A review on the driving forces of water decline and its impacts on the environment in Poyang Lake, China

The recession of water levels of natural lakes and their associated impacts on wetland ecosystems is a serious issue worldwide. Poyang Lake (the largest freshwater lake in China) has experienced a heightened and prolonged water decline since the year 2000, which causes concern for associated ecological impacts. In particular, climate change, operation of the Three Gorges Dam (TGD), and high magnitude sand mining appear to be well-correlated with the occurrence of water decline in Poyang Lake. Though the above factors have been analyzed in previous studies, a comprehensive summary has never been compiled. This paper provides a detailed literary review highlighting the driving forces and possible impacts of the consistent water decline in Poyang Lake. We conclude here that the operation of TGD is a fundamental cause for the lake water decline, aggravated by climate change and sand mining. The water decline has caused a deterioration of water quality, as well as having given rise to a potential threat to the habitat of migratory birds and Yangtze finless porpoises. The paper intends to offer constructive references that can be used in decision-making for effective protection of water resources and lake ecosystems.


INTRODUCTION
It is a well-recognized fact that lakes and their associated watersheds have unique hydrological and biogeochemical cyclical processes, and also provide crucial functions for both local environments and communities, such as biodiversity maintenance, fresh water supply, flood mitigation, agriculture and recreation (Mitsch et al. ; Zedler & Kercher ). However, the maintenance of the natural hydrological regime of many lakes tends to be at odds with globalization, meaning that many lake basin ecosystems are confronting severe challenges (Pekel et al. ; Srinivasan et al. ). Under the influence of 21st-century global climate change and intensified human activities such as enhanced anthropogenic disturbances, hydrological issues facing lake systems have been characterized by water level decline, over-exploitation, frequent floods and droughts, increased eutrophication and pollution, disruption of aquatic ecosystem equilibrium, lake degradation or even extinction (Jiang ; Carpenter et al. ; Paerl et al. ). One of the most severe lake-related tragedies is the irreversible shrinkage of the Aral Sea in Central Asia (Micklin ). In the context of severe changes, the scope of UNESCO's International Hydrological Programme has recognized five major elements as an important multidimensional complex of promoting sustainable development: water resources, biodiversity, ecosystem services, resilience, and culture (Zalewski et al. ).
The Yangtze River in China is the largest and longest river in South Asia, with a water discharge of 900 km 3 /yr and a sediment load of 470 Mt/yr, ranking 5th and 4th in the world respectively (Yang et al. ). To meet the needs of social-economic development, more than 50,000 dams have been established on the river and its tributaries for the purpose of flood prevention, hydroelectric generation and irrigation (Yang et al. ). Such artificial interference has certainly changed the hydrological conditions of the Yangtze River Basin. Hence, many natural lakes in the Yangtze River Basin have faced growing conflicts between socioeconomic development and aquatic environments ( Jiao ). For instance, the lake systems attached to the Yangtze river basin have historically accounted for 21.2% and 24.1% of the country's lake area and the total number of lakes, distributed on both banks of the Yangtze River.
In the middle reaches of the Yangtze River, watershed, marshes, multiple river channels and thousands of lakes are found in the plains, forming a unique and comprehensive ecosystem of rivers and lakes. These lakes are used to supply water to regional agriculture, and also are efficient reservoirs, which are important for flood control (Nakayama & Watanabe ; Yu et al. ; Ma et al. ). Nevertheless, over the past half century, a great number of lakes have vanished due to human disturbances such as the reclamation of lakes and hydraulic engineering projects. As a consequence, Dongting Lake and Poyang Lake are the only remaining large lakes that still maintain a natural connection with the Yangtze River in the middle and lower reaches (Du et al. ). Dongting Lake was once the largest freshwater lake in China with a maximum surface area of about 6,300 km 2 , but due to the adverse land conversion and the establishment of water conservancy facilities, the area of the lake has been reduced by approximately 60% (3,800 km 2 ). Many studies have analyzed this dynamic interaction by way of administration, exploitation, and protection of Dongting Lake, though the trend of shrinkage and fragmentation is still ongoing (Li et  First, Poyang Lake is principally fed by five major tributaries within its basin, and it is connected with the Yangtze River through one outflow. The basin area of Poyang Lake is approximately 162,000 km 3 , accounting for 9% of the total area of the Yangtze River Basin. The average annual outflow to the Yangtze River is 14.6 billion m 3 , accounting for 17% of the average annual runoff of the Yangtze River (Zhu ).
Second, the total population in the Poyang Lake region is over 10 million, with a GDP of more than 500 billion RMB, having increased by 400 billion in the past decade alone. Due to the flat terrain around Poyang Lake (Figure 1), this area has been known as 'the land of plentiful rice' since ancient times. The preservation of Poyang Lake is facing challenges from high-speed urbanization and the collaborative development of industrialization and agriculture in the surrounding areas.
Third, Poyang Lake provides unique and critical habitats and rich food resources for rare species, including the Yangtze finless porpoise, swan goose, and nearly 95% of the entire world population of the endangered Siberian crane (Wu et al. ; Dronova et al. ). Therefore, the variations in the hydrological situation of Poyang Lake not only affect the development of the social economy in surrounding areas, but also determine the regional biodiversity and ecological balance.
Poyang Lake is a unique system, much different from other large lake systems of the world. For example, the Great Lakes in North America are much larger and deeper than Poyang Lake and include a larger watershed with more input tributaries feeding them. Classically, the water levels and area of the Great Lakes are typically much more constant than Poyang Lake as the latter may fluctuate by a large amount seasonally due to a relatively shallow water depth. However, recent developments in the Great Lakes have included excessive low level events in the 2000s, caused by increased water temperature (enhanced evaporation) due to a significant decrease in winter ice cover (Assani et al. ). Conversely, the past few years have shown a reversal of this trend, with historical record high water being observed. Yet, the overall water levels and area of the Great Lakes, even during extreme events, only vary by a small amount when compared to the overall depth and area, and while some elements of the ecology are under greater threats due to these dynamical changes, the overall health of the Great Lakes is strong (Assani ).
The ecohydrology stability of Poyang Lake is more fragile under the interference of climate change and human activities. On one hand, the frequent large floods in Poyang Lake have caused extensive damage to the wetland ecosystem and the social economy (Yu et al. ).
Statistics indicate that from 1950 to 2010, there were 17 years that can be considered major flood events (when the Poyang Lake water level exceeded 20 m), and there were six years that can be defined as severe floods (when the Poyang Lake water level exceeded 21 m) (Li et al. ). It was also found that all of these severe floods happened during or shortly after El Niño events, which shows that the flood phenomenon is closely related to climate change, especially precipitation (Shankman et al. ).
On the other hand, since the beginning of the 21st century, the persistent shrinkage of the lake area and continuous decline of the water level have been well documented (Feng et al. a; Lai et al. a; Yao et al. ).  have studied the water level decline of Poyang Lake which have obtained diverse results. However, the issue of determining the exact nature and contributions of the dominant driving forces is still a subject of strong debate. For instance, some studies illustrated that the enhanced dry season was primarily driven by climate changes while others pointed out that impoundment of water by the TGD rather than precipitation was the major cause of the shrinkage of Poyang Lake (Ye et al. a; Zhang et al. , a). Meanwhile, other studies revealed that the pervasive sand mining in Poyang Lake has increased the discharge factor. Numerous efforts have investigated the cause of drought in Poyang Lake, yet few studies have done a comprehensive analysis of the impact of this constant water decline. In 2019, Poyang Lake has faced the most severe drought conditions in recent history, and the occurrence of the dry season has significantly advanced (by approximately 30 days), according to hydrology department data ( Figure 1). Therefore, proper management strategies of Poyang Lake should be based on careful analyses of hydrological parameters. Our review is mainly based on papers published since 2000, aiming to establish a link between the water decline and its impact on the lake. We examine the current perception of the relevant issues and highlight promising directions for further research. The review attempts to answer the following critical questions: 1. What is the extent of the water decline in Poyang Lake in the past decade? 2. What are the primary mechanisms responsible for the water decline of Poyang Lake? 3. What is the impact of the water decline, and what aspects of future research should be pursued to better understand and mitigate this issue?

DESCRIPTION OF POYANG LAKE
Poyang Lake (at a latitude of 28 24 0 to 29 46 0 N and a longitude of 115 49 0 to 116 46 0 E) is located in the middle reaches of the Yangtze River ( Figure 1) (Lai et al. a). The Yangtze River is the longest river in China and the longest river on the Eurasian continent, and plays an important role not only in the maintenance of the wetlands ecosystem, but also in the social and economic development of China (Du et al. ). It originates in the Tibetan Plateau and flows eastward through central China (Zhang et al. a).
Poyang Lake lies in southeastern China and is connected to the Yangtze River through a narrow channel in Hukou county. The lake receives surface runoff from five major tributaries: Xiushui, Gan jiang, Fuhe, Xinjiang and Raohe, and the total catchment area of the lake accounts for 97% of the territory of Jiangxi Province, home to 46.6 million residents, and classified as a subtropical monsoon climate (Dronova et al. ).
As a natural lake connected to the Yangtze River, the comprehensive effects of the Yangtze River outflow, the inflow of its five tributaries, and various climate conditions form a strongly seasonal characterized water regime of Poyang Lake. Poyang Lake has an inundation area and maximum length which fluctuates from less than 1,000 to over 3,000 km 3 and from 74 to 173 km, respectively, between the summer flood season (June-September) and the relatively drier winter dry season (November-March) (Feng et al. ). As the water level varies from approxi-

PATTERNS OF WATER DECLINE AND SHRINKAGE IN POYANG LAKE
has severely diminished in the dry season, which has led to advanced exposure of marshland, and the long-term ecological water shortage in the dry season has been enhanced These studies reached a consensus that the water level of Poyang Lake has decreased drastically as compared with long-term averages. However, another important issue concerns the shorter-term variation during this extended dry period seen during the past decade. Previous studies used to 2015, and pointed out that Poyang Lake has entered a relatively dry period since the 2000s. In general, previous studies indicated that the decline of lake water level as well as the declining inundation trend have been significant ( Figure 3). Hence, the general consensus is that the prolonged drought, enhanced low water level and shrinkage of the inundation area have all placed immense pressure on water utilization and wetland ecosystem protection of Poyang Lake.

DRIVING FORCES OF HYDROLOGICAL REGIME VARIATIONS IN POYANG LAKE
The above problems are of great concern for governments and local residents. In order to understand if the recent decline is a continuous trend affected by specific factors, it is of critical importance to clarify the mechanisms responsible for these changes. Previous studies have generally focused on the following aspects: Regional climate change, the impact of the Three Gorges Dam, and the impact of sand mining. We will summarize and discuss these factors below.

Impact of three Gorges Dam
As discussed above, the correlation between climate change and water level in autumn is not obvious, and another question can be raised: Are there other driving forces that dominate the water level of Poyang Lake in autumn? One study proposed a perspective that the recent lake water decline was not a long-term trend, as no significant correlation was seen between precipitation and outflow discharge; instead, the shrinkage was considered as a Since Poyang Lake is naturally connected with the middle-lower Yangtze River, it is highly suggested that due to the operation of the Three Gorges Dam, there will be two significant changes in Poyang Lake water levels. The first aspect is that in order to increase the storage capacity for flood mitigation, the TGD water release from May to June will increase the water level of the Yangtze River and inevitably constrain the discharge of Poyang Lake to Yangtze River, called the enhanced blocking effect. The second aspect is that in order to generate power, the rapid water impoundment of TGD from September to November will lower the water level of the Yangtze River, which may accelerate the drainage from Poyang Lake, known as the enhanced empty effect. The empty effect is usually thought to intensify the drought during the dry season of Poyang Lake (Zhang et al. a). However, it was pointed out by

Impact of sand mining
Another important factor that is a likely influence on Poyang Lake water levels comes from sand mining. Sand The impact of sand mining on the water level of Poyang Lake is likely to strengthen with increasing demand of sand and gravel. Another impact of increased mining is that the sediment balance of Poyang Lake is changing significantly.
After 2000, Poyang Lake no longer receives significant loads of sediment from the Yangtze River. (Gao et al. ). Consequently, the sediment load of the Yangtze River entering the sea now contains increasingly more sediment from Poyang Lake. Thus, Poyang Lake has been converted from a depositional to an erosional system with a gross sediment loss of 120.19 Mty À1 .
Thus, it is highly suggested that topography changes caused by sand mining will bring increased impacts to the hydrological features of Poyang Lake such as water level

IMPACTS OF THE DRAMATIC RECESSION IN POYANG LAKE
The majority of recent research on the water decline of Poyang Lake has mainly focused on the driving forces, and little focus has been given to its impact on different aspects of the environment. Yet this is an important consideration, as it can significantly affect the availability and quality of fresh water, causing eco-environmental problems to emerge, thereby triggering modifications to the biota of Poyang Lake. We thereby chose four elements of the Poyang Lake wetland ecosystem (water quality, landscape, migratory birds, and Yangtze finless porpoises) as representative metrics of the direct and indirect impacts of the water decline in Poyang Lake.

Water quality
The primary factor that can be affected by the variation of hydrological conditions is water quality. In China, water quality impairment has been severe in certain regions, with important consequences for ecology as well as sustainable economic and social development (Duan et al. ).
There have been many studies on the water quality of Poyang Lake, but most of these involved sampling analyses of certain positions. The reduction of Poyang Lake's water level will inevitably affect the transport and spread of pollutants and alteration of water residence time. These changes may cause serious ecological damage, such as eutrophication, which has affected many lakes located in the middle and lower Yangtze River. It has been recognized that the  Since Poyang Lake is a shallow lake, the thermal regime of the lake has not received a great deal of research attention. However, Li et al. (Li et al. b) found that the lake is generally stratified during summer and early autumn, and classified as partially mixed and fully-mixed during winter and spring, respectively. Subsurface light penetration and thermal stratification will also likely change with the decline of water level, and research into this aspect should be strengthened. Another crucial parameter affecting water quality is water residence time, which can indicate the transport process of nutrients. The regional Though the current water quality of Poyang Lake is shown to be steady, a deteriorating trend has been observed at the interannual scale in the last decade. This is particularly true in the dry season, as the probability of serious deterioration of water quality has reached 50%-70% (Wu et al. ). This suggests that the water decline is a direct mechanism of water quality deterioration.

Landscape, vegetation and migratory birds
If Poyang Lake's low-water conditions continue, the temporal pattern of wetland submergence or emergence at different elevations will subsequently change, which will in turn alter the spatial landscape structure of the Poyang Lake wetland system, and modify the wetland vegetation belt and dominant communities. These conditions are critical to the sutainability of habitats and feeding locations for wintering migratory birds in Poyang Lake. Therefore, studying the response of the spatio-temporal changes of landscape types such as shoals, mud-flats, and meadows to the water level variation can help improve the stability of the wetland ecosystem of Poyang Lake as well as provide informed suggestions for better protection of wintering migratory birds.
Vegetation is the most extensive class seen during the dry season of Poyang Lake, occupying 41.2-49.3% of the total area, followed by water, mudflats and shoals. Therefore, the response of various migratory birds to the change of the landscape patterns of Poyang Lake is a direction that should be studied in the future, using a combination of biology, ecology, geography, and even epidemiology study methodologies.

The Yangtze finless porpoises
Poyang Lake is an ideal habitat and foraging place for the Yangtze finless porpoises whose populations are threatened

DISCUSSION AND REMARKS
There is a well-known local saying about Poyang Lake, which says 'When the water level is high, we see an expansive lake; when the water level is low, we see isolated rivers'.
However, this regular seasonal variation is destabilizing. In the current year (2019), Poyang Lake has again experienced an advanced dry season and an extremely low water level. This paper has thus reviewed many recent papers concerned with the constant water decline in Poyang Lake. Whereas numerous previous studies usually focused on a specific hydrological feature such as water levels at certain gauging stations or inundation areas, we present here a systematic summary of the important results: After entering 2000, the occurrence of extremely low lake levels (below 10 m) has increased to 25%, while occurrences of extremely high levels (over 22 m) have ceased. The average decline of the lake water level is 0.66 m, and the annual mean inundation area is declining by 30 km 2 /yr, the minimum lake area A thorough temporal analysis of how to quantify the continuous water decline in Poyang Lake was carried out by Huang et al. (), in which a total of 47 hydrological indicators were considered including monthly mean water levels, monthly maximum water levels, monthly minimum water levels, and the rate and frequency of water level changes. These indicators were analyzed to accurately determine the time period when the most dramatic alteration happens through the year. Nevertheless, there still exists a gap in understanding the spatial differentiation within Poyang Lake. Taking the year 2019 as an example, when the low-water period of Poyang Lake was advanced and the minimum water level was continuously refreshed, extreme water decline was also seen in the main tributaries.
Another factor that may have a significant effect on the hydrological and ecological processes of Poyang Lake is the dynamics of seasonal lakes in floodplain regions. Floodplain lakes occupy a relatively small part of Poyang Lake's water volume, yet they contain a large part of the surface water area during low water periods (Tan et al. ). The study by Li et al. () showed that the temporal influences of the seasonal lakes on water levels, lake outflows, and inundation were greater during dry seasons, and the impact of seasonal lakes on the magnitudes of hydrological responses during the dry season is much stronger than during the flood season. Water balance analyses carried out by Li et al. (b) highlighted that the catchment rivers and the associated groundwater system are important parts of Poyang Lake. These findings suggested that the study area should be enlarged to the basin scale, starting from changes in the overall hydrological conditions, and then distinguish the changes in different areas. It is particularly important to take hydrological hysteresis into consideration, which is created by the combined dynamics of the Yangtze River, its associated lakes, and its floodplains. This approach should prove to be more successful in discovering the driving forces with greater accuracy.
Most previous studies mainly discussed specific driving forces, such as climate change, operation of the TGD, sand mining, etc (Table 1) This article chose four elements of ecohydrology (water quality, landscape, migratory birds, and Yangtze finless porpoises) as representative examples to analyze the impacts of the water decline in Poyang Lake. One clear conclusion is that decreasing water level in Poyang Lake has led to the deterioration of the water quality. The advance of Poyang Lake's dry season and the frequent occurrence of extremely low water levels will increase the area of exposed wetlands and may provide more habitat for migratory birds. The habitat and foraging of migratory birds in Poyang Lake is a complicated process, as the gradually exposed wetlands provide rich food resources for birds, while the rapid and largescale exposed wetlands may accelerate wetland desiccation and change plant community structure, which may affect the foraging habits of migratory birds (Jiang et al. ; Zhi et al. ). Moreover, the increased area of exposed wetlands may enlarge the range of human activities, further reducing the habitat of migratory birds. The current research on Yangtze finless porpoise protection focuses on the effectiveness of ex-situ conservation (Zheng et al. ; Wang ), and how changes in hydrodynamic conditions caused by water decline will affect the distribution and survival of finless porpoises in Poyang Lake, this is another issue that should be examined closer in future studies. Fortunately, Poyang Lake is about to enter a long fishing moratorium period (10 years), so that the fish biomass and biodiversity of Poyang Lake are likely to show an increasing trend (Wang et al. a). Though many open research questions remain, it is clear that the impact of water decline on Poyang Lake will affect many secondary systems. Evaluating the ecosystem services of Poyang Lake, and investigating its variation under a decreasing trend of water level can assess the effectiveness of lake ecosystem management. Future studies can address whether the various types of nature reserves on Poyang Lake need to have their locations and ranges adjusted to achieve better protection.
A recent proposal by the government of Jiangxi Province has received much debate, which is to build a hydraulic project on the Poyang Lake to solve the drought problem. On one hand, this project seems to be a direct and effective way to control the water level in the lake. Some scholars have suggested that establishing a dam will be an effective solution to reduce the Yangtze river effect and can bring significant economic benefits. On the other hand, some experts have claimed the dam may trigger a fundamental and possibly detrimental change in the wetland ecosystem (Lai et al. ; Wang et al. c; Wu et al. b). In view of this quandary, specific future research questions may be suggested, such as: Will the construction of a dam solve the influence of water decline in Poyang Lake or it will be enhanced? Will the ten-year fishing moratorium period benefit the hydrobiology condition? To what degree will the hydrological conditions caused by the construction of the dam affect the habitat of birds and finless porpoises?
Overall, the recent water decline has become an undeniably serious problem of the ecohydrology of Poyang Lake.
The changes in the interaction between the river and the lake caused by the operation of the TGD and increasing sand mining operations are the most probable internal and external reasons for the water level decline in Poyang Lake observed since 2000. It will be practical for future studies to focus on the impacts of the decreasing water level to better understand the different responses of the environmental factors of Poyang Lake to the water decline.
These efforts will serve to better protect and ensure the continued health of the wetland ecosystem.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.