Seasonal variation and provenance of organic matter in the surface sediments of the three gorges reservoir: Stable isotope analysis and implications for agricultural management

Perfluoroalkyl acids (PFAAs) and their precursors in sediments and adjacent riparian soils from the Three Gorges Reservoir, China: Contamination characteristics, source apportionment and ecological risks

Information on the occurrence and spatial distribution of perfluoroalkyl acids (PFAAs) and their precursors in sediments and adjacent riparian soils of Three Gorges Reservoir (TGR), which is one of the largest reservoirs in the world, is still limited. In this study, The total concentrations of these per- and polyfluoroalkyl substances (PFASs) ranged from 2220 to 19,300 pg/g in sediments and 298-9540 pg/g in soils. PFOA was the dominant PFAS in sediments and soils, accounting for 23.4% and 30.7% of the total median cocentrations of PFASs, respectively. PFAA precursors, such as 4:2 fluorotelomer sulfonate (4:2 FTS), 6:2 fluorotelomer sulfonate (6:2 FTS), and perfluorooctane sulfonamide (FOSA), were widely detected in sediments and soils. The distribution of PFASs exhibited distinct spatial variations and was more influenced by anthropogenic activities. Positive matrix factorization (PMF) identified fire-fighting foams (AFFF) and legacy fluoropolymer industry/textile treatment were the dominant sources in sediments (31.5%) and soils (30.8%), respectively. Finally, the ecological risk assessment showed that PFOS exhibited low to medium risks. Our findings indicate that the contamination of PFAA precursors must be considered when developing management measures to protect the TGR region.

Deciphering the link between particulate organic matter molecular composition and lake eutrophication by FT-ICR MS analysis

Eutrophication has emerged as a significant environmental problem for global lakes. As an essential carrier of nutrients, particulate organic matter (POM) plays a vital role in the eutrophication process of these aquatic systems. In this study, POM from seven lakes with different trophic states in the middle and lower reaches of the Yangtze River (China) was characterized using carbon and nitrogen stable isotopes and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The aim was to elucidate the relationship between the source and molecular composition of POM during the eutrophication process of lakes. The results indicated that POM was mainly composed of autochthonous (62.7%) and allochthonous (37.3%) sources, with the contribution from autochthonous sources being more pronounced across the different sources. The POM formulas mainly consisted of the subclasses CHO, CHON, CHOP, CHOS, and CHONS. Notably, CHOP formulas had the highest proportion of labile formula compounds, according for 51.56%. The unsaturation, aromaticity, and oxidation of unique POM formulas gradually decreased with increasing trophic states. A significant positive correlation was observed between CHOP and the percentage of labile compounds (MLBL%) in unique POM formulas. The relative abundance of lipid and protein compounds of unique POM formulas showed a positive correlation with lake trophic states, which indicated that with the increase of lake trophic states, the content of autochthonous POM gradually increased. Herein, we inferred that with the intensification of lake eutrophication, the autochthonous POM increased, which was accompanied by a further increase of labile P-containing compounds in POM, thus leading to the increasing eutrophication process of lakes in the form of positive feedback. Overall, this investigation of POM at the molecular level illustrates the deep-rooted mechanism of frequent lake eutrophication. This is of great significance in understanding the fate of POM and effectively controlling lake eutrophication.

Influencing Factors of Phosphorus Mobility and Retention in the Sediment of Three Typical Plateau Lakes

The mechanisms driving changes in the stability of phosphorus (P) in sediments under lake ecosystem degradation remain poorly understood. This study investigated the P-binding forms in sediments from three plateau lakes with different trophic states in Yunnan Province, China, aiming to elucidate the responses of sediment P compositions to human activities, lake trophic status, and dissolved organic matter (DOM) characteristics. The results showed that human activity directly contributed to sediment P retention. The trophic type of lake exerted a discernible effect on P mobility in the sediments, as eutrophic algae-type lakes had a higher content of sediment mobile-P. Moreover, the sediment DOM promoted the adsorption of BD-P and NH4Cl-P. Generally, exogenous pollution caused by human activity leads to lake eutrophication and a decline in lake ecosystem stability. This variation was largely influenced by water depth. A decrease in lake ecosystem stability leads to increased P mobility in sediments, which increases the risk of endogenous pollution. The DOM plays an important role in the mobility of sediment P. These insights offer a novel perspective for understanding how lake ecosystem characteristics are related to endogenous P loads in lakes.

Impact of cascade reservoir on the sources of organic matter in sediments of Lancang river

The construction of dams to intercept natural rivers constitutes the most severe human activity influencing the underlying surface. This study focuses on four cascade reservoirs of the Lancang River and explores their impact on the migration of organic matter in sediments. The research reveals significant spatial variations in total organic carbon (TOC) and total nitrogen concentrations in the sediments of the four reservoirs. The carbon and nitrogen isotopes indicate that terrigenous organic matter is the main source of TOC in the sediments, contributing an average of 66.80%. Endogenous algal-derived organic matter is the second significant source, contributing between 14.30% and 32.91%. The sources contributed from upstream organic matter are the lowest, ranging from 6.36% to 15.33%. Our study demonstrates that the construction of cascade reservoirs may significantly alter the processes of material sources in the river basin ecosystem, particularly in the large reservoir which increased more endogenous algal-derived organic matter.

Cascade reservoirs affected chemical compositions of dissolved organic matter and greenhouse gas dynamics in the Lancang River

Dissolved organic matter (DOM) is an important component in aquatic systems. There has been much debate about the potential effects of cascade reservoirs on the transport and transformation of DOM. Here, through a survey of source to leave-boundary section of Lancang River (LCR) in June and November of 2017-2018, our results revealed that weak spatiotemporal variations were observed for DOC content, whereas DOM parameters were significantly different between natural and reservoir reaches. And DOM showed higher humification degree from allochthonous sources with increasing autochthonous matter in reservoir reach, may due to high particulate organic matter and releasing autochthonous DOM from phytoplankton blooms in the LCR, which can be evidenced by depleted DIC, enriched δ13CDIC and higher BIX. A unique fluorescent fraction (C5) appeared in the reservoir reach and increased along water flow, which was strongly associated with dissolved CO2 and N2O. Meanwhile, BIX value decreased with increasing dam height, hydraulic residence time (HRT), and reservoir capacity, which may promote CH4 production, highlighting variation of DOM compositions in understanding the effect of greenhouse gas (GHG) dynamics in the LCR. The findings were essential for comprehending the influences of cascade reservoirs on carbon cycle, and informed policy development for the sustainable management of transboundary water resources like the LCR.

Spatial and temporal evolution and factors influencing soil aggregate stability in the riparian zone during exposure: A case study of the water-level fluctuation zone of the Three Gorges Reservoir, China

The water-level fluctuation zone (WLFZ) is a unique riparian region that forms in response to the operation of the reservoir. The periodic submersion-exposure conditions severely affect the properties of soil and vegetation, thus also change the spatial and temporal development of soil aggregate stability. Nonetheless, the changes in both space and time concerning soil aggregate stability and the factors influencing it during the exposure period in the WLFZ are still unclear. Consequently, the WLFZ within the Three Gorges Reservoir area (TGRA) was taken as the research object, soil and plant samples were obtained from various elevations along the slope at different times to capture the spatial and temporal changes in soil and plant characteristics throughout the exposure period. Moreover, the wet sieving method was utilized to measure the aggregate size distribution and stability of soil aggregates. To further analyze the effects of both individual and mutual variables on the spatial and temporal changes in aggregate stability, methods such as Pearson's correlation analysis, redundancy analysis (RDA), variation partitioning analysis (VPA), and partial least squares path modeling (PLS-PM) were employed. The findings revealed that aggregate stability increased with increasing elevation until peaking and then decreased spatially, whereas the passage of time contributed to enhanced aggregate stability. The spatial variation in aggregate stability was greater than temporal variation. RDA revealed that soil properties (bulk density, soil water content in mass, clay, silt, sand, pH and soil organic matter), environmental factors (elevation, water level, submersion duration and exposure duration) and plant characteristics (shoot and root biomass) contributed 53.6 %, 45.4 % and 0.9 %, respectively, to the spatiotemporal variability in aggregate stability. VPA and PLS-PM analysis further revealed that the main factors controlling the aggregate stability were soil chemical properties. These results provide a basis for further studies on the soil and water conservation in the WLFZ.

The increasing influence of oyster farming on sedimentary organic matter in a semi-closed subtropical bay

Oyster farming activities play a pivotal role in the biogeochemical cycles of coastal marine ecosystems, particularly in terms of sedimentary carbon cycling. To gain deep insights into the influence of expanding oyster culture on the sedimentary carbon cycle, surface sediments were collected from the Maowei Sea, which is the largest oyster farming bay in south China, based on six filed surveys between July 2010 and December 2022. The sediment samples were analyzed for total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopes (δ13C and δ15N) to evaluate the inter-annual variations in the source contribution to sedimentary organic matter (SOM). The results revealed that the average contents of sedimentary TOC and TN were 0.67 ± 0.41 % and 0.06 ± 0.03 %, respectively. Fluctuations in the C/N molar ratios ranged from 5.8 to 23.6, with an average of 12.6 ± 2.9, indicating a significant terrestrial input contribution to SOM in the study area. Furthermore, the integration of stable isotope analysis and Bayesian mixing model demonstrated a gradual increase in the mean proportion of shellfish biodeposition to SOM, from 12.0 ± 5.6 % in July 2010 to 21.1 ± 7.3 % in December 2022, consistent with the progressive expansion of oyster aquaculture along this coastal area, thereby emphasizing the substantial influence of oyster farming on SOM composition. With the anticipated expansion of oyster farming scale and production in the future, shellfish biodeposition is expected to assume a more important role in shaping SOM dynamics and sedimentary organic carbon cycling in coastal waters. Overall, this study provided an important perspective for better assessing the impact of expanding mariculture scale on coastal biogeochemical cycles, thereby making valuable contributions to future policy formulation concerning mariculture and ecological conservation.

COVID-19 lockdown measures affect polycyclic aromatic hydrocarbons distribution and sources in sediments of Chaohu Lake, China

The COVID-19 pandemic has profoundly impacted human activities and the environment globally. The lockdown measures have led to significant changes in industrial activities, transportation, and human behavior. This study investigates how the lockdown measures influenced the distribution of polycyclic aromatic hydrocarbons (PAHs) in the sediments of Chaohu Lake, a semi-enclosed lake. Surface sediment samples were collected in summer of 2020 (lockdown have just been lifted) and 2022 and analyzed for 16 priority PAHs. The range of ΣPAHs concentrations remained similar between 2020 (158.19-1693.64 ng·g-1) and 2022 (148.86-1396.54 ng·g-1). Among the sampling sites, the west lake exhibited similar PAHs concentrations characteristics over the two years, with higher levels observed in areas near Hefei City. However, the east lake exhibited increased ΣPAHs concentrations in 2022 compared to 2020, especially the area near ship factory. PAHs source analysis using principal component analysis-multiple linear regression (PCA-MLR) revealed an increased proportion of petroleum combustion sources in 2022 compared to 2020. The isotope analysis results showed that organic matter (OM) sources in the western lake remained relatively stable over the two years, with sewage discharge dominating. In contrast, the eastern lake experienced a shift in OM sources from sewage to C3 plants, potentially contributing to the increased PAH levels observed in the eastern lake sediments. Ecological risk assessment revealed low to moderate risk in both 2020 and 2022. Health risk evaluation indicated little difference in incremental lifetime cancer risk (ILCR) values between the two years, with only benzo[a]pyrene (BaP) posing a high risk among the carcinogenic PAHs. Children generally faced higher health risks compared to adults. This study reveals pandemic-induced changes in PAH pollution and sources in lake sediments, offering new insights into the impact of human activities on persistent organic pollutants, with implications for future pollution control strategies.

Different effects of seasonal impoundment and land use change on microbiome in a tributary sediment of the three gorgers reservoir

Anthropogenic activities significantly impact river ecosystem nutrient fluxes and microbial metabolism. Here, we examined the seasonal and spatial variation of sediments physicochemical parameters and the associated microbiome in the Pengxi river, a representative tributary of Three Gorges Reservoir, in response to seasonal impoundment and land use change by human activities. Results revealed that seasonal impoundment and land use change enhanced total organic carbon (TOC), total nitrogen (TN) and ammonium nitrogen (NH4+-N) concentration in the sediment, but have different effects on sediment microbiome. Sediment microbiota showed higher similarity during the seasonal high-water level (HWL) in consecutive two years. The abundant phyla Acidobacteria, Gemmatimonadetes, Cyanobacteria, Actinobacteria and Planctomycetes significantly increased as water level increased. Along the changes in bacterial taxa, we also observed changes in predicted carbon fixation functions and nitrogen-related functions, including the significantly higher levels of Calvin cycle, 4HB/3HP cycle, 3HP cycle and assimilatory nitrate reduction, while significantly lower level of denitrification. Though land use change significantly increased TOC, TN and NH4+-N concentration, its effects on spatial variation of bacterial community composition and predicted functions was not significant. The finding indicates that TGR hydrologic changes and land use change have different influences on the carbon and nitrogen fluxes and their associated microbiome in TGR sediments. A focus of future research will be on assessing on carbon and nitrogen flux balance and the associated carbon and nitrogen microbial cycling in TGR sediment.

Sediment organic matter predicts polycyclic aromatic hydrocarbon distribution in port sediments

We evaluated the influence of organic matter in polycyclic aromatic hydrocarbons (PAHs) in port sediments using multiple linear regression (MLR) and prediction models. Total sediment PAHs ranged between 45 and 3230 ng/g dw (average: 557 ± 962 ng/g dw), with PAHs primarily originating from river inputs, confined to areas near the estuaries. Coal/biomaterial combustion and petroleum mainly contribute to the presence of PAHs along estuaries, with medium-high to high ecological risks. MLR TPAHs prediction model included variables, namely, the marine-derived total organic carbon (TOCmar), terrestrial fraction of organic matter (Fterr), and carbon-to‑nitrogen ratio (CNR). Results indicate that mainly marine- followed by terrestrially-derived organic matter influenced sediment PAH distribution. Total organic nitrogen and CNR were variables in the toxic equivalent (TEQ) prediction model, demonstrating that terrestrial pollution sources primarily influenced TEQ. The study analyzes and predicts the impact of organic matter and its sources on the fate and transport of PAHs in port sediments.

Elevated lead mobility in sediments of a eutrophic drinking water reservoir during spring and summer seasons: Insights from isotopic signatures

Lead (Pb) pollution in sediments remains a major concern for ecosystem quality due to the robust interaction at the sediment/water interface, particularly in shallow lakes. However, understanding the mechanism behind seasonal fluctuations in Pb mobility in these sediments is lacking. Here, the seasonal variability of Pb concentration and isotopic ratio were investigated in the uppermost sediments of a shallow eutrophic drinking lake located in southeast China. Results reveal a sharp increase in labile Pb concentration during autumn-winter period, reaching ∼ 3-fold higher levels than during the spring-summer seasons. Despite these fluctuations, there was a notable overlap in the Pb isotopic signatures within the labile fraction across four seasons, suggesting that anthropogenic sources are not responsible for the elevated labile Pb concentration in autumn-winter seasons. Instead, the abnormally elevated labile Pb concentration during autumn-winter was probably related to reduction dissolution of Fe/Mn oxides, while declined labile Pb concentration during spring-summer may be attributed to adsorption/precipitation of Fe/Mn oxides. These large seasonal changes imply the importance of considering seasonal effects when conducting sediment sampling. We further propose a solution that using Pb isotopic signatures within the labile fraction instead of the bulk sediment can better reflect the information of anthropogenic Pb sources.

Controlling effects of terrestrial organic matter on metal contamination and toxicity risks in port sediments

The contents of metals, total carbon, total nitrogen (TN), total organic carbon (TOC), and stable isotope composition (δ13Corg and δ15N) of sediment organic matter (SOM) were investigated to explore the sources and spatial distribution of metals and SOM in the surface sediments (Kaohsiung Port, Taiwan). Results showed that TOC and metals in estuarine sediments are high, gradually decreasing toward the port entrances. The δ13Corg, δ15N, and TOC/TN ratios indicate that SOM comes mainly from terrestrial sources. This study proposes a befitting model between metal pollution and toxicity risk index and SOM sources in port sediments by combining stable isotope composition, correlation matrix, and multiple linear regression analysis. The model indicates that the degree of metal pollution and toxicity risk in sediments are mainly affected by TOCterr content and SOM source. The results help to understand the influence of organic matter sources in port sediments on metal concentration distribution.

Hydrological regimes and water quality variations in the Yangtze River basin from 1998 to 2018

Understanding the long-term variations in basins that undergo large-scale hydroelectric projects is crucial for effective dam operation and watershed management. In this study, comprehensive analyses were conducted on a dataset spanning over 20 years (1998-2018) of hydrological regime and physicochemical parameters from the Yangtze River basin to evaluate the potential impacts of the Three Gorges Dam. Water level significantly increased from 128.75±58.18 m in 2002 to 136.78±55.05 m in 2005, and the mean flow velocity significantly decreased from 2004 to 2010. However, no significant change in the flow was observed in the basin. Meanwhile, remarkable fluctuations in physicochemical parameters, including dissolved oxygen, chemical oxygen demand, conductivity, hardness, and alkalinity, were mainly observed during impoundment (2003-2009). After that, the above parameters tended to stabilize, and some even returned to their original levels. The dam's retention effect significantly reduced the suspended solids (SS) in both up- and downstream, to only one-third of the pre-operation level. And total phosphorus and chemical oxygen demand also significantly decreased with the decline of SS. Particularly, ammonium also showed a significant downward trend, with the up- and downstream of the dam falling by 36.8 % and 26.1 %, respectively. However, the increasing total nitrogen (7.5 % and 20.0 % up- and downstream of the dam, respectively) still threatened the water quality of the basin, especially in the estuaries. Additionally, the significant decline in dissolved oxygen downstream (from 8.53±1.08 mg/L to 8.11±1.36 mg/L) also exacerbated the hypoxia in the Yangtze River estuary. The results demonstrated the long-term impact of the construction of the Three Gorges Dam on the environmental elements of the Yangtze River basin, which provides reference data and guidance for the construction of big dams in major rivers in the future.

Provenance analyses of silted sediments in Shenzhen Bay: Insights based on rare earth elements and stable isotopes

Shenzhen Bay (SZB) in southern China is a typical eutrophic area, with internal pollution from its sediments representing an important nutrient source. However, the transport paths and sources of sediments in SZB remain unclear, making it difficult to analyze the nutritional budget and propose effective sediment management strategies. To address this, we linked a sediment fingerprinting technique to a Bayesian mixing model (MixSIAR) and conducted provenance analyses. We collected particle samples from SZB sediment and surrounding areas, including the Shenzhen River (SZR), Pearl River Estuary (PRE), and the northern South China Sea (SCS). Two groups of natural tracers were measured to trace different phases of sediments: (1) C and N parameters for the fates of the organic phase of sediments, and (2) rare earth element (REE) patterns for the provenance of mineral fragments. The results showed that the concentrations of total organic C and total N were 0.89-1.44 % and 0.05-0.13 %, respectively. MixSIAR suggested that fluvial inputs from SZR and PRE contributed 46.6 % and 30.3 % of organic matter, respectively. The organic matter in the PRE mainly originated from sewage and the upper reaches of the Pearl River. The concentration range of REEs in SZB sediments was 153.12-480.09 mg/kg with clear enrichment for light REE. MixSIAR results showed that the mineral fragments mainly originated from the outer bay (SCS and PRE, which contributed 57.2 % and 32.7 %, respectively). These results indicate that organic pollution follows a different path from the inorganic base, which is mainly related to anthropogenic input from land. This study highlights that complex sediment transport processes and pollution intrusions from the Pearl River are the issues that must be considered for eutrophication restoration in SZB.