The Impacts of Hydrology and Climate on Hydrological Connectivity in a Complex River–Lake Floodplain System Based on High Spatiotemporal Resolution Images

The Central Yangtze ecoregion in China includes a number of lakes, but these have been greatly affected by human activities over the past several decades, resulting in severe loss of biodiversity. In this paper, we document the present distribution of the major lakes and the changes in size that have taken place over the past 50 years, using remote sensing data and historical observations of land cover in the region. We also provide an overview of the changes in species richness, community composition, population size and age structure, and individual body size of aquatic plants, fishes, and waterfowl in these lakes. The overall species richness of aquatic plants found in eight major lakes has decreased substantially during the study period. Community composition has also been greatly altered, as have population size and age and individual body size in some species. These changes are largely attributed to the integrated effects of lake degradation, the construction of large hydroelectric dams, the establishment of nature reserves, and lake restoration practices.

The variation trend and growth rate of P were analyzed by the concentration of the phosphorus fraction on surface sediment of Dongting Lake from 2012 to 2016, to reveal the cumulative effect of P in the actual environment. Meanwhile, the adsorption kinetics and adsorption isotherm were employed to examine the P-release possibility of sediment, which predicts the yearly released sediment phosphorus in Dongting Lake. The actual growth rate of TP (Total Phosphorus) is 53mg·(kg·year)-1 in East Dongting Lake, 39mg·(kg·year)-1 in South Dongting Lake, and 29mg·(kg·year)-1 in West Dongting Lake, while the sum of the phosphorus fraction growth rates has little difference from the rate of TP in sediments of the three areas of Dongting Lake. Furthermore, the Elovich model and the Langmuir crossover-type equations are established to present the adsorption characteristic of sediment in Dongting Lake; the result shows that the sediments play a source role for phosphorus in East and South Dongting Lake from zero equilibrium phosphorus concentration (EPC0) in the present situation, but an adsorption effect on TP is shown in West Dongting Lake. When the conditions of environment change are ignored, the maximum P-sorption level in sediments of East Dongting Lake will reach in 2040 according to the actual growth rate of sediments, while that in West Dongting Lake and South Dongting Lake will be in 2046 and 2061, respectively.

In order to better understand the effect of the lake shrinkage on the flood process in a large lake,a quantitative analysis of the relationship between the lake shrinkage and the flooding in Dongting Lake was conducted,for which Jingjiang-Dongting Lake hydrodynamic model was applied and different scenarios of water area in the Dongting Lake were assumed. The results show that the water level and peak discharge will increase,and the peak time will occur earlier with the shrinkage of the lake. The water level in the West and South Dongting Lake would increase by around 2.0 m and the water level in the East Dongting Lake would increase by around 0.4 m if the area of Dongting Lake was shrunk from 2 670 km2to 1 380 km2 when the 1996-type flood occurred in Dongting Lake. Moreover,the arrival time of the peak water level at Chenglingji station would be about 11 hours earlier,and the peak discharge would increase by about 4 300 m3/s. Hence,the flood disaster would be more severe in the Dongting Lake area if the lake continues to shrink from now on. In addition,the water surface slope changes a little in the West and South Dongting Lake. But the water surface slope increases a lot near the Lujiao station due to the water level in the East Dongting Lake is affected by both of the lake shrinkage and the water level in Yangtze River.

The Dongting Lake (DTL) area constitutes a naturally interconnected lake wetland. Alterations in river–lake relationships are poised to profoundly modify the ecological condition of the lake region. This study aims to investigate the hydrological interactions and ecological response mechanisms involving the Yangtze River, the Four Rivers within the DTL basin (DFR), and DTL itself during extreme hydrological events. Amidst the trajectory of wetland contraction, the impact of shifts in river–lake relationships on the wetland's state remains unclear. Thus, we employ cross‐wavelet transformation and the multifractal method to establish a framework for the hydrological interactions between rivers and the lake. Through this framework, we analyze the varying progression of hydrological relationships within the river–lake system and their corresponding responses. The findings reveal that instances of low‐water events in DTL predominantly manifest during winter and spring. These low‐water events exhibit a propensity to occur frequently and irregularly, except during July–August in most statistical years, coinciding with the Yangtze River's flood regulation period. Additionally, the DFR–DTL system exhibits a broader range of significant resonance periods and indicates a strong and consistent correlation between the hydrological events in the river and lake systems. The DFR–DTL system serves as the principal impetus behind the ecological evolution of DTL. Simultaneously, alterations in the inundation status across DTL regions display an ascending trend from west to east. The outcomes of this study provide a scientific foundation for elucidating the evolutionary dynamics of river–lake connectivity and informing effective ecological restoration strategies within the DTL region.

This paper conducts an assessment on the social-ecological resilience of China’s Dongting Lake (and its three sections – East, West and South Dongting Lake) in relation to the perturbations of the Three Gorges Dam using a set of resilience-based indicators. Expert scoring is applied to identify the different states of the lake and their resilience levels. Based on equal weighting for all indicators and using the technique of ordering preferences according to similarity to the ideal solution, an assessment of the social-ecological resilience of Dongting Lake is generated. The results show that East Dongting has higher ability to absorb perturbations than South and West Dongting which have relatively low resilience to the changes triggered by the impoundment of the Dam. Effective adaptation measures are needed for the lake to be able to better absorb these perturbations and be sustainable in the long run.

Chlorophyll a variations and responses to environmental stressors along hydrological connectivity gradients: Insights from a large floodplain lake

The Dongting Lake is the largest lake in the Middle and Lower Yangtze River and ranks the second largest freshwater lake in China.Seasonal fluctuation of water-level in the Dongting Lake forms stable wetland resources.A large area of shallow marshes and mudflats,which are exposed over water surface during the non-flood season in the winter,provide habitats for several migratory birds.In recent years,the effects of the hydrological processes on wetland ecological system have aroused worldwide attention.The complex nonlinear relationship between the Yangtze River and the Dongting Lake has led significant changes in hydrological processes in the Dongting Lake after the running of the Three Gorges Project. Data sets of surface area in the Dongting Lake were extracted using MODIS imagery from 2000 to 2010.During interpretation process,NDVI index and NIR band threshold were set to 1000,meaning that NDVI and NIR band values less than 1000 were water wetlands and values higher than 1000 were beach wetlands,including mudflat wetlands,grass wetlands and reed wetlands.Since the water-level of the Chenglingji was proportional to the surface area in the Dongting Lake,the observed water-level data sets were used to validate the rationality of the extracted surface area data sets.The results revealed that correlation coefficient of the extracted surface area and water-level was 0.8058.Mann-Kendall(MK) nonparametric test showed that the Zs statistics were-2.4986,-3.0867 and-2.5421 in the 0.05 level of significance for the whole year,from June to September(flood season) and from October to May in the next year(non-flood season) during 2000 to 2010,respectively.The trends indicated that runoff in the Dongting Lake was reduced dramatically during the last decade.The average surface water area decreased 15.21% and 32.05% during flood season and non-flood season in 2010 compared with that in 2000. In the paper,wetlands in the Dongting Lake were divided into water wetlands and beach wetlands.More attention was paid to the changes of beach wetlands area during non-flood season because of its importance for migratory bird.The data sets of area of water wetlands and beach wetlands indicated that the area of water wetlands decreased dramatically while beach wetlands increased correspondingly from 2000 to 2010.Compared with the data in February,October,and December in 2000,29.98%,26.76%,and 9.02% water wetlands were changed into beach wetlands in the corresponding month in 2010,respectively.According to the statistics of observed water-level of the Chenglingji that firstly and lastly reached 24m,25m,26m,27m and 28m,water-level fluctuation was investigated.The results showed that water-level fluctuation from 24 m to 26 m led to earlier exposure of beach wetlands and later flood of beach wetlands,which prolonged the exposure time of beach wetlands.The change of water wetlands and beach wetlands promoted the evolution of mudflat wetland to grass wetland in the East Dongting Lake at water-level of 24m to 26m.In conclusion,it is multi-factors,including reduction of runoff from Sankou(Songzi,Ouhe,Taiping) and Sishui(River Xiang,River Zi,River Yuan and River Li) and reduction of rainfall etc.,that contributed to the responses of wetlands area to hydrological process of the Dongting Lake.The impoundment of the Three Gorges Project in September to October would further aggravate the trend of water wetlands changing into beach wetlands.

Assessing hydrological connectivity for natural-artificial catchment with a new framework integrating graph theory and network analysis

Since July 2022, the Yangtze River basin has experienced the most severe hydro-meteorological drought since record collection started in 1961, which has greatly affected the ecological environment of the Dongting Lake (DTL) basin. To investigate the effects of drought events on the eutrophication and phytoplankton community structure of DTL, the lake was sampled twice in August and September 2022 based on the water level fluctuations resulting in 47 samples. Furthermore, we combined the comprehensive trophic level index (TLI) and phytoplankton Shannon–Wiener diversity index (H) to characterize and evaluate the eutrophication status. The key influencing factors of the phytoplankton community were identified using redundancy analysis (RDA), hierarchical partitioning, and the Jaccard similarity index (J). Our results showed that the TLI of DTL changed from light–moderate eutrophication status (August) to mesotrophic status (September), whereas the H changed from light or no pollution to medium pollution. The phytoplankton abundance in August (122.06 × 104 cells/L) was less than that in September (351.18 × 104 cells/L) in DTL. A trend in phytoplankton community succession from Bacillariophyta to Chlorophyta and Cyanophyta was shown. The combination of physiochemical and ecological assessment more accurately characterized the true eutrophic status of the aquatic ecosystem. The RDA showed that the key influencing factors in the phytoplankton community were water temperature (WT), pH, nitrogen and phosphorus nutrients, and the permanganate index (CODMn) in August, while dissolved oxygen (DO) and redox potential (ORP) were the key factors in September. Hierarchical partitioning further indicated that temporal and spatial variations had a greater impact on the phytoplankton community. And the J of each region was slightly similar and very dissimilar, from August to September, which indicated a decreased hydrological connectivity of DTL during drought. These analyses indicated that the risk to the water ecology of DTL intensified during the summer–autumn drought in 2022. Safeguarding hydrological connectivity in the DTL region is a prerequisite for promoting energy flow, material cycle, and water ecosystem health.

A framework based on spectral similarity to estimate hydrological connectivity in Juruá River floodplain lakes using 3-m PlanetScope data

Different effects of natural siltation, damming and pollution on two adjacent lakes of Dongting Lake Wetland: Evidence from paleolimnology

Can the “10-year fishing ban” rescue biodiversity of the Yangtze River?

Eight heavy metals including Cr, Ni, Cu, Zn, As, Cd, Hg, and Pb were investigated in surface water and sediments from a river-connected and a river-disconnected lakes (East Dongting Lake and the Honghu Lake, respectively). Zn had the highest mean concentrations in surface water in both lakes. Pearson’s correlation, principal component analysis, and hierarchical cluster analysis revealed that the distributions of Zn, Pb, and As in the East Dongting Lake and Zn, Cd, and As in the Honghu Lake were associated with anthropogenic activities. Nickel and Cr were associated with natural sources, while Cu and Hg originated from both anthropogenic and natural sources. Sediment quality guidelines revealed that both As and Hg could probably have adverse effects in the East Dongting Lake, whereas all detected heavy metals probably could not lead to adverse effects in the Honghu Lake. Potential ecological risks indices (RIs) of heavy metals in two lakes were 999.4 and 151.45, respectively, indicating greater pollution of heavy metals with high risk in the East Dongting Lake as compared to the Honghu Lake with low to moderate risk.

As a large river connected lake, Dongting Lake is influenced by anthropogenic activities and the discharge from its upstream tributaries in the lake basin and by the water recharge via a connection to the Yangtze River (YR) outside the basin. This makes the lake phosphorous cycle more complex than that in other disconnected lakes. Here, we calculated section fluxes and ran a hydrodynamic model to investigate the phosphorus (P) variations in response to the changing interactions in the water and sediment between the YR, four tributaries, and the lake. Results show that particulate P was the dominant form with a significant linear relationship with suspended sediment (r 2=0.906). The sediment input reduction from the YR through three water inlets, which is closely related to the Three Gorges Reservoir operation since 2003, led to a decrease in the total P (TP) concentration in the western Dongting Lake. However, the impact and range of this decrease were fairly limited. Compared with the limited effect of the YR, the raised TP flux from the Yuanjiang tributary controlled the TP concentration at the outlet of the western Dongting Lake. Apart from the influence of the YR and the tributaries, anthropogenic activities (sand dredging) in the eastern Dongting Lake also contributed to a high TP concentration around the S10 area through sediment resuspension. We suggest that, compared with the reduction in TP flux and sediment load from the connected Yangtze River outside the basin, the elements within the basin (increased TP input from tributaries and sand dredging) have a greater effect on the variations of TP in Dongting Lake.

ABSTRACTSurface sediment samples were collected from 10 typical locations throughout the Dongting Lake, China, in January 2009. Samples were assayed by atom absorption spectrophotometer and cold atomic fluorescent spectrophotometer for Pb, Cd, As, Hg. In order to investigate the spatial distribution characteristics, sources, and potential ecological risks of heavy metals, the geostatistics method, potential ecological risk index, and multivariate statistical analysis were applied. The results showed that except for the content of Hg, the contents of Pb, Cd, and As had similar spatial distribution characteristics. The average contents of Cd and As exceeded the second class contents of the National Standard for Soil Environment Quality (GB15618-1995), especially that for Cd. The potential ecological risk posed by these heavy metals decreased in the order of the outlet of the Dongting Lake > the East Dongting Lake > the South Dongting Lake > the West Dongting Lake spatially. From the results of multivariate statistical analysis, Pb and Cd, as the first group, were considered to be rooted in mining smelting processes for developed mining and heavy industry. And Hg, as the second group, was mainly derived from parent material weathering, while As was probably considered to originate from both sources above.