Ecological Continuum from the Changjiang (Yangtze River) Watersheds to the East China Sea Continental Margin

Energy and material fluxes from watersheds converge in coastal zones and at open ocean boundaries, and climate variability, biogeochemical processes, and anthropogenic perturbations in the coastal zone have strong feedbacks with other components of Earth’s physical and social systems. Land–ocean interactions, forced by dynamic processes and natural and human activities, are concentrated in the zone between upstream watersheds, in this study the Changjiang (Yangtze River) Estuary, and the open boundary of the East China Sea. Studies of land–ocean interfaces in this region (e.g., Changjiang Estuary) have been carried out since the 1940s, and the knowledge based on previous research forms an important foundation for an integrated discussion of the hydrography, geochemistry, geography, ecology, and biology of the system. The sustainable development of coastal ecosystems depends on how well we understand the function and response of these systems under combined natural and human forcings, and adaptation and management policies depend on the understandings of the way that land–ocean interactions respond to these external forcings.

In this chapter, we use our previous observational and modeling studies to analyze some basic dynamic features of the Changjiang (Yangtze River) Estuary, including the tidal and subtidal motion, saltwater intrusion, and the extension of a river plume into the ocean. The dynamics of this estuary are strongly regulated by tides on both intertidal and subtidal scales. Harmonic analysis shows that the tidal regime has one dominant semidiurnal constituent. The tidal range has significant seasonal variations, but is typically between 4 and 5 m during the spring tide and ~2 m during the neap tide. Tidal forcing induces significant subtidal circulation, resulting in a net landward flow into the North Branch of the Changjiang Estuary when river discharge is low. This residual transport forms a type of saltwater intrusion known as the saltwater-spilling-over (SSO). This causes high-salinity water in the North Branch to intrude upstream and finally spill over into the South Branch, which is the major channel of the estuary. The SSO has a major impact on the Changjiang Estuary and on the availability of freshwater for nearby cities. In areas near the river mouth, the tide-induced residual transport also results in significant inter-channel water mass exchange. Freshwater output from the Changjiang forms a large-scale plume, which plays a key role in physical processes in the adjacent East China Sea and Yellow Sea. We found that, in the region close to the river mouth, tidal forcing has a marked effect on the extension of the plume into the open ocean. Wind forcing also plays an important role in the dynamics of the Changjiang Estuary. Northerly winds can enhance the SSO and saltwater intrusion around the river mouth. In the plume-affected region, the wind significantly modulates the plume shape, especially farther from the river mouth.

This chapter deals with river flow and sediment transports in the Changjiang River continuum. We first briefly introduce riverine flow and sediment discharges acting on the estuary and their variations in the past half century. There is an ongoing decrease in sediment discharge since the 1980s, which has accelerated since 2003 when the Three Gorges Dam (TGD) commenced operation. In situ flocculation of suspended sediment is detected in the freshwater river and the blackish estuary, affecting sediment transport in the system. In the estuary, changes in sediment concentrations and grain sizes are observed in response to reduced river-borne sediment load. Analysis of sediment grain sizes, suspended sediment concentration (SSC), and the clay–silt–sand content of the sediments in the estuary reveals the presence of a nearshore depocenter featured by high sediment exchange ratio off the South Passage. Numerical experiments of estuarine circulation and water age suggest that large-scale engineering works have had marked impact on the regional hydrodynamics. Overall, the river, the estuary, and the marginal sea are inherently connected through the water and sediment linkage; therefore, systematical insights on their hydro-, morpho-, and eco-dynamics are required.

This chapter focuses on anthropogenic impacts on the water and sediment discharge of the Changjiang over the past 150 years and the sedimentary response of its delta and adjacent waters. Increased water consumption and dam construction have reduced the annual water discharge by ~10 % in the past 150 years. This decrease is mainly attributable to the reduction between August and November each year when the reservoirs store water. In contrast, the water discharge in January and February each year has shown a significant increasing trend, presumably due to water release from reservoirs in the dry season. Human impacts on the riverine sediment load are even more pronounced than impacts on river discharge. The annual sediment load has decreased by more than 70 % since the 1970s and by more than 50 % since the 1990s, due to dam construction and soil conservation in the drainage basin. The decline in sediment discharge has caused a drastic decrease in the progradation of intertidal wetlands on the delta front and has generated erosion on the subaqueous delta. Sediment resuspension from delta erosion partly offsets the decrease in riverine sediment supply, so that the suspended sediment concentration (SSC) in the estuary and adjacent waters only decreased by an average of 26 % over the period 1992–2009. It is predicted that these temporal trends will continue to occur in response to anthropogenic impacts for the remainder of this century.

Over the last several decades, the Changjiang (Yangtze River) watersheds has suffered from an acceleration of economic activities and changing land-use patterns. Monitoring programs at downstream hydrological stations and data from field campaigns have shown that these changes have caused an increase in the loss of chemical elements (e.g., plant nutrients such as N, P, and Si) from catchment areas and changes in pollutant discharges (e.g., trace elements). Changes in plant nutrients and trace elements in the river have altered the partitioning of chemical species between water and solid phases (e.g., total suspended matter) in aquatic systems, in both freshwater and marine environments. Irreversible element partitioning in rivers and coastal waters can affect the bioavailability of chemical species in adjacent marine environments, which can in turn impact ecosystem structure and functioning. In the East China Sea, the continual increase in plant nutrient fluxes from land sources, such as the Changjiang, has caused eutrophication in surface waters of the coastal environment. The degradation of organic matter, either allochthonous or autochthonous in origin or both, has fueled heterotrophic processes that induce the seasonal depletion of dissolved oxygen (DO) and even hypoxia in the Changjiang Estuary and the inner shelf of the East China Sea. Based on observational hydrography and chemistry data, a box model approach is used in this study to describe the plant nutrient and trace element budgets. The results illustrate that incursion across the shelf break of the western boundary current system (i.e., Kuroshio) has a significant influence on the inventory and residence time of chemical elements such as plant nutrients on the East China Sea Shelf.

As one of most populated river–estuary–shelf regions, the biogeochemistry of organic matter in the Changjiang Estuary and East China Sea (ECS) is critical for a quantitative understanding of global biogeochemical cycles. This chapter summarizes the spatial variation of organic matter and biomarkers from the watershed to the shelf. Monthly data collected from the downstream of the watershed help to elucidate the seasonal variation of biomarkers (e.g., pigments and amino acids) along with hydrological and biological processes. The geochemical characterization of biomarkers in the ECS is well studied, and this chapter provides an overview of the potential controls on biomarkers. Hydrological sorting, in situ primary production, and diagenetic processes contribute to the variable biomarker distribution, composition, and burial in the ECS. Anthropogenic disturbance is illustrated by an evaluation of the impact of the Three Gorges Dam (TGD) on the composition of terrigenous organic matter and pollutants in the study system. Climate change (e.g., extreme drought), impoundment by the TGD, and the effects of tributaries and lakes in the middle and lower streams of the river may play different roles in the delivery of organic carbon. Although there is high pressure from anthropogenic activities compared with other regions in the world, pollutant concentrations are relatively low and potential ecological risks are limited. With changing fluxes from rivers, and significant modification and burial of organic matter on the shelf due to global change, long-term observations are necessary to develop our knowledge of the biogeochemistry of organic matter in this highly dynamic shelf region.

The enormous quantities of sediment produced by the Changjiang (Yangtze River) have created extensive coastal wetlands in the estuarine region, which have been colonized by various types of salt marsh vegetation. As the Changjiang Estuary is located adjacent to the megacity of Shanghai, human activities exert substantial pressures on the coastal wetlands and cause coastal ecosystem degeneration and consequent decreases or loss in ecosystem services. The major stresses from human activities on the coastal wetlands in the Changjiang Estuary include urbanization, reclamation, the introduction of invasive species, overfishing, and environmental pollution. In this chapter, the succession of plant communities in the coastal wetlands of the Changjiang Estuary is described. The spatial and temporal variation of plant communities on salt marshes are analyzed on the basis of remote sensing (RS) and mapping, combined with field inventories of sampling plots. The population expansion of an exotic species, Spartina alterniflora, after its introduction to the coastal wetlands in the Changjiang Estuary in the 1990s, is described and analyzed. The impacts of the invasion of S. alterniflara on benthic communities are examined along intertidal salt marsh gradients, and the invasion history is described. A cellular automaton (CA) model was developed to simulate the expansion of S. alterniflora and to study the interactions between the spatial pattern of expansion and salt marsh vegetation ecosystems. Field experiments and a demonstration project were carried out to control the invasion of S. alterniflora. Finally, this chapter discusses the future development, management, and conservation of coastal wetlands and biodiversity in the Changjiang Estuary.

This chapter focuses on the biological components of the Changjiang (Yangtze River) Estuary and adjacent East China Sea shelf, including phytoplankton, zooplankton, benthic macro-invertebrate, and fish faunas. During recent decades, the marine ecosystems in this area have experienced marked disturbances induced by human activities and changing climate. An increase in the abundance of dinoflagellates constitutes the most important change in phytoplankton populations over the last 20 years. The accelerated eutrophication related to human activity, particularly increased anthropogenic inputs into the Changjiang, and the increasingly skewed ratio of nutrient species that has accompanied global warming, might be partially responsible for these changes. The Changjiang Estuary and the adjacent East China Sea shelf are situated in the subtropical–warm temperate biogeographic boundary zone of the NW Pacific Ocean. Warming of surface waters in this area has been occurring since the middle to late 1980s. Over this same period, the abundance of temperate and warm temperate zooplankton has significantly decreased, and the abundance and frequency of occurrence of subtropical and tropical zooplankton have significantly increased. The Changjiang Estuary and the adjacent East China Sea shelf are the habitats of many resident and migratory fish species (present in various life cycle stages) that are of commercial and ecological value. However, because of human-induced disturbances (such as over-fishing) and environmental changes, many of the larger and commercially important demersal species occupying higher trophic levels have been replaced by smaller commercially marginal pelagic species occupying lower trophic levels. Thus, the recent intensification of human-induced disturbances and those of climate change necessitate and demand an integrated estuarine, coastal, and marine management strategy for this region.

The four coastal provinces (Jiangsu, Zhejiang, Fujian, and Taiwan) and autonomous city (Shanghai) adjacent to the East China Sea are among the most developed and the densely populated areas in China. Over the last 30 years, these areas have experienced rapid urbanization and industrialization. With increasing incomes and improved living standards in China, so tourism, traffic, and related industries have also developed rapidly. The rapid urbanization of these provinces along the coast of the East China Sea has adversely impacted the marine environment. Domestic sewage discharge per capita has increased significantly, especially in metropolitan cities such as Shanghai. Illegal reclamation and dumping have also had adverse effects on the marine environment. Increased use of fertilizer to promote higher agricultural crop yields has directly impacted water quality in the East China Sea, leading to widespread eutrophication. Incident such as oil spills has had serious impacts on the marine ecosystem. River inputs represent the largest source of water pollution in the East China Sea, but sewage outfalls, mariculture, engineering activities, agriculture, and nonpoint source pollutants are other key contributors. At the present time, water quality in the East China Sea meets national standards, but it has been deteriorating for the last 10 years. Measures must be implemented to prevent further deterioration in water quality as a result of human activities.