Trends in nutrients in the Changjiang River

Constraining the composition and biochemical activity of organic carbon in a large eutrophic estuary using size-fractionated analysis

The presence of refractory dissolved organic carbon (RDOC) can result in the misestimation of organic pollution, and documentation regarding the characteristics of organic carbon (OC) and its relationship with pollution is limited. This study employed physical separation, biological incubation, and chemical analysis to examine the size-fractionated composition and bioavailability of OC in the Yangtze River Estuary, one of the most polluted estuarine areas in China. Results revealed that OC chemical features were highly diverse, with RDOC constituting approximately 65.8% ± 9.2% of dissolved organic carbon (DOC). During incubation, less than 10% of CHO molecules (molecules composed solely of carbon, hydrogen and oxygen atoms) identified by ultra-high resolution mass spectrometry were degraded. A significant positive linear relationship between OC and RDOC in size-fractionated OC indicated greater recalcitrance in smaller size fractions. The OC present in the >0.45 μm fraction was notably important for labile OC, including the particulate fraction of OC, which is relevant to chemical oxygen demand (COD) assessments. Excluding RDOC allows for a more accurate estimation of the contribution of labile OC to COD, as represented by the equation: CODLabile = 0.47 × CODBulk - 0.03. Approximately 0.44 ± 0.10 Gt of refractory OC, including 0.31 ± 0.07 Gt of RDOC, is transported annually into the ocean via rivers. This linear relationship of COD reveals an overestimation in current assessments of organic pollution and a neglect of RDOC's role in carbon preservation, thereby necessitating a revision of the COD evaluation practices in estuaries. This study highlights the differentiated impacts of refractory and labile OC on the quantification of OC pollution in a large eutrophic estuary.

Nitrogen uptake and nitrification in the Changjiang estuary

Anthropogenic activities have collectively resulted in high loads of nutrients exported into the estuaries, significantly affecting the nitrogen (N) cycling. Yet nitrification and N uptake, the central processes of the N cycle, have rarely been reported simultaneously in the estuaries. Here we report on pelagic nitrification and uptake rates of ammonia (NH4+) and nitrate (NO3-) based on 15N labeling techniques in the Changjiang estuary (CJE). N uptake rates in the surface layer were higher than in the bottom layer, while nitrification rates were the opposite. Light inhibited nitrification from 67.7 % to 100.0 %, while dark depressed NH4+ uptake and NO3- uptake from 29.2 % to 79.7 % and 50.9 %-100.0 %, respectively. Phytoplankton and nitrifiers competed for the substrate NH4+, with uptake being more competitive in the surface layer and nitrification being more competitive in the bottom layer. Across the CJE, the rates of nitrification and N uptake were higher at the phytoplankton bloom zone and turbidity maximum zone (TMZ) in intermediate-salinity. This suggests that high NO3- concentrations support high productivity, while high productivity and high turbidity enhance the NH4+ regeneration process. The autochthonous NH4+ played a key role in both nitrification and NH4+ uptake processes, and exhibited a higher preference for phytoplankton and a shorter turnover time compared to NO3-. Moreover, in-situ nitrification could have contributed between 0.1 % and 50.5 % of the daily phytoplankton NO3- requirements in the surface layer. Our study provides a basic knowledge of a central N-cycle process, which were essential for understanding the role of such estuarine ecosystems in the N cycle and their responses to human activities.

Spatial and temporal distribution characteristics and source apportionment of biogenic elements using APCS-MLR model in the main inlet tributary of Danjiangkou Reservoir

Danjiangkou Reservoir has been widely concerned as the water source of the world's longest cross basin water transfer project. Biogenic elements are the foundation of material circulation and key factors affecting water quality. However, there is no comprehensive study on the biogenic elements in tributaries of Danjiangkou Reservoir, hindering a detailed understanding of geochemical cycling characteristics of biogenic elements in this region. Guanshan River, one of the main tributaries that directly enter the Danjiangkou Reservoir, was token as the research object. Spatiotemporal distribution characteristics of basic water quality parameters and biogenic elements were studied. Water quality was comprehensively evaluated through water quality index (WQI). Absolute principal component score-multiple linear regression (APCS-MLR) model was adopted to explore the main sources of biogenic elements. Results showed that, in terms of season, the concentrations of total nitrogen (TN), total phosphorus (TP), and dissolved organic carbon (DOC) were significantly higher in wet season than in dry season, while no significant differences were found for dissolved inorganic carbon (DIC) and dissolved silica (DSi). Spatially, the concentrations of dissolved carbon, DIC, TN, and TP in the middle and lower reaches were higher than that in the upstream. DOC concentration peaked in the middle reaches, while DSi showed higher concentrations in the upstream. WQI values indicated that the river water quality was between good and excellent, although the water quality in wet season was slightly worse than that in the dry season. PCA extracted five potential sources, which accounting for 84.12% of the total variance, including rock weathering, mixed source of sewage discharge and agricultural non-point source pollution, dissolved soil CO2, seasonal factor, and agricultural non-point source pollution. These sources contributed 38.96%, 12.33%, 13.54%, 23.95%, and 11.21% to river water quality parameters, respectively. Strengthening the monitoring of biogenic elements, controlling pollutant discharge, and exploring the relationship between biogenic elements and other pollutants are important for the water environment management in this basin.

Influence of cascade reservoirs on the distribution, transport, and retention patterns of biogenic elements in the Jinsha River

Cascade reservoirs construction can greatly alter flow regime and sediment transport of rivers, further affecting migration and transformation processes of biogenic elements. The Jinsha River (JSR) is the China's largest hydropower base and the main runoff, sediment suspension, and nutrient source areas of the Yangtze River. However, the distribution, transport, and retention patterns of biogenic elements in the JSR are still unclear under the influence of cascade reservoirs. Therefore, monthly concentration monitoring work was conducted from November 2021 to October 2023 for various forms of carbon (C), nitrogen (N), phosphorus (P), and silicon (Si). Results showed that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) exhibited continuous decreasing trends along the reservoirs cascade, whereas N exhibited contrasting trends. The concentrations of dissolved total carbon (DTC), dissolved inorganic carbon (DIC), and total silicon also showed decreasing trends from upstream to downstream, whereas their fluxes were primarily influenced by runoff and exhibited upward fluctuations. Compared with other biogenic elements, there was a more pronounced retention effect on TP and PP by reservoirs, with average retention rates of 8.29 % and 16.01 %, respectively. Longer hydraulic retention time (HRT) can retain more TP and PP. Meanwhile, the retention rates of DTC, DIC, and particulate silicon were positively correlated with HRT, while the retention rate of dissolved silicon (DSi) showed a positive correlation with reservoir age. Moreover, the higher ratios of dissolved inorganic nitrogen to dissolved inorganic phosphorus (DIP) and DSi to DIP have occurred, resulting in apparent P limitation, particularly during the non-flood season due to lower DIP concentration. Overall, cascade reservoirs construction exists great influences on the spatial allocation, fluxes transport, and biogeochemical cycles of biogenic elements, potentially affecting the stability of rivers ecosystem along the food chain network.

Seasonal and interannual variations of nutrients in the Subei Shoal and their implication for the world's largest green tide

The world's largest "green tide" (Ulva prolifera) has occurred every year since 2007 in the Yellow Sea. The Subei Shoal area is thought to be the origin of the green tide. Based on field data from 2016 to 2023, seasonal and interannual variations of dissolved nutrients and their ecological effects in the Subei Shoal were analyzed. Spatial distribution of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicate (DSi) showed clear terrestrial sources, while ammonia (NH4-N) and dissolved organic nitrogen (DON) were not solely controlled by terrestrial sources. The seasonal variations of NH4-N, DIN, DON, DIP and DSi concentrations were significant, and the interannual variations of DIN, DON, DIP and DSi concentrations showed general decreasing trends from 2016 to 2023. The key factors affecting the seasonal and interannual variations of DIN and DIP concentrations were terrestrial input, aquaculture wastewater discharge, atmospheric deposition, submarine groundwater discharge and macroalgae absorption, while the dominant factor determining the variations of DSi concentrations was terrestrial input. NH4-N and DON concentrations were mainly influenced by aquaculture wastewater discharge and the absorption and release of macroalgae. The high nutrient concentrations in the Subei Shoal throughout the year provided sufficient material basis for the growth of Ulva prolifera in the source area of green tide outbreak.

Multi-machine learning methods to predict spatial variation characteristics of total nitrogen at watershed scale: Evidences from the largest watershed (Yangtze River Watershed), Asian

Nitrogen pollution has emerged as a significant threat to the health of global river systems, garnering considerable attention. However, numerous challenges persist in understanding the characteristics and predicting the spatial changes of total nitrogen (TN) at the catchment scale. We leveraged data from 530 monitoring sections to calculate a land-use composite index and perform statistical analyses to explore the primary factors influencing nitrogen enrichment in the Yangtze River Watershed. We developed three machine learning models to forecast future TN concentrations at monitoring points. Our results showed that agricultural activities and rainfall were the primary drivers of monthly variations in TN concentrations. The upstream region of the watershed exhibited larger variations in TN concentrations (0.097 to 11.099 mg/L), significantly higher than the middle and downstream areas (0.348 to 6.844 mg/L). Microbial-mediated organic matter decomposition in sediment and changes in land-use were identified as key contributors to regional differences in nitrogen enrichment. Potential nitrogen sources include sediment release, urban sewage, and agricultural fertilization. Random Forest model achieved a prediction accuracy of 77.6 %, surpassing the BP and LSTM models. We identified 37 high-risk areas of nitrogen enrichment, concentrated in the Chengdu-Chongqing, Yunnan-Central urban cluster, and the Chaohu Lake sub-watershed. Increased urban land-use and industrial inputs primarily influenced nitrogen enrichment in the upstream area, while agricultural inputs were the main drivers in the middle and downstream regions. Our multi-machine learning models identified the relationship between TN and influencing factors, providing a reliable method for assessing nitrogen enrichment risk in the watershed.

Deciphering the impact of cascade reservoirs on nitrogen transport and nitrate transformation: Insights from multiple isotope analysis and machine learning

Construction of cascade reservoirs has altered nutrient dynamics and biogeochemical cycles, thereby influencing the composition and productivity of river ecosystems. The Lancang River (LCR), characterized by its cascade reservoir system, presents uncertainties in nitrogen transport and nitrate transformation mechanisms. Herein, we conducted monthly monitoring of hydrochemistry and multiple stable isotopes (δ15N-NO3-, δ18O-NO3-, δ18O-H2O, δD-H2O) throughout 2019 in both the natural river reach (NRR) and cascade reservoirs reach (CRR) of the LCR. Through the monthly detection of nitrogen forms and runoff in the import (M2) and export (M9) section, the average annual retention ratios for Total nitrogen (TN), Nitrate nitrogen (NO3--N), Particulate Nitrogen (PN) and Ammonium Nitrogen (NH4+-N) were about -35%, -53%, 48% and -65%, respectively. The retention rates were positively correlated with hydraulic retention time and negatively correlated with reservoir age, especially in the flood season. Compared to the NRR, the reservoir had significantly affected the nitrogen transport characteristics, especially for the large reservoirs (like Xiaowan and Nuozhadu), which enhanced phytoplankton uptake of NO3--N to form PN capabilities in the lentic environment and subsequently to precipitate or intercept it at the reservoir. This led to the overall decreasing trend of TN and PN concentrations along the CRR. The Bayesian stable isotope model quantified NO3--N sources from the NRR to the CRR. During this transition, soil nitrogen (SN) ratios decreased from 69.3% to 61.8%, while Manure & sewage (M&S) increased from 24.0% to 31.3%. Anthropogenic and natural factors, including urban sewage discharge, population density, and precipitation, were selected as key predictor variables. The eXtreme Gradient Boosting (XGBoost) model exhibited superior predictive performance for NO3--N concentrations, achieving an R2 of 0.70. These findings deepen our understanding of the impact of reservoirs on river ecology.

Composition and transport of silicon in rivers of the Bohai rim with implications for the coastal environment

The transportation of silicon (Si) by rivers to the sea plays a vital role as an external source of Si budget for coastal environments, impacting the carbon cycle in the ocean. Nevertheless, the transport of reactive silica (RSi) from small rivers to the coastal sea has been frequently disregarded in scientific investigations. This research focused on 24 rivers situated along the Bohai Sea (BS) Rim, encompassing small rivers (SR) and the largest river in the region, the Yellow River (YR), to analyze their concentrations and fluxes of dissolved silicate (DSi), biogenic silica (BSi) and other amorphous forms of Si. The findings indicated seasonal variations in DSi concentrations, with higher levels observed during the flood season. Annually, approximately 105 × 103 t DSi and 200 × 103 t BSi were transported to the BS, with SR and YR contributing equally to the total riverine BSi flux. The smaller rivers were found to increase the BSi fraction of RSi due to elevated biological fixation. The ratios of average DSi and BSi fluxes to the river watershed area of SR were 3.5 and 6 times higher, respectively, compared to those of YR. SR play a critical role in the terrestrial Si export in the BS Rim. Human activities have led to significant deviations in the Si ratios to nitrogen and phosphorus in these rivers from the Redfield-Brzezinski ratio. This discrepancy could impact the phytoplankton community, primary production, and the environment of the BS. The study highlights the substantial contribution of SR to coastal environments, particularly in semi-closed marine environments like the BS.

Impact of cascade reservoirs on nutrients transported downstream and regulation method based on hydraulic retention time

Cascade reservoirs construction has modified the nutrients dynamics and biogeochemical cycles, consequently affecting the composition and productivity of river ecosystems. The Jinsha River, as the predominant contributor to runoff, suspended sediment (SS), and nutrients production within the Yangtze River, is a typical cascade reservoir region with unclear transport patterns and retention mechanisms of nutrients (nitrogen and phosphorus). Furthermore, how to regulate nutrients delivery in the cascade reservoirs region is also an urgent issue for basin water environment study. Therefore, we monitored monthly variations in nitrogen and phosphorus concentrations from November 2021 to October 2022 in the cascade reservoirs of the Jinsha River. The results indicated that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) decreased along the cascade of reservoirs, primarily due to PP deposited with SS, while opposing trends for total nitrogen (TN) and dissolved total nitrogen (DTN), which might be the consequences of human inputs and the increase of dissolved inorganic nitrogen discharged from the bottom of the reservoirs. Moreover, the positive average annual retention ratios for TP and PP were 10% and 16%, respectively, in contrast to the negative averages of -8 % for TN and -11% for particulate nitrogen (PN). The variability in runoff-sediment and hydraulic retention time (HRT) of cascade reservoirs played crucial roles in the retention of TP and PP. A regulatory threshold of HRT = 5.3 days in the flood season was obtained for controlling the balance of TP based on the stronger relationship between HRT and TP retention ratio. Consequently, the HRT of these reservoirs could be managed to control nutrients delivery, which was of particular significance for basin government institutions. This study enhances our comprehension of how cascade reservoirs influence the distribution and transport patterns of nutrients, offering a fresh perspective on nutrients delivery regulation.

Basin-wide tracking of nitrate cycling in Yangtze River through dual isotope and machine learning

The nitrate (NO3-) input has adversely affected the water quality and ecological function in the whole basin of the Yangtze River. The protection of water sources and implementation of "great protection of Yangtze River" policy require large-scale information on water contamination. In this study, dual isotope and Bayesian mixing model were used to research the transformation and sources of nitrate. Chemical fertilizers contribute 76 % of the nitrate sources in the upstream, while chemical fertilizers were also dominant in the midstream (39 %) and downstream (39 %) of Yangtze River. In addition, nitrification process occurred in the whole basin. Four machine learning models were used to relate nitrate concentrations to explanatory variables describing influence factors to predict nitrate concentrations in the whole basin of Yangtze River. The anthropogenic and natural factors, such as rainfall, GDP and population were chosen to take as predictor variables. The eXtreme Gradient Boosting (XGBoost) model for nitrate has a better predictive performance with an R2 of 0.74. The predictive models of nitrate concentrations will help identify the nitrate distribution and transport in the whole Yangtze River basin. Overall, this study represents the first basin-wide data-driven assessment of the nitrate cycling in the Yangtze River basin.

Nutrient changes in the Bohai Sea over the past two decades

With the growing problem of eutrophication in the Bohai Sea, actions have been taken to reduce nutrient inputs, but it remains to be seen whether nutrient levels and structure have been ameliorated. In this study, the nutrient trends in the Bohai Sea are re-examined based on observations from 2000 to 2019. The results suggest that dissolved inorganic nitrogen (DIN) concentrations and DIN/DIP (dissolved inorganic phosphate) ratios gradually increased from 2000 to 2013 but dramatically decreased from 2013 to 2019. The increase and decrease rates of DIN concentrations decreased with increasing water depth, indicating that DIN concentrations in nearshore waters responded more rapidly to changes in human activities. However, DIP concentrations responded weakly to nutrient inputs, with their trends uncoupled. The DIN/DIP ratios have declined close to and in some seasons even below the canonical Redfield ratio in areas with water depths >20 m recently, implying that relative nutrient limitation in these areas may be shifting from relative phosphorus (P) limitation to absence of relative nutrient limitation or relative nitrogen (N) limitation. Atmospheric deposition, wastewater discharge, and riverine input were responsible for 66 %, 21 %, and 13 % of the variance in the decline of DIN concentration, respectively. Several environmental indicators responded positively to the decrease in DIN concentrations and DIN/DIP ratios, with varying degrees of recovery recently. Our study proves the phased success of various nutrient reduction measures taken by the Chinese government to improve the environment of the Bohai Sea over the past decade.

Surface-Water Nitrate Exposure to World Populations Has Expanded and Intensified during 1970-2010

Excessive nitrate in surface waters deteriorates the water quality and threatens human health. Human activities have caused increased nitrate concentrations in global surface waters over the past 50 years. An assessment of the long-term trajectory of surface-water nitrate exposure to world populations and the associated potential health risks is imperative but lacking. Here, we used global spatially explicit data on surface-water nitrate concentrations and population density, in combination with thresholds for health risks from epidemiological studies, to quantify the long-term changes in surface-water nitrate exposure to world populations at multiple spatial scales. During 1970-2010, global populations potentially affected by acute health risks associated with surface-water nitrate exposure increased from 6 to 60 million persons per year, while populations at potential chronic health risks increased from 169 to 1361 million persons per year. Potential acute risks have increasingly affected Asian countries. Populations potentially affected by chronic risks shifted from dominance by high-income countries (in Europe and North America) to middle-income countries (in Asia and Africa). To mitigate adverse health effects associated with surface-water nitrate exposure, anthropogenic nitrogen inputs to natural environments should be drastically reduced. International and national standards of maximum nitrate contamination may need to be lowered.

Long-term variation in nutrients in the South Yellow Sea in response to anthropogenic inputs

Based on historical data from 1976 to 2019, the effects of anthropogenic activities on long-term changes in nutrients and their ecological effects in the South Yellow Sea were investigated. The dissolved inorganic nitrogen (DIN) concentrations increased continuously from 1990 until the mid-2000s, followed by a shift from an upward trend to a downward trend. The phosphate (PO₄-P) and silicate (SiO₃-Si) concentrations also showed obvious interannual variations throughout the study period. The concentrations of DIN, PO₄-P and SiO₃-Si have decreased significantly in recent decade and more. These changes mainly resulted from the reduction in terrestrial input, while the main reason for the decrease in DIN and PO₄-P concentrations is the reduction in anthropogenic input. The long-term nutrient changes in the South Yellow Sea have potential ecological impacts on green tide features.