Connectivity of floodplain influences riverine carbon outgassing and dissolved carbon transport.

Rivers not only function as a conduit for the delivery of terrestrial constituents to oceans, but they also serve as an essential medium for biogeochemical processing of the constituents. While extensive research has been conducted on carbon transport in many rivers, little is known about carbon transformation in engineered rivers reconnected with their floodplain network. Being the largest distributary of the levee-confined Mississippi River (MR), the Atchafalaya River (AR) carries 25 % of the MR water, flowing through North America's largest freshwater swamp basin and emptying into the Gulf of Mexico. Previous studies reported that this 200-km long, 5-30-km wide river basin can remove a substantial amount of riverine nutrients and organic carbon. This study aimed to test the hypothesis that the AR emits significantly higher CO2 into the atmosphere as it flows through its extensive floodplain network than the levee-confined MR does. From January 2019 to December 2021, we conducted biweekly - monthly in-situ measurements in the lower AR at Morgan City and in the lower Mississippi River at Baton Rouge. Field measurements included partial pressure of dissolved CO2 (pCO2), water temperature, chlorophyll a, colored dissolved organic matter, dissolved oxygen, pH, and turbidity. During each field sampling, water samples were collected and analyzed for concentrations of dissolved organic and inorganic carbon (DOC and DIC). Mass transport of DOC and DIC and outgassing of CO2 were quantified for the two rivers. We found that pCO2 levels were significantly higher in the AR (mean: 3563 µatm; min-max: 1130-8650 µatm) than those in the MR (1931 µatm, 836-3501 µatm), resulting in a doubled CO2 outgassing rate in the AR (486 mmol m2 d-1) than in the MR (241 mmol m2 d-1). The AR had higher DOC (8.5 mg L-1) but lower chlorophyll a (153.9 AFU) when compared with the MR (7.5 mg L-1 and 164.0 AFU). Water temperature was constantly higher in the AR than in the MR, especially during the wintertime. Since the Mississippi-Atchafalaya River system is among the world's largest and most engineered river systems, our assessment offers a field case study to inform on the potential implications of reconnecting rivers with their floodplains networks.

Dissolved carbon in effluent of wastewater treatment plants and its potential impacts in the receiving karst river.

The dissolved carbon cycling in river system fueled by wastewater treatment plant effluent have been a research hotspot. However, the composition of dissolved carbon (DC) in wastewater effluents from karst regions remains poorly understood, resulting in a lack of clarity regarding its impact on the dynamics of dissolved carbon in karst rivers. To address this knowledge gap, this study investigated variations of dissolved inorganic (DIC) and organic C (DOC) components in effluent in karst regions and preliminarily discussed their influence on the DC cycling in karst rivers. The results showed that bicarbonate (HCO3-) in WWTP effluents makes more than 90% of the total dissolved inorganic carbon (DIC). The partial pressure of aqueous CO2 (pCO2) of the effluent reached 14450 ± 10084µtam, and pCO2 level declined with increasing river distance from the effluent discharge, effluent acted as a strong CO2 emitter to the atmosphere. Stable carbon isotope and water chemistry evidence revealed that organic matter degradation made important contributions to the high CO2 concentrations in effluent. PHREEQC mixing simulation together with filed samples data indicated that the DIC species can be changed, and pCO2 increased in receiving karst river water after mixed with effluent. The dissolved organic carbon (DOC) of effluent contained humic-like and protein-tryptophan-like, both of them appeared important and recent autochthonous, which could interfere the distinguish the sources of DOC in receiving karst river water. Thus, these findings highlight that the effluent can be an essential factor for the changes of the karst riverine DC pool, which advance our understanding on karst riverine DC evolution under anthropogenic activities. As more than 30% of the earth surface in China, northern America, and Europe are covered by carbonate rocks, this study has relevant implications for other karst regions as it underscores the influence of WWTP effluents on the carbon cycle in karst rivers. Such information and knowledge are valuable for monitoring and managing effluent-receiving river in other karst regions in the world.

Response of dissolved carbon dioxide and methane concentration to warming in shallow lakes.

Shallow lake ecosystems are highly sensitive to temperature fluctuation because of their high water surface-to-volume ratios. Shallow lakes have been increasingly identified as a hotspot of CO2 and CH4 emissions, but their response to temperature variation remains unclear. Here, we report from a 5-month outdoor mesocosm experiment where we investigated the impacts of a projected 3.5 °C future warming and monthly temperature changes on lake CO2 and CH4, as well as the key drivers affecting the lake carbon cycling. Our results show that CO2 and CH4 concentrations had a significantly positive correlation with monthly temperatures. CH4 concentration was primarily regulated by monthly temperature, while nutrients effects on CO2 concentration overrode climate warming and temporal temperature changes. These findings imply the varied roles that temperature and nutrient levels can play on CO2 and CH4 dynamics in shallow lake systems. The relationship between temperature and CO2 concentration was nonlinear, showing a threshold of approximately 9 °C, at which CO2 concentration could be strongly modified by nutrient level in the lake systems. Understanding this complex relationship between temperature with CO2 and CH4 concentrations in shallow lakes is crucial for effective lake management and efficient control of greenhouse gases (GHGs) emissions.

Dual stable isotopes to rethink the watershed-scale spatiotemporal interaction between surface water and groundwater.

The interaction between groundwater and surface water, including their recharge relationship and ratio, is crucial for water cycling, management, and pollution control. However, accurately estimating their spatiotemporal interaction at the watershed scale remains challenging. In this study, we used dual stable isotopes (δ18O, δ2H, d-excess, and lc-excess) and hydrochemistry methods to rethink spatiotemporal interaction at the Yiluo River watershed in central China. We collected 20 groundwater and 40 surface water samples over four periods in two seasons (dry and wet). Our results showed that in the downstream region, groundwater recharged surface water in the dry season while surface water recharged groundwater in the wet season, with average recharge ratios of 89.82% and 90.02%, respectively. In the midstream region, surface water recharged groundwater in both seasons with average ratios of 93.79% and 91.35%. In contrast, in the upstream region, groundwater recharged surface water in both seasons with ratios of 67.35% and 76.89%. Seasonal changes in the recharge relationship between surface water and groundwater in the downstream region also been found. Our findings provide valuable insights for watershed-scale water resource and pollution management.

Intense methane diffusive emissions in eutrophic urban lakes, Central China.

Urban lakes are hotspots of methane (CH4) emissions. Yet, actual field measurements of CH4 in these lakes are rather limited and our understanding of CH4 response to urban lake eutrophication is still incomplete. In this study, we measured dissolved CH4 concentrations and quantified CH4 diffusion from four urban lakes in subtropical China during wet and dry seasons. We found that these lakes were constantly CH4-saturated, contributing the greenhouse gas (GHG) to the atmosphere. Nutrient enrichment significantly increased CH4 concentrations and diffusive fluxes. Average CH4 flux rate in the highly-eutrophic lake zones (4.18 ± 7.68 mmol m-2 d-1) was significantly higher than those in the mesotrophic (0.19 ± 0.18 mmol m-2 d-1) and lightly/moderately-eutrophic zones (0.72 ± 2.22 mmol m-2 d-1). Seasonally, CH4 concentrations and fluxes were significantly higher in the wet season than in the dry season in the mesotrophic and the lightly/moderately-eutrophic lake zones, but an inverse pattern existed in the highly-eutrophic lake zones. CH4 concentrations and fluxes increased with elevated levels of nitrogen, phosphorus and dissolved organic carbon (DOC). The accumulation of nutrients provided autochthonous substrate for methanogenesis, indicated by a negative correlation between CH4 and the C:N ratio. Ammonium-nitrogen (NH4+-N) was the best predictor for spatial fluctuation of CH4 concentrations and diffusive fluxes in the mesotrophic and the lightly/moderately-eutrophic lake zones, while total nitrogen (TN) and total phosphorus (TP) levels showed the highest predictability in the highly-eutrophic lake zones. Based on the findings, we conclude that nutrient enrichment in urban lakes can largely increase CH4 diffusion, and that urban sewage inflow is a key concern for eutrophication boosting CH4 production and diffusive emission. Furthermore, our study reveals that small urban lakes may be an important missing source of GHG emissions in the global C accounting, and that the ratio of littoral-to-pelagic zones can be important for predicting lake-scale estimation of CH4 emission.

Changes in CO2 concentration and degassing of eutrophic urban lakes associated with algal growth and decline.

Urban lakes are numerous in the world, but their role in carbon storage and emission is not well understood. This study aimed to answer the critical questions: How does algal growing season influence carbon dioxide concentration (cCO2) and exchange flux (FCO2) in eutrophic urban lakes? We investigated trophic state, seasonality of algal productivity, and their association with CO2 dynamics in four urban lakes in Central China. We found that these lightly-to moderately-eutrophic urban lakes showed a shifting pattern of CO2 source-sink dynamics. In the non-algal bloom phase, the moderately-eutrophic lakes outgassed on average of 12.18 ± 24.37 mmol m-2 d-1 CO2; but, during the algal bloom phase, the lakes sequestered an average 1.07 ± 6.22 mmol m-2 d-1 CO2. The lightly-eutrophic lakes exhibited lower CO2 emission in the algal bloom (0.60 ± 10.24 mmol m-2 d-1) compared to the non-algal bloom (3.84 ± 12.38 mmol m-2 d-1). Biological factors such as Chl-a (chlorophyll a) and AOU (apparent oxygen utilization), were found to be important factors to potentially affect the shifting pattern of lake CO2 source-sink dynamics in moderately-eutrophic lakes, explaining 48% and 34% of the CO2 variation in the non-algal and algal bloom phases, respectively. Moreover, CO2 showed positive correlations with AOU, and negative correlations with Chl-a in both phases. In the lightly-eutrophic lakes, biological factors explained a higher proportion of CO2 variations (29%) in the non-algal bloom phase, with AOU accounting for 19%. Our results indicate that algal growth and decline phases largely affect dissolved CO2 level and exchange flux by regulating in-lake respiration and photosynthesis. Based on the findings, we conclude that shallow urban lakes can act as both sources and sinks of CO2, with algal growth seasonality and trophic state playing pivotal roles in controlling their carbon dynamics.

Nitrogen addition effect overrides warming effect on dissolved CO2 and phytoplankton structure in shallow lakes.

Shallow lakes are numerous in all climate zones, but our knowledge about their dissolved carbon dioxide (CO2) response to future climate change and nutrient enrichment is rather limited. Here we performed a mesocosm experiment with four treatments to investigate how warming and nitrogen addition will impact the partial pressure of CO2 (pCO2) and phytoplankton community individually and combined. We found that warming alone had no significant effect on pCO2, while nitrogen addition increased pCO2 significantly. The combined effects of nitrogen addition and warming on pCO2 level were prevalent, indicating that eutrophic shallow lakes would be double-jeopardized in the future climate. Warming and nitrogen addition together also showed to have changed the phytoplankton community structure, suggesting a potential shifting of biological system in shallow lakes under changing climate. These findings highlight the importance of reducing nitrogen pollution to shallow lake systems for sustainable development goal.

Optimizing environmental flow based on a new optimization model in balancing objectives among river ecology, water supply and power generation in a high-latitude river.

Environmental flow plays an important role in maintaining the health of river ecosystems and aquatic habitats. Although ecological regulation of environmental flow has attracted the attention of scientists, managing the world's reservoir-regulated rivers to better meet the needs of human being and ecosystems is a complex social challenge. To address the above issues, we constructed a model for optimizing reservoir operation based on a balance in achieving multi objectives among environmental flow, water supply and power generation (EWP). The model was solved using an intelligent multi-objective optimization algorithm (ARNSGA-III). The developed model was demonstrated in a large reservoir, Laolongkou Reservoir in the Tumen River. The results showed that the reservoir altered environmental flows mainly in terms of flow magnitude, peak, times, duration and frequency, which result in a sharp decrease in spawning fish, and degradation and replacement of vegetation along the channels. In addition, the mutual feedback relationship between the objectives of environmental flows, water supply and power generation is not static, but varies over time and space. The constructed model based on Indicators of Hydrologic Alteration (IHAs) can effectively guarantee the environmental flow at daily scale. In detail, the river ecological benefit increased by 64% in wet year, 68% in normal year, 68% in dry year after optimizing regulation of reservoir, respectively. This study will provide a scientific reference for the optimizing of the management in other rivers affected by dams.

Streamflow and sediment load changes from China's large rivers: Quantitative contributions of climate and human activity factors.

Riverine water and sediment discharge drive global material circulation and energy transfer, and they are crucial to the biogeochemical cycle. We investigated the changes in water-sediment fluxes in six major rivers from north to south in China from the mid-1950s to 2020 under the influence of climate change and human activities, and quantified the contributions of these specific influencing factors to water-sediment flux changes. Results showed that streamflow of the Songhua, Liao and Yellow rivers decreased significantly (p < 0.05). The sediment load of all rivers reduced significantly (p < 0.01) except the Songhua River. Streamflow or sediment fluxes to the oceans have increased or stabilized since around 2000, and the terrestrial sediment yielding center in China has shifted southward from the Yellow River to the Yangtze and Pearl rivers. The contribution of precipitation to the streamflow and sediment load changes decreased from north to south across the six rivers. From the mid-1950s to 2020, the underlying land surface change was the dominant contributor (>70 %) to reducing streamflow in the Songhua and Yellow rivers, while climate change (>50 %) was responsible for decreased streamflow in the Liao and Huai rivers. The sediment load reduction of the six rivers was attributed mainly to human activities. Among them, dam construction, human water consumption and catchment land surface change have reduced the total sediment load into the sea by 49 %, 25 % and 19 %, respectively. These results highlight that north-south variability in water and sediment flux are driven by both natural and anthropogenic forcing agents.

Land use and hydrological factors control concentrations and diffusive fluxes of riverine dissolved carbon dioxide and methane in low-order streams.

We analyzed the impacts of land use/land cover types on carbon dioxide (CO2) and methane (CH4) concentration and diffusion in 1st to 4th Strahler order tributaries of the Longchuan River to the upper Yangtze River in China by using headspace equilibration method and CO2SYS program. Field sampling and measurements were conducted during the dry and wet seasons from 2017 to 2019. The average of calculated CO2 partial pressure (pCO2, mean ± SD: 2389 ± 3220 µatm) by CO2SYS program was 1.9-fold higher than the value (mean ± SD: 1230 ± 1440 µatm) 10 years ago in the Longchuan River basin, where the urban land area increased by a factor of 7 times. Further analysis showed that corrected pCO2 by headspace method and dissolved CH4 (dCH4) decrease as the stream order and flow velocity increase. The pCO2 and dCH4 in the wet season was lower than that in the dry season. The explanatory ability of land use types on the variation of corrected pCO2 and dCH4 was stronger at the reach scale than at the riparian and catchment scales in two seasons. Urban land at reach scale further showed much higher explanation on corrected pCO2 and dCH4 than cropland, grassland and forest land in the wet season. The Longchuan River emits approximately 112.5 kt CO2-C and 1.0 kt CH4-C per year, being 1.7-fold of the total lateral export of dissolved inorganic and dissolved organic carbon (68.3 kt C y-1). The findings highlight the scale effects of land use on the observed seasonality in dissolved carbon gases in low-order streams.

Riverine nitrate source and transformation as affected by land use and land cover.

A mixed land use/land cover (LULC) catchment increases the complexity of sources and transformations of nitrate in rivers. Spatial paucity of sampling particularly low-resolution sampling in tributaries can result in a bias for identifying nitrate sources and transformations. In this study, high spatial resolution sampling campaigns covering mainstream and tributaries in combination with hydro-chemical parameters and dual isotopes of nitrate were performed to reveal spatio-temporal variations of nitrate sources and transformations in a river draining a mixed LULC catchment. This study suggested that point sources dominated the nitrate in the summer and winter, while non-point sources dominated the nitrate in the spring and autumn. A positive correlation was observed between proportions from sewage and land use index (LUI). However, negative correlations between soil nitrogen/nitrogen fertilizer and LUI were observed. With an increase of urban areas, the increased contribution from domestic sewage resulted in an increase of NO3- concentrations in rivers. Both urban and agricultural inputs should be considered in nitrate pollution management in a mixed LULC catchment. We concluded that the seasonal variations of nitrate sources were mainly affected by flow velocity conditions and agricultural activities, while spatial variations were mainly affected by LULC. In addition, we found a novel underestimation of dominated sources from Bayesian model because of mixing effect of isotope values from the tributaries to mainstream, however, high spatial resolution sampling can make up for this shortcoming. δ15N and δ18O values of nitrate indicated that nitrate originated from nitrification in soils. The nitrate concentrations and correlation between δ15N and 1/[NO3-] suggested little contribution of nitrate removal by denitrification. Thus, the nitrate reduction in the Yuehe River basin needs to be strengthened. The study provides new implications for estimation of nitrate sources and transformations and basis for nitrate reduction in the river with mixed LULC catchment.

Source and quality of dissolved organic matter in streams are reflective to land use/land cover, climate seasonality and pCO2.

Sources and quality of dissolved organic matter (DOM) in streams may be largely controlled by the landscape and season. In this study, we attempted to answer three critical questions: 1) Do land use/land cover (LULC) types affect DOM characteristics? 2) Is there a seasonal fluctuation in DOM components? 3) How do DOM quality and LULC types influence aqueous carbon dioxide partial pressure (pCO2). To achieve this, we investigated the fluorescence characteristics of DOM and its implication for pCO2 in three streams draining land with different urban intensities under distinctive dry and wet seasons. Four fluorescence components were identified, including two terrestrial humic-like components, one protein-like component and one microbial humic-like component. We found a significant positive relationship of the maximum fluorescence intensity (Fmax) of the four components and fluorescence index (FI370) with urbanization intensity in both the dry and wet seasons. The mean Fmax, biological index (BIX) and FI370 all exhibited an increasing trend from upstream to downstream in the stream with highest proportions of urban and cropland. The fluorescence characteristics were negatively related to proportion of forested land in the both seasons. The terrestrial humic-like DOM was dominating in the studied streams. Moreover, the seasonality altered the DOM composition, with protein-like component emerging only in stream waters during the dry season, while microbial humic-like component exclusively occurred during the wet season. pCO2 values were positively related to terrestrial humic-like and biological protein-like components, and urban land. The dry season had much higher pCO2 than the wet season. Results from the Partial Least Squares Path (PLS-PM) models further indicated that LULC types were important in mediating fluorescence DOM whilst pCO2 was more sensitive to the direct effect from FDOM dynamics. We conclude that DOM source and quality in streams are reflective to LULC and climate seasonality, and are good indicators of pCO2 via source tracer and quality of fluorescence components.

Spatial and Temporal Changes of Sand Mining in the Yangtze River Basin since the Establishment of the Three Gorges Dam.

The global demand for sand and gravel is at 50 billion tons per year, far exceeding global resource capacities. It reached 7.6 billion tons in 2021 in the Yangtze River Basin (YRB), China. However, production is severely limited in the YRB. Therefore, the incongruity between the supply and demand of river sand is prominent. Wise management of decreasing sand resources in the YRB has become critical since the Three Gorges Dam became operational in 2003. This study synthesized spatial and temporal changes in sand mining activities and quantities along the Yangtze River and its major tributaries from 2004 to 2020. Results from the study show that the mining amount during the period reached 76.2 million tons annually. At the same time, riverine suspended sediment discharge (SSD) downstream of the Three Gorges Dam decreased largely. SSD reduction leads to riverbed erosion, further limiting the riverine sand and gravel sources for mining. Thus, alternative sand and gravel resources, as well as optimizing supply/demand balance, are necessary for sustainable development. There is an urgent need to assess the relationship between river sand resources and exploitation in the YRB for creating a sand and gravel data management system in order to cope with the increasing incongruity between their supply and demand.

Riverine dissolved organic matter (DOM) as affected by urbanization gradient.

Rapid urbanization has considerably altered carbon biogeochemical cycle and river hydrology. However, the influences of urban land use and urban-induced nutrient increase on dissolved organic matter (DOM) characteristics are poorly understood. Here we hypothesize that the alterations significantly change sources and levels of DOM in river systems that drain the urban areas. To test the hypothesis, we investigated DOM in headwater rivers with varied urban intensities in the Three Gorges Reservoir Area (TGRA), China, through field sampling conducted in the dry and wet seasons. We found positive relationships of urban land (%Urban) with DOC concentration and chromophoric DOM (CDOM) absorption coefficients a254, a280 and a350, as well as fluorescence index (FI370), indicating the significantly increased levels of DOM and autochthonous sources along an urbanization gradient. A stepwise regression analysis demonstrated that occurrences of DOC and CDOM can be predicted by %Urban, while increasing autochthonous source is predictable by the increase in riverine nitrogen. Moreover, a254, a280 and FI370 values showed distinct seasonal variations, with significantly higher CDOM concentration in the wet season and with much higher autochthonous signal in the dry season with high nitrogen loading. Based on the findings, we conclude that urbanization influences occurrences and sources of DOM, with increasing urbanization making an important and direct contribution to DOM, and an indirect effect of urban induced nutrient enrichment, i.e., enhanced nutrient loadings increase autochthonous DOM production in rivers.

Watershed scale spatiotemporal nitrogen transport and source tracing using dual isotopes among surface water, sediments and groundwater in the Yiluo River Watershed, Middle of China.

Nitrogen pollution has been shown to have strong potential threaten to the human drinking water and agriculture. However, identifying the nitrogen and spatial-temporal variation and nitrogen pollution sources among surface water, sediments and groundwater at the watershed scale is still of insufficient understanding. In this study, multi-methods (dual isotopes, hydraulic, hydrogeochemical methods) have been used and 400 samplings (40 sediments, 20 shallow groundwater and 40 surface waters in four periods in dry and wet seasons) were collected from 2018 to 2020. The results showed that the concentration of NO3--N, NH4+-N, NO2--N and total nitrogen (TN) had variable spatial and temporal changes in whole watershed. The concentration of TN, NO3--N, NH4+-N and NO2--N in downstream was higher than midstream and upstream both in dry and wet seasons. The concentration of TN, NO3--N, NH4+-N and NO2--N of the whole watershed in wet season was higher than dry season. The dual isotope values indicate that nitrogen sources were mainly derived from manure and sewage waste input (MSI), agriculture chemical fertilizers (ACFI) and sediments nitrogen input (SNI). Those nitrogen sources have different proportion in downstream, midstream and upstream in dry and wet seasons (the largest proportion: MSI 95.24% in downstream and ACFI 86.26% in upstream both in dry season, SNI 31.75% in midstream in wet season). Water exchange has positive correlation with the nitrogen concentration. High level of nitrogen in river also can be a diver in different location and seasons. Those results can be useful for developing regional management strategies and plans for water pollution control and treatment at watershed-scale.

Unravelling the spatiotemporal variation of pCO2 in low order streams: Linkages to land use and stream order.

Headwater streams make the majority of cumulative stream length in a river basin, carbon dioxide (CO2) emission from headwater (low order) streams is thus an essential component. Anthropogenic activities in headwater areas such as land use change and land use practices can strongly modify terrestrial carbon and nutrient input, which could affect the level of partial pressure of dissolved carbon dioxide (pCO2) and CO2 degassing from streams. However, there are large uncertainties in estimates due to the lack of data in subtropical rivers of rapidly developing rural regions. The spatiotemporal variation and driving factors of the pCO2 and CO2 degassing from low-order streams remain to be explored. In this study, we assess multi-spatial scale effects of land use on pCO2 dynamics in seven headwater tributary rivers in Central China during 2016, 2017 and 2018 in rainy and dry seasons. Our results reveal that the stream pCO2 level consistently increases as the stream order increases from 1 to 3 under apparent seasonal variations. Riverine pCO2 is positively related to the percentage of urban land and cropland surrounding the river segments, but is negatively related to the percentage of forest land. The stream pCO2 is more closely correlated with the 1000 and 2000 m diameters of circular buffers at upstream sampling sites than the circular buffers with 100 and 500 m diameters. There exist significant relationships of pCO2 with the concentrations of TN, TP, DO, and DOC in the low-order streams. The partial redundancy analysis quantifies the relative importance of anthropogenic land uses, natural factors and water chemical variables in mediating stream pCO2, showing that influences of anthropogenic land uses (urban and cropland) on pCO2 decrease, with a percentage role of 34%, 14%, and 4% in the 1st-, 2nd- and 3rd-order streams, respectively. The impact of nutrients on pCO2, however, increases as the stream order increases. Urban influence on stream pCO2 also decreases as stream order increases. Our study highlights the effect of land use/land cover types and stream order on riverine pCO2 and provides new insight into estimating CO2 emission in headwater streams. Future studies are needed on the linkage between riverine CO2 degassing and stream orders under changing land use conditions.

Hydrological seasonality and nutrient stoichiometry control dissolved organic matter characterization in a headwater stream.

Dissolved organic matter (DOM) is a diverse and highly complex mixture of organic macromolecules, and thus plays a central role in aquatic ecosystems. However, responses of components and sources of DOM to hydrological processes and trophic levels (nutrient stoichiometric ratios) are poorly understood, particularly in monsoonal headwater streams of Asia that are vulnerable to catchment physical characteristics. In this study, the excitation - emission matrix florescence spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) was used to explore the DOM characters in a headwater stream, where seasonal rainfalls and nutrient levels vary largely. The EEM-PARAFAC modelling identified one autochthonous protein-like fluorescence substance (C1) and two allochthonous fulvic- and humic-like fluorescence compounds (C2 and C3). The allochthonous compounds dominated the overall DOM signal in the headwaters. The hydrological seasonality coupled with nutrients was key in modulating headwater DOM sources and components. Seasonal rainfall events contributed more allochthonous terrestrial-derived DOM flushing into river waters, resulting in higher fulvic- and humic-like organic matter (C2 + C3) in the wet season. In the dry season, longer water residence time accompanying with higher C:P stoichiometric ratio was responsible for higher autochthonous microbial- and plant-derived DOM (tryptophan and tyrosine fractions), also reflected by higher C1, biological index (BIX) and freshness index (ß:α). In-stream microbial metabolism of labile DOM fractions largely contributed to autochthonous DOM and partial pressure CO2 increase in the headwater stream. Our findings indicate that quality and quantity of DOM in headwater streams play a crucial role in downstream carbon cycle. Furthermore, the evidence combined from PARAFAC components, pCO2 and spectral slope clearly highlights the importance of microbial metabolism of carbon in lotic systems, especially during a dry season with increased residence time.

Hot spot of CH4 production and diffusive flux in rivers with high urbanization.

Rivers and streams play a central role in global carbon budget, but our knowledge is limited on the magnitude and extent of urbanization influence on riverine methane (CH4) dynamics. In this study, we investigated dissolved CH4 (dCH4) concentration and CH4 diffusive fluxes in 27 river segments of two 4th-order and three 3rd-order tributary rivers to the Yangtze River in China, which drained land areas with varied urbanization intensities. We found that urban development was the key factor responsible for high fluvial dCH4 concentration and diffusive flux, exceeding the influence of agricultural farming, and these headwater rivers were over-saturated in CH4 with respect to atmospheric equilibrium. dCH4 concentration (3546 ± 6770 nmol L-1) in the river segments draining higher urban area (20% ≤ urban land proportion ≤ 46%) was 5-6 times higher than those (615 ± 627 nmol L-1 and 764 ± 708 nmol L-1) in the river segments draining less urban area (0.1% ≤ urban land proportion < 2% and 2 ≤ urban land proportion < 20%). River segments draining higher urban area also acted as important sources of CH4 to the atmosphere (8.93 ± 14.29 mmol m-2 d-1). Total nitrogen (TN) concentration in river water showed the best prediction capacity when compared to other water parameters. Based on urban land use grouping, nutrient elements could predict dCH4 well in rivers draining higher urban areas (urban ≥ 2%), which also reflected the lateral input of pollutants (TN, ammonia nitrogen, and total phosphorus). River bottom sediment fraction contributed to trapping organic matter and nutrients as well as to oxic and anoxic conditions, thereby determining reach-scale spatial patterns of dCH4 concentration. These findings highlight that combining distal geomorphic and hydrologic drivers can be effective in determining the relationship between riverine CH4 and the proximal controls (e.g., nutrients, dissolved oxygen, dissolved organic carbon), as well as in identifying their key drivers. Being rapid urbanization a common feature of catchments worldwide, our results suggest riverine CH4 emissions will increase into the future.

Investigating the potential use of Sentinel-1 data for monitoring wetland water level changes in China's Momoge National Nature Reserve.

BACKGROUND: Interferometric Synthetic Aperture Radar (InSAR) has become a promising technique for monitoring wetland water levels. However, its capability in monitoring wetland water level changes with Sentine-1 data has not yet been thoroughly investigated. METHODS: In this study, we produced a multitemporal Sentinel-1 C-band VV-polarized SAR backscatter images and generated a total of 28 interferometric coherence maps for marsh wetlands of China's Momoge National Nature Reserve to investigate the interferometric coherence level of Sentinel-1 C-VV data as a function of perpendicular and temporal baseline, water depth, and SAR backscattering intensity. We also selected six interferogram pairs acquired within 24 days for quantitative analysis of the accuracy of water level changes monitored by Sentinel-1 InSAR. The accuracy of water level changes determined through the Sentinel-1 InSAR technique was calibrated by the values of six field water level loggers. RESULTS: Our study showed that (1) the coherence was mainly dependent on the temporal baseline and was little affected by the perpendicular baseline for Sentinel-1 C-VV data in marsh wetlands; (2) in the early stage of a growing season, a clear negative correlation was found between Sentinel-1 coherence and water depth; (3) there was an almost linear negative correlation between Sentinel-1 C-VV coherence and backscatter for the marsh wetlands; (4) once the coherence exceeds a threshold of 0.3, the stage during the growing season, rather than the coherence, appeared to be the primary factor determining the quality of the interferogram for the marsh wetlands, even though the quality of the interferogram largely depends on the coherence; (5) the results of water level changes from InSAR processing show no agreement with in-situ measurements during most growth stages. Based on the findings, we can conclude that although the interferometric coherence of the Sentinel-1 C-VV data is high enough, the data is generally unsuitable for monitoring water level changes in marsh wetlands of China's Momoge National Nature Reserve.

Assessment of water quality of best water management practices in lake adjacent to the high-latitude agricultural areas, China.

A major inland alkalinity lake in Northeast China, the Chagan Lake, was studied for the changes of its water qualities over the past three decades. Water quality data, including total nitrogen (TN), total phosphorus (TP), pH, dissolved oxygen (DO), and fluoride (F-), were analyzed to derive key indices for guiding water quality management. Our study found that the Chagan Lake had an average trophic state index (TSI) ranging 50 to 70; the average TSI for TP ranging between 70 and 80, and the average TSI for TN being 50. Over the past three decades, the TSI values generally trended lower, but there was a slight uptrend from 2012 onwards. Seasonal variations in the concentrations of TN and TP were identified. The TSI values in September were higher than those in May, while the values of un-ionized ammonia (UIA) during rainy seasons were higher than those during dry seasons. The average values of alkalinity and F- in the lake water exceeded the upper limits set in the Chinese water quality standards, i.e., 20 mg/L and 1 mg/L, respectively. It was defined that the evolution of lake water quality proceeded in four consecutive periods, namely natural, deterioration, improvement, and risk period; the improvement period benefitted from a historical water conservation project. Our study concluded that the amount of irrigation discharge into the Chagan must be monitored, and controlled, in order to sustain the critical ecological functions currently provided by the Chagan Lake.