ① Sediment stability improves with an increase of clay content in the bare flat. ② Sediment stability in the salt marsh zone is significantly higher than that in the adjacent bare flat because of the “reinforcing” effect of the root system. ③ Underground biomass determines sediment stability for the same type of vegetation. The sediment becomes more stable with an increase of the underground biomass in vegetation. The sediment stability of different vegetation is determined by characteristics of the vegetation root system. The sediment stability of Spartina alterniflora vegetation zone with coarser roots was worse than that of Scirpus mariqueter with finer roots. Our results not only advance theoretical research on sediment stability in intertidal flats, but also provide scientific guidance for the construction of Green Sea Defence and other coastal green protection measures.
To understand the stability of the estuarine ecosystem and nitrogen balance in the context of global climate change, it is important to investigate the temperature sensitivity of the microbial nitrogen fixation process. Until now, there have been few studies in the literature on the response of the nitrogen fixation process to temperature changes and its influencing factors. We selected six sampling sites around the Yangtze River Estuary (including four sites inside and two sites outside the Yangtze River Estuary) for the scope of the study; in particular, we explored the temperature sensitivity and influencing factors of the nitrogen fixation process on sediments of the Yangtze River Estuary using slurry incubation experiments and the 15N2 isotope tracer technique. The results showed that the in-situ temperature nitrogen fixation rate in the sediments of the Yangtze River Estuary ranged from 0.72 to 2.85 nmol·g–1·h–1. At 5 ~ 10 ℃ and20 ~ 30 ℃, the nitrogen fixation rate was inhibited by an increase in temperature. However, in the range of 10 ~ 20 ℃, the nitrogen fixation rate was significantly promoted with an increase in temperature. The sensitivity of the nitrogen fixation rate to temperature is relatively consistent, although the physical and chemical properties of the sediments vary significantly. Correlation analysis showed that the contents of sulfide, ferrous iron, nitrate, and total organic carbon were the main environmental factors affecting nitrogen fixation.
River regime changes have a substantial impact on estuary hydrodynamics and the incidence of saltwater intrusion. Based on measured water depth data in the North Branch of the Yangtze River Estuary from 2007 and 2016, we analyzed the river regime changes of the North Branch across a 10-year timespan; in addition, we numerically simulated and analyzed the influence of the river regime change on hydrodynamics and saltwater intrusion. The water volume increased by 4.4% in the upstream section, decreased by 8.8% in the midstream, and decreased by 20.5% in the downstream of the North Branch from 2007 to 2016. Overall, the data reflects an overall net erosion in the upstream section and deposition in the midstream and downstream sections of the North Branch. In fact, a new sand body with deposition thickness of 4 ~ 6 m appeared in the bifurcation of the North Brach and South Branch. The numerical simulation results, moreover, show that the new sand body caused a 15.0% increase in saltwater intrusion, a change in net water diversion ratio from –2.8% to –3.2% in the upstream section of the North Branch during spring tide, and clear enhancement of saltwater intrusion; if the new sand body continues to deposit up to 0.85 m, the saltwater intrusion will not increase further. At the water intakes of the three reservoirs in the South Branch, the new sand body caused the average salinity during spring tide to increase by 0.14 at Dongfengxisha reservoir, 0.12 at Chenhang reservoir, and 0.11 at Qingcaosha reservoir; similarly, the average salinity during the subsequent middle tide increased by 0.15 at Dongfengxisha, 0.11 at Chenhang, and 0.09 at Qingcaosha. The deposition in the downstream section of the North Branch led to tidal prisms cross section at the port of Lianxin during flood tide and ebb tide to decrease by 15.2% and 16.4%, respectively, in spring tide, and decrease by 21.2% and 19.0%, respectively, in neap tide. As we move further upstream along the North Branch: the amount of rising and falling tides decreases and shows a relative downward trend, the high tide level drops, the low tide level rises, and the tidal range decreases during spring tide. Moreover, the tidal prisms in the midstream and downstream sections of the North Branch decreased, and the saltwater spillover from the North Branch into the South Branch weakened. The deposition in the downstream section of the North Branch caused the salinity decrease at the water intakes of the three reservoirs of the South Branch. On the whole, the saltwater spillover from the North Branch to the South Branch weakened significantly due to the river regime changes in the North Branch from 2007 to 2016; the salinity decreased by 2 ~ 3 in the upstream section of the North Branch and 1 ~ 2 in the downstream section of the North Branch; the salinity decreased at the water intakes of the three reservoirs of the South Branch; and the average salinity during spring tide and the subsequent middle tide decreased by 0.41 and 0.21, respectively, at Dongfengxisha reservoir, decreased by 0.34 and 0.18, respectively, at Chenhang reservoir, and decreased by 0.28 and 0.17, respectively, at Qingcaosha reservoir.
Under high-speed wind conditions, cross-polarization synthetic aperture radar (SAR) is not affected by signal saturation. Hence, SAR can be used to observe expansive, high-speed wind fields under all-weather, day- and night-time conditions and offers great potential for monitoring typhoons. Sentinel-1, which was launched by the European Space Agency (ESA), is one of the few available SAR satellites in orbit at present that can provide cross-polarization data. Based on Sentinel-1 cross-polarization data, seven different cross-polarization models, including the C-band cross polarization ocean model (C-2PO), C-band cross-polarization coupled-parameters ocean model (C-3PO), and quad-polarization stripmap cross-polarization model (QPS-CP), developed from 2011 to 2021 were used to estimate the typhoon wind fields of Higos and Molave. A denoising method was applied to remove the noise from extra wide (EW) mode SAR images. The results show that the denoising method can effectively reduce the noise and improve the retrieved wind fields. The C-3PO model performs well in monitoring high-speed winds, but does not obtain reliable results for low- to moderate-speed winds compared with the Sentinel-1 Level-2 Ocean (OCN) product. By merging results from the cross-polarization model and the OCN wind product, the combined wind field can effectively reproduce the inner high-speed winds and outer relative low-speed winds. This study is of significant value for forecasting, data assimilation, and research of typhoon disasters.
In this study, we analyzed the evolution of saltwater intrusion in Changjiang Estuary since the 1970s based on: salinity data collected at the Wusong, Gaoqiao, and Baogang stations; days of saltwater intrusion at the water intakes of the Wusong water plant, Chenhang reservoir, and Qingcaosha reservoir; river discharges at Datong station; and satellite remote sensing data of estuarine topography changes. The measured salinity changes at Wusong, Gaoqiao, and Baogang stations in the dry seasons showed that the saltwater intrusion in the Changjiang Estuary was serious in the 1970s, became weak in the 1980s, and was weak from 1990 to 1996. The peak salinity at Baogang station occurred prior to Wusong station, and the peak salinity at Wusong station occurred prior to Gaoqiao station; these observations indicate that the saltwater intrusion originated from upstream saltwater spilling over from the North Branch. The annual days of saltwater intrusion at the water intakes of the Wusong water plant, Chenhang reservoir, and Qingcaosha reservoir indicate that the saltwater intrusion was serious from 1974 to 1981 and particularly acute in 1974, 1979 and 1980; in these cases, the days of saltwater intrusion at the water intake of Wusong water plant exceeded 70 days. The saltwater intrusion was relatively weak from 1982 to 1995. The saltwater intrusion intensified from 1996 to 2002, and serious saltwater intrusion occurred in 1996, 1999, and 2001. The saltwater intrusion from 2003 to 2020 decreased significantly. The construction of the Three Gorges reservoir in 2003 and the cascade reservoirs in the upper reaches of the Changjiang Basin after 2003 resulted in a significant increase in river discharge during the dry season; this phenomenon was the main driver for the weakening saltwater intrusion. The changes in estuarine topography from 1974 to 2013 were detected by satellite remote sensing images; in particular, the North Branch was a wide river in the 1970s. With the successive reclamations of Yonglongsha, Xinglongsha, and Xincunsha, as well as the reclamation of the south shoal in the lower reaches of the North Branch, the North Branch became narrow and the tidal capacity decreased; the sequence of events subsequently led to the gradual weakening of saltwater spillover from the North Branch into the South Branch in a long time scale. The topography changes of the North Branch also explain the drivers for the serious saltwater intrusion that occurred in the 1970s and the relative weakening of saltwater intrusion over time, particularly since the beginning of this century. River discharge and estuarine topography changes are the main drivers for the long-term changes in saltwater intrusion in the Changjiang Estuary. With the construction of more reservoirs in the upper reaches of the Changjiang River and further shrinkage of the North Branch, saltwater intrusion will continue to weaken. These changes are conducive to the safety of freshwater resources in the Changjiang Estuary.
Extreme precipitation and floods may occur during a storm surge hazard, accompanied by typhoon conditions and high tide levels. The combination of these factors intensifies the risk of flooding in coastal regions suffering from a storm surge. Thus, multi-impact analysis should be applied to determine flood risk during a storm surge. River networks play an important role in flood processes. The storage and transportation capacity provided by rivers can directly change the distribution of a flood. In this paper, a 1-D river network model and a 2-D surface model were respectively established and coupled to simulate the flood processes during an assumed storm surge in Jinshan District, Shanghai. The cumulative influence of the concurrent storm surge, typhoon, rainfall, and upstream flooding was explored to support hazard risk analysis for Jinshan District. The coupled model’s simulation indicated a clear decrease in the number of waterlogged areas in Jinshan District after considering the river network’s storage and transportation capacity during a storm surge event. The distribution of predicted waterlogged areas also changed; according to the simulation results, the flood risk grade decreased in the central and northern Jinshan District and rose in the Northwest corner.
In this study, four typical typhoons that significantly affected Shanghai were selected based on their respective intensity and the water level along the Shanghai Coast. The RMW (Radius of Maximum Winds) formula, moreover, was determined using in-situ data from recent typhoons. The typhoon model was built and validated using in-situ wind speeds from the four typhoons selected. The peak wind speed and the forward peak wind speed along the Shanghai Coast were calculated, case by case, during all typhoons over the period from 1949 to 2014 as well as the four typical typhoons selected. Finally, the range and distribution of the peak (forward peak) wind speed were quantitatively studied.
In this study, we analyzed the spatial and temporal variations in the residual water level of the Changjiang Estuary and evaluated the respective causes. To achieve this objective, we used data from the hourly water level at the Chongxi, Nanmen, and Baozhen hydrological stations in the Changjiang Estuary; daily river discharge levels at the Datong hydrological station; and wind speed and direction at the Chongming eastern shoal weather station in 2016 and 2017. The results showed that the residual water level was the highest at Chongxi station and the lowest at Baozhen station among the three hydrological stations in each month. The drops in residual water level among the hydrological stations became smaller during low river discharge and tended to become larger during high river discharge. Higher levels of river discharge were associated with a larger drop in the residual water level. In 2016, the residual water levels at the Chongxi, Nanmen, and Baozhen hydrological stations were lowest in February with values of 2.09, 1.96, and 1.93 m, respectively; similarly, the residual water levels were the highest in July with values of 2.91, 2.62, and 2.50 m, respectively. The residual water level was mainly affected by river discharge, while the wind was also an important influencing factor in the variations observed. Southerly wind made the residual water level decrease, and northerly wind made it increase. In 2017, the minimum monthly mean residual water level occurred in December with values of 2.04, 1.91, and 1.87 m at the Chongxi, Nanmen, and Baozhen hydrological stations, respectively; this coincided with the lowest annual river discharge observed during the same period. The maximum monthly mean residual water level occurred in October with values of 2.79, 2.58, and 2.49 m at the Chongxi, Nanmen, and Baozhen hydrological stations, respectively. Although the river discharge was lower in October than the one in July by 24214 m3/s, the residual water level was higher in October than that in July. The explanation for this phenomenon is the persistent strong northerly wind observed in middle to late October, which produced strong landward Ekman water transport, and resulted in the water level rise. The spatial and temporal variation in the residual water level of the Changjiang Estuary is remarkable, and should be considered in engineering design and theoretical research.
Qingcaosha Reservoir is the main water source for Shanghai, providing approximately 55% of its high-quality raw water needs, and effectively guarantees the safety of water supply for Shanghai. The waters near the Qingcaosha Reservoir experience saltwater spillover from the North Branch into the South Branch; the nearby waters, moreover, suffer from direct saltwater intrusion from the open sea. In this study, a large number of measured salinity data in the upstream and downstream sluice was used to statistically analyze the characteristics of direct saltwater intrusion near the Qingcaosha Reservoir waters in recent decades. The analysis results show that direct saltwater intrusion neat the Qingcaosha Reservoir waters in recent decades was closely related to the river discharge, tide, and wind. There were a total of 16 instances of direct saltwater intrusion at the upstream sluice that occurred from September to March of the following year; likewise, there were a total of 41 instances of direct saltwater intrusion at the downstream sluice that occurred from September to May of the following year. The direct saltwater intrusions at the upstream and downstream sluices appeared primarily in December, January, and February of each year. We found that saltwater intrusions occurred most commonly when the river discharge was less than 18 000 m3/s during neap tide and middle tide (after neap tide) accompanied by persistent northerly or northwesterly winds. We found that the strength and duration of the northerly or northwesterly winds in the days preceding saltwater intrusion had an important role on direct saltwater intrusion.
Twenty-two surface sediments collected from the Changjiang Estuary and neighboring shelf were subjected to particle-size measurements, with the intent of understanding the implications for provenance, transport, and depositional dynamics. The results showed that Changjiang River-derived sediments, relict sands, and Yellow River-derived sediments were the primary sources controlling the magnetic properties of sediments in the study area. The three areas, however, exhibited different spatial distributions. Spatial variations of magnetic parameters, including magnetic susceptibility (χ), saturation isothermal remanent magnetization (SIRM), hard isothermal remanent magnetization (HIRM), and anhysteretic susceptibility (χARM), suggest that sediments from the Changjiang River are transported towards the south and southeast when they move out of the river mouth. According to bi-plots of SIRM versus χ and S-ratio (S–100) versus SIRM, the > 63 μm fraction is roughly bounded by the 30 m isobaths that separates the Changjiang River sediment from the relict sands on the shelf. The < 16 μm fraction is derived mainly from the modern fluvial sources of the Changjiang and Yellow Rivers; in particular, the Changjiang River-derived sediment dominates the inner estuary and the Yellow River-derived sediment dominates the northern coast of the shelf. The other areas of the shelf are characterized by mixed sources of the < 16 μm fraction, with a majority being Changjiang River-derived sediment. Spatial variations of particle size compositions and magnetic properties reflect the role of hydrodynamic sorting on particle size as well as mineral density; this results in differences in magnetic properties among the sedimentary units as well as the contribution of different sized fractions to the bulk SIRM values. Particle size separation could reduce the effect of particle size on bulk magnetic properties and lead to more precise provenance discrimination. Our results have great potential in the study of geomorphological changes and quantitative source identification in delta environments.
Flux footprint analysis is an important step in studying the carbon, water vapor, and heat flux exchange of land-atmosphere interactions based on the eddy covariance (EC) method. In this research, we used the flux source area model (FSAM) to investigate seasonal flux footprints with different wind directions and atmospheric conditions on the basis of half-hourly EC measurements throughout 2018. The results showed that: ① The flux footprint area changes with the seasons. The largest flux footprint area, ordered highest to lowest, was found in autumn, summer, spring, and winter under stable stratification; meanwhile, under unstable stratification, the flux footprint area did not change significantly between seasons. The daily variation in the footprint, moreover, was obvious and the footprint was found to be larger comparatively at nighttime than that observed during the daytime. ② The flux source area under non-prevailing wind conditions was larger than that under the prevailing wind condition. ③ The flux source area was much larger under stable stratification. The distance between the location of the maximum value of the flux footprint and the station was also found to be much larger under stable stratification.
Surface sediments were collected from five representative areas—the floodgate entrance, the north and south sides of the reclamation area, and the central and downstream sections—of Qingcaosha Reservoir; the pollution characteristics and potential ecological risk of seven heavy metals (Cu, Zn, Pb, Cr, Cd, As and Hg) in these sediments were subsequently investigated. Results showed that the heavy metal content in the surface sediments showed spatial variations: the content was relatively higher in the center of the reservoir and was low in the north and south sides of the reclamation area. Heavy metals in the surface sediments, in addition, were mainly in the residual fraction; the content of heavy metals in the exchangeable fraction was extremely low. A potential ecological risk assessment indicated that the comprehensive potential ecological risk index (ERI) of the investigated heavy metals ranged from 55 to 113. The maximum ERI value was observed around the floodgate of the reservoir entrance, and low ERI values were observed at the north and south sides of the reclamation area. The ERI was lower than the threshold for low ecological risk, indicating that heavy metals in the surface sediments of the Qingcaosha Reservoir have low potential ecological risk.