Effects of dam construction and increasing pollutants on the ecohydrological evolution of a shallow freshwater lake in the Yangtze floodplain

Environmental fate of microplastics in alpine and canyon-type river-cascade reservoir systems: Large-scale investigation of the Yalong River in the eastern Qinghai-Tibet Plateau

Reservoirs are regarded as potential collection sites for microplastics (MPs), and ample water resources in plateau regions provide favorable natural conditions for hydroelectric power generation. However, research on the impact of cascade reservoir construction in the plateau region on the fate of MPs within the watershed is limited. In this study, the Yalong River, an alpine canyon river in the eastern Qinghai-Tibet Plateau, was selected as the research area. This study explored the distribution of MPs at various depths in water, sediment, and riverbank soil as well as the formation of "MP communities" within the river-cascade reservoir system. Furthermore, the effects of dam construction on MPs' migration in different environments were analyzed. The results revealed that the abundance of MPs in the water and sediment within the cascade reservoir area (CRA) was significantly higher than that in the river area (RA) (P < 0.001). Additionally, the trend of increasing MPs in water with decreasing altitude was notably slower in CRA. Regarding shape, the proportion of fibers in the water within the CRA was significantly lower than that in the RA, with a smaller vertical migration rate in the water than in the sediment. The proportion of MPs < 500 μm in the water within the CRA was significantly higher than that in the RA. High-density MPs were notably deposited in the reservoir sediments. The analysis of the MP communities revealed that the construction of cascade dams led to relative geographical isolation between different sampling sites, reducing the similarity of MP communities in the CRA. This study established a theoretical foundation for understanding the impact of cascade dam construction on the fate characteristics of MPs and their potential risks in plateau areas.

Occurrence and ecological risk assessment of 16 phthalates in surface water of the mainstream of the Yangtze River, China

Phthalic acid esters (PAEs), a class of typical endocrine disruptors, have received considerable attention due to their widespread applications and adverse effects on biological health. In this study, 30 water samples, along the mainstream of the Yangtze River (YR), were collected from Chongqing (upper stream) to Shanghai (estuary) from May to June in 2019. The total concentrations of 16 targeted PAEs ranged from 0.437 to 20.5 μg/L, with an average of 1.93 μg/L, where dibutyl phthalate (DBP, 0.222-20.2 μg/L), bis (2-ethylhexyl) phthalate (DEHP, 0.254-7.03 μg/L), and diisobutyl phthalate (DIBP, 0.0645-0.621 μg/L) were the most abundant PAEs. According to the pollution level in the YR to assess the ecological risk posed by PAEs, the results showed medium risk level of PAEs in the YR, among which DBP and DEHP posed a high ecological risk to aquatic organisms. The optimal solution for DBP and DEHP is found in ten fitting curves. The PNEC_SSD of them is 2.50 μg/L and 0.34 μg/L, respectively.

Centurial changes in sedimentary phosphorus forms and trace elements in response to damming and anthropogenic pollution in a floodplain lake, central China

A massive increase in dam construction has decreased fluvial sediment discharge at a global scale. In order to explore potential effects of the Three Gorges Dam (TGD) on floodplain lakes in the middle Yangtze reaches (central China), this study investigated phosphorus forms (i.e., Ca-bound phosphorus, Fe/Al-bound phosphorus, and organic phosphorus) and trace elements (i.e., Sc, Ba, Be, Pb, and Zn) in a ²¹⁰Pb-dated sediment core collected from East Dongting Lake, a hydrologically open lake proximal to the TGD. Sedimentary records revealed that the fluxes of phosphorus in different forms and trace elements were high before 2005. Thereafter, the fluxes of Ca-bound phosphorus, Sc, Ba, and Be declined sharply, probably due to declining supply of riverine detritus from the upstream after the TGD operation. In contrast, the fluxes of Fe/Al-bound phosphorus and heavy metals remained high after 2005, indicating the impacts of industrial sewage inputs. Our results underscore that river damming and anthropogenic pollution have altered sedimentary geochemical composition in East Dongting Lake. This phenomenon might be widespread in similar floodplain lakes due to increasing human disturbance during recent decades.

Insights into Shallow Freshwater Lakes Hydrology in the Yangtze Floodplain from Stable Water Isotope Tracers

Stable isotopes of lake waters are widely used to identify the relative importance of hydrological processes on the lake water balance across the ungauged landscape via the coupled-isotope tracer model. The isotopic compositions of twenty shallow freshwater lakes across the mid-lower reaches of Yangtze floodplain (MLY) were investigated in January and May of 2018. The lake-specific input water (δI) and evaporation-to-inflow (E/I) ratios were estimated to explore the specific lake hydrology across the MLY. Results showed that distinct isotopic enrichment trends in May compared with those in January, which was indicative of stronger evaporation in May. The δ18OI values of specific lakes exhibited large variability across the MLY, which may be related to the watershed properties, such as watershed area and elevation, and rainfall. The estimated E/I ratios of lakes across the MLY were below 1, which suggested that these lakes (code 1–15) are flood-dominated in the middle reaches of Yangtze River where lakes are susceptible to Three Gorges Dams regulations. By contrast, the relatively lower variability of lake E/I ratios were observed from the Yangtze River Delta (code 17–20) because these lakes with developed river network systems are highly exchanged by artificial regulation. Our investigation of lake types and corresponding isotopic evolution patterns are likely typical of other floodplain landscapes and their identification could be used to better predict hydrological responses to ongoing climate change and artificial regulations by dams.

Metacommunity Concepts Provide New Insights in Explaining Zooplankton Spatial Patterns within Large Floodplain Systems

Flood pulse related physical variables (FLOOD) can affect zooplankton community structure through local factors directly and can also influence through regional dispersal factors of metacommunity concepts indirectly. Therefore, we infer that spatial patterns of zooplankton communities could be related to metacommunity concepts and their importance may depend on the size of the aquatic/terrestrial transition zone (ATTZ). Herein, we explored the relative importance of limnological (LIMNO) and FLOOD variables in zooplankton community by analyzing data from 272 sites across three floodplain lakes in the middle reaches of the Yangtze River. Our results showed that the variation in the zooplankton community can be well explained by the LIMNO and FLOOD variables in all of the lakes under the low water level season. However, during the high water level season, neither LIMNO nor FLOOD can explain the spatial variances of zooplankton. Therefore, our results indicated that testing biogeographical theories and macroecological laws using zooplankton should consider temporal aspects of flood pulse. Furthermore, we noted that the number of explained variance by local variables is negatively correlated with the size of the ATTZ. Metacommunity concepts provide complementary insights in explaining zooplankton spatial patterns within large floodplain systems, which also provide a theoretical basis for ATTZ protection in floodplain management.

River connectivity and climate behind the long-term evolution of tropical American floodplain lakes

This study presents the long-term evolution of two floodplains lakes (San Juana and Barbacoas) of the Magdalena River in Colombia with varying degree of connectivity to the River and with different responses to climate events (i.e., extreme floods and droughts). Historical limnological changes were identified through a multiproxy-based reconstruction including diatoms, sedimentation, and sediment geochemistry, while historical climatic changes were derived from the application of the Standardised Precipitation-Evapotranspiration Index. The main gradients in climatic and limnological change were assessed via multivariate analysis and generalized additive models. The reconstruction of the more isolated San Juana Lake spanned the last c. 500 years. Between c. 1,620 and 1,750 CE, riverine-flooded conditions prevailed as indicated by high detrital input, reductive conditions, and dominance of planktonic diatoms. Since the early 1800s, the riverine meander became disconnected, conveying into a marsh-like environment rich in aerophil diatoms and organic matter. The current lake was then formed around the mid-1960s with a diverse lake diatom flora including benthic and planktonic diatoms, and more oxygenated waters under a gradual increase in sedimentation and nutrients. The reconstruction for Barbacoas Lake, a waterbody directly connected to the Magdalena River, spanned the last 60 years and showed alternating riverine-wetland-lake conditions in response to varying ENSO conditions. Wet periods were dominated by planktonic and benthic diatoms, while aerophil diatom species prevailed during dry periods; during the two intense ENSO periods of 1987 and 1992, the lake almost desiccated and sedimentation rates spiked. A gradual increase in sedimentation rates post-2000 suggests that other factors rather than climate are also influencing sediment deposition in the lake. We propose that hydrological connectivity to the Magdalena River is a main factor controlling lake long-term responses to human pressures, where highly connected lakes respond more acutely to ENSO events while isolated lakes are more sensitive to local land-use changes.

Large lake sluice operations during an extreme rainfall season greatly affect circulation and water quality dynamics of a shallow eutrophic lake

Large hydraulic infrastructures have been constructed globally to address water challenges. Past studies have well documented their effects on downstream aquatic ecosystems, which have included disrupting hydrological regimes as well as nutrient delivery, cycling and mediating processes that affect primary production. However, how these infrastructure operations affect lake ecosystems where the infrastructures are situated remains poorly understood. In the present study, we used a three-dimensional hydrodynamic-biogeochemical lake model to quantify the potential effects of large lake sluice operations under extreme high water levels on current structure and water quality parameters of Lake Chaohu in China. We designed and simulated multiple operation strategies based on actual operation curves during the 2016 extreme rainfall season. The model successfully captured the water quality dynamics of Lake Chaohu during both the calibration and validation phases. Our results indicate that higher lake water release rates led to overall accelerations of the current velocity; however, the deceleration of along-shore current velocity along the shorelines was also evident. Higher release rates also resulted in rapid rises ammonium nitrogen (NH₄-N), total nitrogen (TN) and total phosphorous (TP) concentrations in the eastern lake basin, as well as a lake-wide rise of chlorophyll-a (Chla) concentration. When the lake sluice was operated at its full capacity, mean concentrations of these four parameters increased by 5.21%, 5.58%, 9.6% and 7.46%, respectively. Modeling results demonstrate that the effects of lake sluice operations were still quite pronounced for four months after the operations. Modeling results also revealed that higher release rate during the operation phase may help decease TN and TP concentrations during the subsequent period. This study provides a useful perspective on how to support the planning and operation of large infrastructures in the face of climate change induced extreme events.

Total mercury and methylmercury in human hair and food: Implications for the exposure and health risk to residents in the Three Gorges Reservoir Region, China

Three Gorges Dam (TGD) is the largest hydroelectric construction in the world, and its potential impacts on the ecological environment and human health risks have invoked considerable global concern. However, as a mercury (Hg) sensitive system, limited work was conducted on the Hg exposure level of local residents around the Three Gorges Reservoir (TGR). Thus, 540 human hair samples and 22 species of local food samples were collected to assess the Hg exposure and human health risk to the residents located in the Three Gorges Reservoir Region (TGRR) and to investigate their dietary exposure to Hg. The results showed that the geometric mean concentrations of total mercury (THg) and methylmercury (MeHg) in hair were 0.42 ± 0.43 μg g^-1 and 0.23 ± 0.32 μg g^-1, respectively, lower than the reference level (1.0 μg g^-1) recommended by the United States Environmental Protection Agency (US EPA), indicating a low level Hg exposure for residents around the TGR. No significant difference in the accumulation of Hg in hair between the gender subgroups was observed, whereas age difference, smoking and alcohol drinking behavior, and fish consumption frequency were significant predictors of hair Hg level. Besides, THg and MeHg of all the investigated food samples did not exceed the corresponding Chinese national standard. The average probable daily intakes (PDIs) of THg and MeHg were 0.032 μg kg^-1 day^-1 and 0.007 μg kg^-1 day^-1, which were obviously below the recommended values of 0.57 μg kg^-1 day^-1 and 0.1 μg kg^-1 day^-1, respectively. The cereal (mainly rice) contribution of THg (76.0%) and MeHg (74.4%) intakes to the local residents around the TGR was much higher than that of fish (10.7% and 22.9%, respectively) due to the considerable rice consumption. Overall, residents around the TGR were at a low Hg exposure and rice consumption was the major pathway for Hg exposure.

Sedimentary DNA record of eukaryotic algal and cyanobacterial communities in a shallow Lake driven by human activities and climate change

Global freshwater lakes are changing due to human activities and climate change. Unfortunately, sufficient long-term monitoring is lacking for most lakes. However, lake sedimentary archives can extend the instrumental record and reveal historical environmental trends. In particular, sedimentary DNA analysis of lacustrine sediment cores can aid the reconstruction of past trends in eukaryotic algal and cyanobacterial communities, as was conducted in this study for Lake Chaohu in China. The results presented here indicate that the construction of the Chaohu Dam in 1963 is associated with decreased richness of eukaryotic algal and cyanobacterial communities. Several groups, including the eukaryotic algal taxa, Chlorophyceae, and cyanobacterial groups like Dolichospermum, Microcystis, Planktothricoides, Cyanobium, Pseudanabaena, and Synechococcus, increased in abundance following inferred historical nutrient enrichment. Nutrient concentrations and hydrologic conditions were further implicated as the dominant controls on communities based on Random Forest and generalized additive modeling statistical analyses. In particular, significant increases in lake hydraulic residence times after the construction of the Chaohu Dam were significantly associated with altered biological community structures. Further, phosphorus enrichment was positively associated with increased richness and diversity of these communities following the 1980s. In addition, effects from increased atmospheric temperatures on eukaryotic algal and cyanobacterial communities were apparent. Here, high-throughput sequencing analysis of sedimentary DNA allowed the inference of long-term biodiversity dynamics of Lake Chaohu. These results underscore the important impacts of anthropogenic activities and climate change on aquatic ecosystems at the decadal scale.

Long-term responses of epiphytic midges (Diptera, Chironomidae) to emergent macrophytes removal and P concentrations in a shallow hypertrophic lake ecosystem

Chironomid larvae are used as indicators of environmental changes in neolimnological and paleolimnological research. In the present study, we evaluated the responses of epiphytic chironomids to changes in environmental conditions over a long time scale. We intended to decipher whether changes in the trophic status of a lake (hypertrophic-eutrophic) would affect the taxonomic structure of epiphytic chironomids by influencing their food availability (structure of periphytic algae) and whether the responses of chironomids are taxa specific. In a shallow hypertrophic lake ecosystem, epiphytic chironomids associated with the emergent macrophyte Phragmites australis were studied from 2001 to 2018. In the autumn of 2006, emergent macrophyte removal led to an improved water transparency and reduced phytoplankton biomass. Epiphytic chironomids responded clearly to the shift from hypertrophic to eutrophic conditions. Under hypertrophic conditions larvae of detritivorous Cricotopus sp. (gr. sylvestris) and filter-feeder larvae of Glyptotendipes sp. prevailed. After macrophyte removal, we observed high relative abundances of Endochironomus albipennis and Paratanytarsus austriacus, which are classified as grazers, utilise periphytic algae (mainly diatoms) as a food source. The results indicated that the density of P. australis, relative abundances of cyanobacteria and diatoms in periphyton communities, Secchi disc depth, and periphytic and planktonic Chl-a are significant determinants of the diversity of epiphytic chironomids. The results can provide a reliable reference to the neolimnological and paleominological studies on chironomid responses to multiple environmental stressors in shallow lakes.

Distribution, partitioning behavior, and ecological risk assessment of phthalate esters in sediment particle-pore water systems from the main stream of the Haihe River, Northern China

The distribution, partitioning behavior and risk assessment of phthalate esters (PAEs) in the surface sediment-pore water system of the Haihe River were investigated. The total cumulative concentrations of 21 PAE species (Σ₂₁PAEs) in the surface sediment ranged from 45.9 to 1474.1 ng·g^-1 dry weight (dw) and were from 17.9 to 2628.8 ng·mL^-1 in the pore water. Di (2-ethylhexyl) phthalate (DEHP), dibutyl phthalate (DBP), and diisobutyl phthalate (DIBP) were the dominant components, and their sum accounted, on average, for 88.4% and 72.0% of Σ₂₁PAEs in the surface sediment and pore water, respectively. The spatial distributions of Σ₂₁PAEs in the surface sediment and pore water indicated that large amounts of the consumed products contained plasticizers in the urban and nearshore areas and increased the discharge of PAEs into the Haihe River. The river dam also affected PAEs distributions. The organic carbon normalized partitioning coefficient (logK_OC) followed a sequence as dry season (2.47 ± 0.35 mL·g^-1) > wet season (2.02 ± 0.45 mL·g^-1) > normal season (1.98 ± 0.42 mL·g^-1). The risk quotient (RQ) method was employed to assess the potential ecological risk from specific species. High ecological risks of DEHP to the sensitive algae, crustacean, and fish species along with high ecological risks of DIBP to sensitive fish species were found in the surface sediment and pore water for all sampling seasons. In addition, DBP in the surface sediment and pore water exhibited moderate and high ecological risks to sensitive aquatic species. The highest RQ values for PAEs were found in the surface sediment and pore water in suburban and urban areas, respectively, and indicated that anthropogenic activities may cause severe river pollution and high risk to the local aquatic ecosystem. CAPSULE: High levels and ecological risks from PAEs were found in the urban river, and the partitioning behaviors of PAEs between the surface sediment and pore water were not significantly affected by their hydrophobicity, especially for species with low K_OW.

Regime shifts in shallow lake ecosystems along an urban-rural gradient in central China

Due to differential exploitation pressure, ecosystems along the urban to rural gradients often exhibit different status in ecological structure and function. This can be challenging for lake restoration, given the relative strengths, magnitudes and speed of the exploitation. In this paper, we reconstructed the ecological changes over the past century and identified the regime shifts based on subfossil aquatic biota (chironomid records) in three shallow lakes (Shahu, Yanxi and Futou Lake) along an urban-rural gradient in the Yangtze floodplain, China. Our results illustrated the differences among lakes in trajectories, timing of critical transition and current ecological status. Eutrophic chironomid taxa increased markedly and replaced macrophyte-related taxa in urban Shahu Lake and suburban Yanxi Lake, indicated by the shift from a stable, vegetation-dominated state to an alternative, algal-dominated state in 1963 CE and 1975 CE respectively. The ecological regime in rural Futou Lake transited around 1980 CE but it is still in a relatively clear water state with abundant macrophytes due to anthropogenic hydrological controls. The greatest variance of chironomid compositional changes in both Shahu and Yanxi Lake was captured by anthropogenic pollutants, and analyses show that when these pressures are high they may be further amplified by climate warming. Responses along the urban-rural gradient are exemplified by urban Shahu Lake having shifted to a fragile regime with weak resistance and resilience, while rural Futou Lake has stabilized in a new regime with improved ecological resilience. Suburban Yanxi Lake is still moving toward a new state, and as such is unstable, because the types and magnitudes of external stressors are changing with urbanization in the city. It is suggested that active and precise management strategies for lakes should be established along the urban-rural gradient given their distinct development trajectories, drivers and current status.

A century of limnological evolution and interactive threats in the Panama Canal: Long-term assessments from a shallow basin

Large tropical river dam projects are expected to accelerate over the forthcoming decades to satisfy growing demand for energy, irrigation and flood control. When tropical rivers are dammed the immediate impacts are relatively well studied, but the long-term (decades-centuries) consequences of impoundment remain poorly known. We combined historical records of water quality, river flow and climate with a multi-proxy (macrofossils, diatoms, biomarkers and trace elements) palaeoecological approach to reconstruct the limnological evolution of a shallow basin in Gatun Lake (Panama Canal, Panama) and assess the effects of multiple linked factors (river damming, forest flooding, deforestation, invasive species, pollution and hydro-climate) on the study area. Results show that a century after dam construction, species invasion, deforestation and salt intrusions have forced a gradual change in the study basin from a swamp-type environment towards a more saline lake-governed system of benthic-littoral production likely associated with the expansion of macrophyte stands. Hydrology still remains the most important long-term (decades) structural factor stimulating salinity intrusions, primary productivity, deposition of minerals, and reduction of water transparency during wet periods. During dry periods, physical-chemical conditions are in turn linked to clear water and aerobic conditions while nutrients shift to available forms for the aquatic biota in the detrital-rich reductive sediments. Our study suggests that to preserve the natural riverine system functioning of this area of the Panama Canal, management activities must address long-term ecosystem structural drivers such as river flow, runoff patterns and physical-chemical conditions.

Canonical Correlation Study on the Relationship between Shipping Development and Water Environment of the Yangtze River

The sustainable development of the Yangtze River will affect the lives of the people who live along it as well as the development of cities beside it. This study investigated the relationship between shipping development and the water environment of the Yangtze River. Canonical correlation analysis is a multivariate statistical method used to study the correlation between two groups of variables; this study employed it to analyze data relevant to shipping and the water environment of the Yangtze River from 2006 to 2016. Furthermore, the Yangtze River Shipping Prosperity Index and Yangtze River mainline freight volume were used to characterize the development of Yangtze River shipping. The water environment of the Yangtze River is characterized by wastewater discharge, ammonia nitrogen concentration, biochemical oxygen demand, the potassium permanganate index, and petroleum pollution. The results showed that a significant correlation exists between Yangtze River shipping and the river’s water environment. Furthermore, mainline freight volume has a significant impact on the quantity of wastewater discharged and petroleum pollution in the water environment.

Long-Term Water Quality Patterns of a Flow Regulated Tropical Lowland River

Floodplain ecosystems in Africa are under threat due to direct anthropogenic pressure and climate change. The lower Phongolo River and associated floodplain is South Africa’s largest inland floodplain ecosystem and has been regulated by the Pongolapoort Dam since the 1970s. The last controlled flood release from the dam occurred in December 2014, after which a severe drought occurred and only a base flow was released. The central aims of this study were to determine the historic and present water quality state of the middle and lower Phongolo River and assess the possible effects of the most recent drought may have had. Historic water quality data (1970s to present) were obtained from monitoring stations within the Phongolo River catchment to assess the long-term water quality patterns. Using multivariate statistical analyses as well as the Physicochemical Driver Assessment Index (PAI), a water quality index developed for South African riverine ecosystems, various in situ and chemical water variables were analysed. Key findings included that the water quality of the middle and lower Phongolo River has degraded since the 1970s, due to increased salinity and nutrient inputs from surrounding irrigation schemes. The Pongolapoort Dam appears to be trapping nutrient-rich sediments leading to nutrient-depleted water entering the lower Phongolo River. The nutrient levels increase again as the river flows through the downstream floodplain through input from nutrient rich soils and fertilizers. The drought did not have any significant effect on water quality as the PAI remained similar to pre-drought conditions.

Terrestrial eutrophication of building materials and buildings: An emerging topic in environmental studies

Eutrophication has been analyzed mostly in aquatic or soil environments to date. Direct terrestrial eutrophication of building materials and buildings, contrary e.g. to their biodeterioration or biodegradation, was studied so rarely that even its exact definition does not exist yet. In this paper, eutrophication of building materials and buildings as an emerging topic in environmental studies is analyzed in detail and future developments in the field are contemplated. The analysis includes a survey of directly and indirectly related research studies, identification of basic mechanisms and principal factors, and a critical assessment of current methodologies potentially applicable for recognition and classification of eutrophication of building materials and buildings. A definition of direct terrestrial eutrophication of building materials and buildings is proposed afterwards and an alternative method for the calculation of their eutrophication potential is suggested. Finally, recommendations for solving the most urgent problems in future research are formulated.

Policy-driven changes in enclosure fisheries of large lakes in the Yangtze Plain: Evidence from satellite imagery

Enclosure fisheries have accommodated the widespread expansion of aquaculture in many lakes throughout the Yangtze Plain (YP), China, for over four decades. Such practices have increased food provision but have also triggered various detrimental environmental consequences. To restore ecosystem functions, the Chinese government recently implemented specific regulations to remove enclosure fences from lakes throughout the YP. However, little information is available on the spatial and temporal distributions of the enclosure fences, particularly in relation to the enforcement of recent policy changes. Using synthetic aperture radar (SAR) satellite images taken between 2002 and 2018, we conducted the first comprehensive assessment of the interannual changes in enclosure fences in 17 large lakes throughout the YP. Consistent decreases in fence density were found in most lakes after 2015; 15 lakes had >50% of their fences removed, while 9 lakes had >90% removed. The timing and implementation of the development and destruction of enclosure fisheries were related to government policy; before 2015, regional dynamics in enclosure fisheries were attributed to provincial policies, whereas the nearly ubiquitous fence demolition after 2015 was likely a response to national policy. This study represents remotely sensed evidence that demonstrates the importance of both local and national environmental policies and their effectiveness in mitigating ongoing human impacts on vulnerable and valuable natural resources. These findings provide valuable baseline information for future lake environmental monitoring and restoration in the YP region, and the methods used here could be applied to other lacustrine and coastal regions experiencing similar aquaculture activities.