Dongting Research Articles

The Current Distribution of Oncomelania hupensis Snails in the People’s Republic of China Based on a Nationwide Survey

Schistosomiasis is a helminth infection caused by the genus Schistosoma, which is still a threat in tropical and sub-tropical areas. In the China, schistosomiasis caused by Schistosoma japonicum is mainly endemic to the Yangtze River valley. The amphibious snail Oncomelania hupensis (O. hupensis) is the unique intermediate host of S. japonicum; hence, snail control is a crucial approach in the process of schistosomiasis transmission control and elimination. In 2016, a nationwide snail survey was conducted involving all snail habitats recorded since 1950 in all endemic counties of 12 provinces. A total of 53,254 existing snail habitats (ESHs) were identified, presenting three clusters in Sichuan Basin, Dongting Lake, and Poyang Lake. The overall habitat area was 5.24 billion m2, of which 3.58 billion m2 were inhabited by O. hupensis. The area inhabited by snails (AIS) in Dongting and Poyang Lakes accounted for 76.53% of the population in the country. Three typical landscape types (marshland and lakes, mountains and hills, and plain water networks) existed in endemic areas, and marshland and lakes had a predominant share (3.38 billion m2) of the AIS. Among the 12 endemic provinces, Hunan had a share of nearly 50% of AIS, whereas Guangdong had no ESH. Ditches, dryland, paddy fields, marshland, and ponds are common habitat types of the ESH. Although the AIS of the marshland type accounted for 87.22% of the population in the whole country, ditches were the most common type (35,025 or 65.77%) of habitat. Six categories of vegetation for ESHs were identified. A total of 39,139 habitats were covered with weeds, accounting for 55.26% of the coverage of the area. Multiple vegetation types of snail habitats appeared in the 11 provinces, but one or two of these were mainly dominant. Systematic sampling showed that the presence of living snails was 17.88% among the 13.5 million sampling frames. The occurrence varied significantly by landscape, environment, and vegetation type. The median density of living snails in habitats was 0.50 per frame (0.33 m × 0.33 m), and the highest density was 40.01 per frame. Furthermore, two main clusters with high snail densities and spatial correlations indicated by hotspot analysis were identified: one in Hunan and Hubei, the other in Sichuan. This national survey is the first full-scale census on the distribution of O. hupensis, which is significant, as transmission interruption and elimination are truly becoming the immediate goal of schistosomiasis control in China. The study discerns the detailed geographic distribution of O. hupensis with the hotspots of snail density in China. It is beneficial to understand the status of the snail population in order to finally formulate further national control planning.

Modulation of sediment load recovery downstream of Three Gorges Dam in the Yangtze River

The sediment load in the Yangtze River downstream of the Three Gorges Dam (TGD) has substantially declined in recent decades. The decrease is more profound below the TGD, e.g., a 97% decrease at Yichang, compared with that at the delta apex, 1200 km downstream, e.g., a 75% decrease, implying along-river sediment recovery. Two large river-connected lakes, i.e., Dongting and Poyang Lakes, may play a role in the re-establishment of the river’s morphodynamic equilibrium, but a quantitative data-based understanding of this interaction is not yet available. In this work, we collected a series of field data to quantify the sediment gain and loss in the river-lake system in the middle-lower Yangtze River, and evaluate the lake’s response to the reduction in riverine sediment supply. We find that Dongting Lake and Poyang Lake shifted from net sedimentation to erosion in 2006 and 2000, and back to a sedimentation regime again after 2017 and 2018, respectively. Natural morphodynamic adaptation and sand mining play an important role in the regime changes in the Dongting Lake whereas sand mining dominates the abrupt changes in the Poyang Lake. The Dongting and Poyang Lake contributed maximum by 38% (2015) and 17% (2006) (respectively) to the sediment recovery in the erosion regime, whereas the riverbed erosion dominates the main sediment source. These changes in the relative contribution of sediment sources also indicates a response time of ~ 20 years in the lakes towards a new equilibrium state. It is noteworthy that the lakes’ buffer effects may be overestimated as the supplied sediment from the lakes is rather small compared to the significant dam trapping in the upstream basin and sediment source from downstream degradation. The results imply that river management and restoration should take into account of the river-lake interactions and feedback impact at decadal time scales.

Hydrological drought in two largest river-connecting lakes in the middle reaches of the Yangtze River, China

Abstract Poyang and Dongting Lakes are two important river-connecting lakes with a complicated water system pattern in the middle reaches of the Yangtze River, China. Recently, extreme drought events occurred frequently in the two lakes. This study analyzed the characteristics and differences of hydrological droughts in Poyang and Dongting Lakes during the period of 1964–2016 and explored the correlation between drought and large-scale climate indices. The results showed that the hydrological droughts of Poyang and Dongting Lakes became increasingly serious. Especially after 2003, both lakes entered the dry season earlier and the intensity of drought was increased. The hydrological drought of Poyang Lake was more serious than that of Dongting Lake in spring (16.40%) and winter (14.26%), while the autumn drought in Dongting Lake (32.46%) was severer than that in Poyang Lake (27.65%). The spring droughts in the two lakes were significantly associated with droughts in their local catchments with the joint probabilities of 10.84 and 9.52%, while the autumn droughts were consistent with the hydrological droughts of the Yangtze River with large joint probabilities (26.39 and 27.76%). The changes in large-scale climate indices more significantly affected the drought in Poyang Lake than that in Dongting Lake, especially in autumn and winter.

Evolutionary patterns of the sedimentary environment signified by grain size characteristics in Lake Dongting during the last century

湖泊沉积物储存着长时期的流域环境信息，是百年尺度湖泊环境演变研究的高分辨率记录器。本文通过²¹⁰Pb_ex、¹³⁷Cs组合定年法，分析放射性核素剖面特征，研究沉积速率时空变化规律，并解析沉积物粒度指示的环境信息，揭示百年尺度湖泊沉积环境演变。结果表明：洞庭湖²¹⁰Pb_ex剖面分布呈现多段衰变型、混合-衰变-平衡型、衰变-本底型和无沉积型4种类型。3大湖体200 cm的沉积深度对应的年代跨度为127~162年，构建了洞庭湖沉积物深度-年代关系。洞庭湖平均质量沉积速率为1.47~3.11 g/（cm²·a），整体上呈现南洞庭湖>西洞庭湖>东洞庭湖的空间特征；自1900年后，质量沉积速率增加了3~7倍，呈时序增加趋势。沉积年代上，沉积物粒度特征表明洞庭湖沉积环境呈3个阶段2个过渡期的演变特征，与同时期区域内毁林开荒、水利建设、植树造林等人为活动有较好的对应关系，说明近百年来人为活动是洞庭湖泥沙沉积的重要因素。本研究结果可为强化湖泊及其流域生态环境保护提供理论参考。;Lake sediments record large amount of environmental information from the watershed over a long time, and has become a high-resolution archives for investigating the lake environmental change at the centennial scale. In this paper, we analyzed the features of radionuclide profiles through the combined dating method of ²¹⁰Pb_ex and ¹³⁷Cs, investigated the temporal and spatial changes of sedimentation rate, and extracted the environmental information indicated by sediment particle size to reveal the patterns of the sedimentary environment during the last century. The results showed that ²¹⁰Pb_ex profiles presented four stages:a multi-stage decay, a mixed-decay-equilibrium, a decay-background and a no-deposition. The sediment depths of 200 cm in the three major lake bodies covered a chronological span of 127-162 years, and a depth-age relationship was constructed for Lake Dongting sediments. The mean mass accumulative rate (MAR) ranged from 1.47 to 3.11 g/(cm²·a), demonstrating an overall spatial characteristic of the highest MARs in the south, and higher in the west than the east. Since 1900, MARs in Lake Dongting increased 3-7 times, showing an upward trend in the chronology. The depositional change of sediment grain size in Lake Dongting was characterized by three stages and two transitional periods, which corresponded well with anthropogenic activities such as deforestation, hydrological construction, and afforestation in the basin in the period. These findings suggested that anthropogenic activities had been an important factor for sediment deposition in Lake Dongting. The foundings in our study can provide a theoretical reference for enhancing the ecological protection of the lake and its watershed.

Distinct bacterial communities associated with photosynthetic picoeukaryotes in Lake Dongting throughout the seasonal cycle as revealed by flow cytometry sorting

Associations and interactions between bacteria and phytoplankton are prevalent in aquatic systems. However, the coexistence of bacteria with the smallest phytoplankton groups is mostly unknown. Here, the associated bacteria (AB) of photosynthetic picoeukaryotes (PPEs) in the Yangtze-connected Lake Dongting were studied using flow cytometry sorting combined with high-throughput sequencing of the 16S rRNA gene throughout the seasonal cycle. Bacteria in the corresponding whole water (WWB) were also investigated for comparison. Our results revealed a distinct bacterial community of AB compared to WWB. The taxonomic and phylogenetic diversity indices, as well as the richness, of AB were significantly lower than those of WWB; however, AB showed higher Pielou’s evenness. Proteobacteria (53.8%) was the most abundant phylum in AB, whereas Actinobacteria was predominant in WWB (36.0%). Several abundant clades, including Flavobacterium, Pseudomonas, Rheinheimera, Novosphingobium, and Sphingomonas, were significantly enriched in AB. Functional prediction analysis identified 116 KEGG pathways differing significantly between AB and WWB. Particularly, functional properties involved in cellular processes (mainly bacterial chemotaxis), environmental information processing, organismal systems and xenobiotics biodegradation and metabolism were greatly enriched in AB. Total nitrogen, temperature and chlorophyll a correlated significantly with the community dissimilarity between AB and WWB. Our results shed new light on the bacterial diversity and function associated with the smallest phytoplankton.

Assessment of integrity of macrobenthos in Lake Dongting (1988-2021) and impacts from environmental stress

Assessment of integrity of macrobenthos in Lake Dongting (1988-2021) and impacts from environmental stress

Driving factors of wetland evolution in Lake Dongting during 1900-2020

Driving factors of wetland evolution in Lake Dongting during 1900-2020

The Climate Adaptive Characteristics of Urban Inside/Outside Water Bodies Based on Their Cooling Effect in Poyang and Dongting Lake Regions, China

The Climate Adaptive Characteristics of Urban Inside/Outside Water Bodies Based on Their Cooling Effect in Poyang and Dongting Lake Regions, China

Water depth resources and driving mechanism of navigation hindrance of Jingjiang Reach watershed after Three Gorges Project operation

系统解析长江中游河湖交汇区域航道水深资源的变化,可支撑“黄金航道”的可持续发展。本研究以洞庭湖-荆江交汇段为对象,研究洞庭湖分汇区域水沙条件、荆江河段滩槽演变与河湖交汇段航道水深资源的关系。研究表明:三峡工程运行以来,洞庭湖三口分流量和分沙量延续了三峡工程运行前的减少态势,伴随洞庭湖分流量减少,相对增加的长江干流径流量增强了河床冲刷强度,反馈使得洞庭湖三口分流量和分沙量均处于较低值;荆江河段河床冲刷给航道尺度提升奠定了有利基础,河湖分汇区域不满足4.5 m×200 m(水深×宽度)的长度为12.6 km,占荆江河段碍航总长度的68.35%。碍航驱动机制上:松滋口分流区段(枝城-昌门溪)的4.5 m水槽贯通但宽度不足200 m,汊道分流关系不稳定及洲滩萎缩制约航道条件稳定,枯水位下降及“坡陡流急”现象仍然严峻,不利于航道条件保障及船舶安全航行;太平口分流区段水位下降、洲滩萎缩、汊道交替发展使得枯水航路不稳定或水深不足4.5 m;藕池口分流区域的洲滩崩退、洲滩冲刷引起的向下游泥沙输移,碍航表现为航宽不足200 m或出现水深不足4.5 m的浅滩;洞庭湖入汇影响区段(熊家洲-城陵矶)受弯道冲淤特性、湖泊出流顶托作用等影响,航槽内出现水深不足4.5 m的浅滩,但顶托作用存在趋缓趋势,有利于航道条件的改善。;A systematic analysis on the effects of the changed interactive relationships between rivers and lakes on fairway depth can help to support the sustainable development of the “Gold waterway” of the Yangtze River. In this study, we investigate the variation characteristics of water and sediment, and the relationships between the evolution of shoals and channels and the fairway depth, in the Lake Dongting-Jingjiang Reach confluence of the Yangtze River. The results indicate that since the operation of the Three Gorges Project, the split ratio of watersplit and the sediment distribution of the three outlets of Lake Dongting continued to decrease. Along with the decreasing flow of Lake Dongting, the relative increase in the runoff of the Jingjiang Reach increases the riverbed erosion scouring intensity, which keep the split ratio of water split and the sediment distribution of the three outlets at a lower value. The riverbed erosion scouring of the Jingjiang Reach has created a promising foundation for an increase in the waterway dimensions. However, the length of the reach fail to meet the requirement of 4.5 m× 200 m (water depth × width) is 12.6 km in the river-lake confluence, accounting for 68.35% of the total length of obstructed navigation. The mechanisms of the obstructed navigation are as follow: at the reach affected by the first outlet (Songzikou), a 4.5 m flume has been formed, but its width is less than 200 m. The instability of water split and the shrinkage of bottomland restrict the stability of channel conditions. Moreover, the reductions of low-flow water level and the phenomenon of “steep slope and rapid current” affect the ships' safety. At the reach (Taipingkou) affected by the second outlet, the shrinkage of the bottomland, and the intersecting braided river channels make the low-flow routes unstable, the water depth being less than 4.5 m. At the reach (Ouchikou) affected by the third outlet, the shoreline of the reach has collapsed and retreated significantly. Additionally, point bar beach erosion scouring has led to the downstream movement of sediment, which have caused the downstream waterway width at the reach to be reduced to less than 200 m or formed shoals with a water depth of less than 4.5 m. Owing to the characteristics of scouring and deposition in the bends reaches and the jacking effect of the lake's outflow backwater, the water depth of some shoals are less than 4.5 m in the navigation waterway. However, the jacking effect of the lake's outflow backwater is weakening, which benefits the improvement of channel conditions.

Contrasting nutrients input along with groundwater discharge to east Dongting Lake, central China: A geological perspective

The spatial patterns of lacustrine groundwater discharge (LGD) and associated nutrient inputs are crucial for effective management and protection of lakes. Multiple factors have been found to influence the spatial differences in LGD rates and associated nutrients loads, but the influence of geological conditions on the differences have not been well understood. In this study, we quantified LGD rates and associated nutrients loads on the west and east of the lake with contrasting geological conditions of East Dongting Lake (EDL) within central Yangtze catchment and discuss the influence of geology on the spatial differences, through 222Rn mass-balance model, water chemistry coupled with existing geological data. The results showed that LGD rates were 34.76 ± 23.36 mm/d in the east EDL which is characterized by hilly geomorphy, deep/fast/narrow flowing, coarse-grained lakebed and large hydraulic gradients (0.004–0.006). Surprisingly, LGD rates were higher (71.47 ± 52.16 mm/d) in the west EDL which is characterized by alluvial-lacustrine plain geomorphology, shallow/sluggish flowing, clayey or silty lakebed and low hydraulic gradients (0.0002–0.0015). The remaining factor determining the higher LGD rates in the west EDL is the permeability of the porous aquifer connected with the lake. The groundwater around the east EDL existed in a less confined environment, and frequent flushing led to low concentrations of nutrients. By contrast, rapid burial of sediments and deposition of paleo-lake sediments since Last Deglaciation formed an organic-rich and reducing environment, which facilitated the enrichment of geogenic nutrients. As a result, the loads of LGD-derived nutrients in the west generally exceeded that in the east by one order of magnitude. The study provides new understanding of the spatial differences in LGD and associated nutrients input to the large lakes resulting from variations in geological conditions, thus serving as a reference for ecological protection of lakes.

Morphological Adjustment and Diversion Discharge Prediction in the Three Outlets Channel of the Jingjiang River

The Three Outlets Channel (TOC) consisting of three anabranching rivers formed due to natural avulsion by extremely high floods, is the primary water–sediment channel connecting the Jingjiang in the middle Yangtze River and Dongting Lake (the second largest freshwater lake in China). Since the impoundment of the Three Gorges Reservoir (TGR) in 2003, the decreasing diversion of runoff and sediment load of the TOC has an impact on the hydrological connectivity of the river–lake relation. Nonetheless, it lacks complete understanding on the diversion of runoff and sediment load, erosion and deposition amount, and the calculation method of diversion runoff in the TOC. We used remote sensing images, hydrological data, and channel topography to analyze the change in runoff and sediment load, channel morphology, local erosion and deposition amounts. The main results show that: (i) Meander cutoffs of the Lower Jingjiang accelerated the reduction process of the Ouchi River’s runoff and sediment load and the increase in the number of dry days. After the impoundment of the TGR, the diversion of sediment load of the TOC was greatly affected, but the decreasing trend of the runoff diversion slowed down. (ii) The morphological change of the inlet zone of Ouchi River is larger than that of Songzi River and Hudu River. The morphological evolution of the inlet area led to the change in the diversion of runoff and sediment load of the TOC. (iii) In the dry season, the water level drop in the inlet zone of the TOC leads to a decrease in the diversion discharge. Therefore, considering the water level drop and channel width adjustment in the inlet zone, five empirical formulae for the diversion discharge of the five hydrological stations in the TOC are proposed. These empirical formulae can be used as a short-term forecast for future changes in the hydrologic regime and the dynamics of the Jingjiang–Dongting Lake relation.

Mitochondrial diversity and phylogenetic structure of Yellowcheek (Elopichthys bambusa) in the Yangtze River

The immense hydrological changes in the Yangtze River basin during the last decades have had drastic effects on the Yellowcheek Elopichthys bambusa migration rate and its population structure. As a consequence, the declining population size of the E. bambusa populations raises concern in terms of sustainability of their genetic integrity and conservation management. The E. bambusa is described as data-deficient by IUCN Red List of Threatened Species. In this research, the species demographic structure and molecular phylogeny were examined from five populations in the Yangtze River basin's center and lower parts. A total of 73 individuals were collected, sequenced and analyzed for the mitochondrial cytochrome b gene (cyt b) variation. Three distinct mitochondrial lineages of E. bambusa were revealed, with 39 different haplotypes which nucleotide divergence ranged from 0.87–0.98. The analysis of molecular variance (AMOVA) suggested the existence of historically limited gene flow among the lineages. The stocks of Dongting Lake (DTL), Poyang Lake (PYL) and Donghu lake Wuhan (WHN) belong to the same lineage while, the samples collected from Danjiang Reservoir (DJR) and Taihu Lake (THL) appeared as separate lineages. The results suggest an underestimated genetic diversity of Elopichthys species which may refer to each of the mtDNA lineages in three different groups. Future studies should add more information on this species genetic and conservation status.

Eutrophication has a greater influence on floodplain lake carbon cycling than dam installation across the middle Yangtze region

Carbon cycling in shallow floodplain lakes is complex due to variability in delivery of flood-derived allochthonous organic matter (OM). Human activities have potential to significantly modify the carbon balance of lakes by damming which restricts external OM inputs and via eutrophication which can increase the in-lake production of algae and/or aquatic plants. In order to understand how these human activities influence carbon cycling in shallow floodplain lakes over decadal-centennial timescales, we analysed C/N ratios and δ13C from terrestrial plants, catchment soils, aquatic plants and dated sediment cores from six heavily modified lakes in the middle Yangtze floodplain. Submerged macrophytes (−21.4 ± 4.6 ‰) had higher δ13C than C3 plants from the catchment (−26.6 ± 0.6 ‰) and emergent and floating plants (−26.6 ± 4.0 ‰). Increases in sedimentary chlorophyll a (from primary producers) were associated with a decline in sedimentary δ13C in the severely eutrophic Dongting, Luhu, Wanghu and Poyang Lakes after the 1980s. In contrast, sedimentary δ13C increased in Honghu and Futou Lakes which have abundant submerged macrophytes. The timing and scale of sedimentary δ13C changes indicated stronger responses to eutrophication than damming, with eutrophication responses ranging from a macrophyte proliferation to the dominance of phytoplankton.

Importance and main ecological and environmental problems of lakes in China

<p indent="0mm">Lakes including reservoirs are sensitive to global environmental change and human activities in the basin. They are sensitive sentinels and regulators of global environmental change and regional climate. Meanwhile, lakes are important constituent of “landscape, forest, farmland, lake, grassland and sand” life community. Remote sensing monitoring showed that there were 2670 lakes (≥<sc>1 km²)</sc> with a total water area of <sc>80662.4 km²,</sc> and 5156 reservoirs (≥<sc>1 km²)</sc> with a total water area of <sc>39697.1 km²</sc> in China in 2020. China Water Statistical Yearbook 2021 showed that the number and storage capacity of completed reservoirs increased from 86822 to 98566, and from <sc>416.9</sc> to <sc>930.6 km³</sc> from 1980 to 2020. Therefore, there is a total lakes and reservoirs area of <sc>120359.5 km²,</sc> but only accounting for 1.25% of China’s land area. Although the area of lakes and their proportion of land area are very limited, lakes play a vital role in ensuring the ecological civilization construction and promoting the economic and social development of the basin. Unfortunately, the importance and ecological service values of lakes are seriously underestimated and lack the real quantification. Therefore, this study systematically explores and quantifies the importance and ecological services of lakes in China from seven different perspectives. First, lakes are the most and the best centralized drinking water sources (CDWSs) contributing 40.6% of the CDWSs vs. river (30.8%) and groundwater (28.6%), and act as the backbone of the safety of drinking water in China based on 1093 CDWSs from 340 prefecture-level cities and 55 county-level cities (population ≥1 million). Second, lakes have important regulation and storage functions and play a key role in flood control and drought resistance. Based on the national lake survey data, the total amount of water regulation of natural lakes (≥<sc>1 km²)</sc> is close to <sc>150 km³.</sc> Meanwhile, the joint operation of the Yangtze River Basin in 2021 indicated a total regulation capacity of <sc>106.6 km³</sc> and a total flood control capacity of about <sc>69.5 km³</sc> of 47 control reservoirs. Third, lake basins play an important role in the national economic development and food security. Of China’s nine major commercial grain production areas, seven are located in Jianghuai lakes zone and northeast plain lake zone. Fourth, lakes are one of the richest biodiversity ecosystems on earth. Among the 62 international important wetlands in China, there are 26 lake wetlands including Lake Poyang, Lake Dongting, Lake Qinghai etc accounting for 41.9%. Fifth, lake fishery plays an important role in China’s fishery production evidenced by China Fishery Statistical Yearbook 2021, which showed that the aquaculture area of lakes was 2186662 hectares, accounting for 42.7% of the national freshwater aquaculture area and 30.8% of the national aquaculture area in 2019. Sixth, lakes have important tourism value and contribute to beautiful business cards of many cities. Among the 45 national tourist resorts in China, there are 15 lakes tourist resorts, accounting for one third, highlighting the rich tourism value resources of lakes. Seventh, lakes are an important part of the northern ecological security barrier. Lake Hulunhu, Lake Wuliangsuhai, Lake Daihai, Lake Qinghaihu, Lake Bositenghu, and Lake Aibihu act as the key nodes of northern ecological security barrier. In addition, lakes play a big role in energy storage and power generation, agricultural irrigation, inland navigation, carbon sequestration and pollution reduction and regional climate regulation. Large numbers of lakes with different types are widely distributed throughout China exhibiting obvious regional differentiation. Moreover, lakes are facing many ecological and environmental problems which reduce their value of providing ecological services. Specifically, eutrophication and cyanobacteria bloom have not been fundamentally alleviated, which threats the water quality safety of lake drinking water sources. The aquatic vegetation of the lakes is seriously degraded, which weaken their purification capacity and reduce their biodiversity. The synergistic effects of rapid climate change and intense human activities cause either lake flooding, shrinkage or salinization. Warm and humid climate results in a rapid expansion of the lakes on the Tibetan Plateau, which significantly increase the risk of glacial lake outburst. Aiming the above-mentioned ecological and environmental problems, this study puts forward the strategies and countermeasures for lake protection and management including lake-basin system studies, scientific regulation of water resources, cyanobacteria bloom early warning and control, national lake ecological restoration project, basin industrial structure upgrading, and lake protection legislation, which will promote the improvement of lake environmental and the construction of beautiful lakes.

Paleolimnological records for tracking dam-induced changes in the composition and supply of sediment to middle Yangtze floodplain lakes

A global boom in dam construction has reduced sediment loading of large rivers, as well as their connected floodplains. In order to explore potential effects of hydrological regulation on floodplain ecosystems, this study presents concentrations of inorganic elements, organic matter content and grain size spectra in 210Pb-dated sediment cores of five lakes in the middle Yangtze River floodplain. These lakes had free hydrological connectivity with the Yangtze River before the operation of local sluice gates, and hence K-rich but Al-poor particulates from the upper Yangtze reaches can be transported into lakes during periods of high river discharge. After hydrological regulation, sedimentary K/Al ratios registered substantial decreases in the study lakes, probably due to the declining supply of fine-grained and K-enriched riverine particulates from the Yangtze River. In East Dongting Lake, a hydrologically open lake proximal to the Three Gorges Dam (TGD), the sharp decrease in K/Al ratio after 2003 mainly responded to declining sediment supply from the Yangtze River after the TGD operation. In addition, prolonged water retention time probably promoted aquatic production and greater deposition of organic matter, as indicated by recent increases in organic matter content in the upper strata of the dammed lakes. Taken together, sedimentary K/Al ratios provide essential information on effects of past hydrological changes on sediment supply and aquatic productivity over multi-decadal timescales in these floodplain lakes with scarce monitoring data. The impacts of dam construction during recent decades on sediment composition in middle Yangtze floodplain lakes might be widespread in other similar floodplains worldwide. Thus sedimentary records can be utilized to inform and direct floodplain management.

Characteristics and Sources of DOM in Lake Sediments Under Different Inundation Environments

The characteristics and sources of DOM in sediments are significantly affected by fluctuations in lake water levels. However, the impact of spatial differences on water levels remain unclear. Here, 36 sediment samples were collected from the flood passage and coastal beach of East Dongting Lake. The differences in the composition and source of DOM in sediments under perennial inundation and seasonal inundation were studied using UV-visible absorbance (UV-Vis) and fluorescent excitation-emission matrix (EEM)-parallel factor analysis (PARAFAC). Three fluorescent components of DOM in the sediment were identified. The relative abundance of protein-like components was as high as (72.95±8.94)%, including tryptophan (C2) and tyrosine (C3). However, the humic-like component (C1) abundance was (27.05±8.94)%. Compared with that in perennial inundation, DOM in seasonal inundation had a higher and lower relative abundance of protein-like components and humic-like components, respectively. Further, the aromatic and hydrophobic components were higher in perennial inundation, showing a spatial pattern of the middle>entrance>outlet of the lake, which was more conducive to the migration of pollutants. The high FI (1.93) and BIX (0.91) and low HIX (1.57) indicated that the DOM in sediments had the mixed characteristics of being mainly endogenic and relatively weakly terrigenous. This was mainly influenced by human input and sediment characteristics. The direct effect of sewage discharge was intensified by sediment exposure in the seasonal inundation zone. Additionally, the contents of clay and total nitrogen (TN) were significantly positively correlated with FI, indicating that high nutrients and clay in sediments enhanced the endogenous input of DOM (FI>1.9). The perennial inundation zone was influenced by external runoff input. At the same time, the pH and C/N were significantly positively correlated with HIX and C1, indicating that DOM in the sediments had higher terrigenic characteristics (HIX=1.38±0.57) than those in the seasonal inundation zone owing to the alkaline environment (pH>7.5) and runoff input. The results above revealed the relevant theories of the response of DOM in sediment to water quality and pollution in the process of hydrology and human activities and provide a scientific basis for the prevention and control of sediment pollution in lakes.

Water Occurrence in the Two Largest Lakes in China Based on Long-Term Landsat Images: Spatiotemporal Changes, Ecological Impacts, and Influencing Factors

Poyang Lake and Dongting Lake are the two largest freshwater lakes in China. They play a critical ecological role in regional sustainable development, which is closely related to water occurrence (WO). This study aimed to characterize the spatiotemporal changes, ecological impacts, and influencing factors of WO in the two lakes. First, this study utilized the Google Earth Engine (GEE) platform and all available Landsat images to retrieve the time series of WO in Poyang Lake and Dongting Lake on a two-year scale from 1989 to 2020. Then, the study explored the landscape variation intensity, landscape structure, and their ecological impacts on net primary production (NPP) and total nitrogen (TN) based on the landscape transfer rate (LTR), landscape indices, and geostatistical connectivity analysis. Finally, the study investigated the natural and human factors that influence the changes in WO. The major conclusions include the following: (1) the mean WO showed a significantly decreasing trend in the two lakes from 1989 to 2020. According to the LTR, the landscape variation in Dongting Lake was more intense before 2002, while after 2002, the landscape variation in Poyang Lake was more intense. (2) The landscape structure of WO gradually fragmented and diversified, along with decreasing hydrological connectivity in the two lakes. (3) The increase in NPP in Poyang Lake and the increase in TN in Dongting Lake were closely correlated with the changes in WO. (4) The upstream runoff showed a significantly positive correlation with the mean WO in Poyang Lake and the annual precipitation showed a significantly positive correlation with the mean WO in Dongting Lake; the influences of natural factors on WO changes became weaker. This study reinforces the understanding of long-term changes in the water body in Poyang Lake and Dongting Lake.

Spatial and Temporal Evolution Characteristics of Low Water Level Based on 70 years’ Measured Data: A Case Study of Dongting Lake in China

In recent years, the operation of cascade reservoirs in the upper reaches of the Yangtze River led by the Three Gorges Reservoir has significantly changed the low water level of Dongting Lake, which has had an adverse impact on the water resources and ecological security in the lake area. As a result, the low water level situation of Dongting Lake is drawing more and more attention. Based on the abundant and long-term daily observation data of Dongting Lake and main stream of the Yangtze River, the spatial and temporal evolution characteristics of low water level in the Dongting Lake area since the 1950s were analyzed in this study, especially during the storage period of the Three Gorges Reservoir. In addition, the influence of the jacking effect of the Yangtze River on Dongting Lake was also discussed to analyze the impact of the Three Gorges Reservoir. The results show that the annual minimum water level in the Dongting Lake area over the years generally occurs from January to February and December. Since it is not within the storage period of the Three Gorges Reservoir, it is not affected by the Three Gorges Reservoir. However, the average water level in September and October during the main storage period of the Three Gorges Reservoir has significantly reduced, and the closer it is to the main stream of the Yangtze River, the greater the influence is. In recent years, affected by the advance of the impounding period, the average water level has decreased further in September. In addition, the low water level of Dongting Lake is obviously affected by the jacking effect of the main stream of the Yangtze River. After the impoundment of the Three Gorges Reservoir, the water surface slope of the Dongting Lake area has decreased, while that of the main stream of the Yangtze River has increased. This has strengthened the jacking effect of the main stream. The research results can provide reference for the solution of water resource and ecological crises in the Dongting Lake area.

Monitoring Surface Water Inundation of Poyang Lake and Dongting Lake in China Using Sentinel-1 SAR Images

High-temporal-resolution inundation maps play an important role in surface water monitoring, especially in lake sites where water bodies change tremendously. Synthetic Aperture Radar (SAR) that guarantees a full time-series in monitoring surface water due to its cloud-penetrating capability is preferred in practice. To date, the methods of extracting and analyzing inundation maps of lake sites have been widely discussed, but the method of extracting surface water maps refined by inundation frequency map and the distinction of inundation frequency map from different datasets have not been fully explored. In this study, we leveraged the Google Earth Engine platform to compare and evaluate the effects of a method combining a histogram-based algorithm with a temporal-filtering algorithm in order to obtain high-quality surface water maps. Both algorithms were conducted on Sentinel-1 images over Poyang Lake and Dongting Lake, the two largest lakes in China, respectively. High spatiotemporal time-series analyses of both lakes were implemented between 2017 and 2021, while the inundation frequency maps extracted from Sentinel-1 data were compared with those extracted from Landsat images. It was found that Sentinel-1 can monitor water inundation with a substantially higher accuracy, although minor differences were found between the two sites, with the overall accuracy for Poyang Lake (95.38–98.69%) being higher than that of Dongting Lake (95.05–97.5%). The minimum and maximum water areas for five years were 1232.96 km2 and 3828.36 km2 in Poyang Lake, and 624.7 km2 and 2189.17 km2 in Dongting Lake. Poyang Lake was frequently inundated with 553.03 km2 of permanent water and 3361.39 km2 of seasonal water while Dongting Lake was less frequently inundated with 320.09 km2 of permanent water and 2224.53 km2 of seasonal water. The inundation frequency maps from different data sources had R2 values higher than 0.8, but there were still significant differences between them. The overall inundation frequency values of the Sentinel-1 inundation frequency maps were lower than those of the Landsat inundation frequency maps due to the severe contamination from cloud cover in Landsat imagery, which should be paid attention in practical application.

Poyang and Dongting Lakes, Yangtze River: tributary lakes blocked by main-stem aggradation

During its 6,300-km course from the Tibetan Plateau to the ocean, the Yangtze River is joined by two large lakes: Dongting Lake and Poyang Lake. We explain why these lakes exist. Deglaciation forced the ocean adjacent to the Yangtze mouth to rise ∼120 m. This forced a wave of rising water surface elevation and concomitant bed aggradation upstream. While aggradation attenuated upstream, the low bed slope of the Middle-Lower Yangtze River (∼2 × 10-5 near Wuhan) made it susceptible to sea level rise. The main stem, sourced at 5,054 m above sea level, had a substantial sediment load to "fight" against water surface level rise by means of bed aggradation. The tributaries of the Middle-Lower Yangtze have reliefs of approximately hundreds of meters, and did not have enough sediment supply to fill the tributary accommodation space created by main-stem aggradation. We show that the resulting tributary blockage likely gave rise to the lakes. We justify this using field data and numerical modeling, and derive a dimensionless number capturing the critical rate of water surface rise for blockage versus nonblockage.