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Mountainous rivers (MRs) are one of the critical systems in transporting dissolved organic carbon (DOC) from terrestrial environments to downstream ecosystems. However, how geographical factors and anthropogenic impacts control the composition and export of DOC in mountainous rivers remains largely unclear. Here, we explore DOC dynamics in three subtropical mountainous catchments (i.e., the Yinjiang, Shiqian, and Yuqing catchments) in southwest China which are highly influenced by anthropogenic activities. Water chemistry, stable and radioactive carbon isotopes of DOC (δ13CDOC and Δ14CDOC) and optical properties (UV absorbance and fluorescence spectra) for river water were employed to assess the biogeochemical processes and controlling factors of DOC. The radiocarbon ages of the DOC in the Yinjiang River varied widely, ranging from 928 years before present to modern. Both allochthonous and autochthonous sources had an important effect on riverine DOC export. Results from carbon isotopes suggested that in-stream processing of POC is also an important source of DOC. DOC in catchments with higher slope gradients and lower annual air temperature was characterized by lower concentration and more aromatic, which was distinct from those with gentle slopes and higher temperature. Variabilities in DOC concentrations and δ13CDOC were also explained by land use, showing that higher DOC concentrations with 13C-depleted characters were observed in urban and agricultural land use areas. Moreover, DOM was less aromatic, less recently produced and had a higher degree of humification in catchments with a higher proportion of urban and agricultural land use area. This research highlights the significance of incorporating geographical controls and anthropogenic impacts into the MRs to better understand their DOC dynamics and quality of dissolved organic matter (DOM).

The increase of affected river reaches by reservoirs has drastically disturbed the original hydrological conditions, and subsequently influenced the nutrient biogeochemistry in the aquatic system, particularly in the cascade reservoir system. To understand the seasonal variation of nitrogen (N) behaviors in cascade reservoirs, hydrochemistry and nitrate dual isotopes (δ15N-NO3- and δ18O-NO3-) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO3--N accounted for almost 90% of the total dissolved nitrogen (TDN) concentration with high average concentration 3.8 ± 0.4 mg/L among four cascade reservoirs. Higher N concentration (4.0 ± 0.8 mg/L) and larger longitudinal variation were observed in summer than in other seasons. The relationship between the variation of NO3--N and dual isotopes in the profiles demonstrated that nitrification was dominated transformation, while assimilation contributed significantly in the epilimnion during spring and summer. The high dissolved oxygen concentration in the present cascade reservoirs system prevented the occurrence of N depletion processes in most of the reservoirs. Denitrification occurred in the oldest reservoir during winter with a rate ranging from 18 to 28%. The long-term record of surface water TDN concentration in reservoirs demonstrated an increase from 2.0 to 3.6 mg/L during the past two decades (~ 0.1 mg/L per year). The seasonal nitrate isotopic signature and continuously increased fertilizer application demonstrated that chemical fertilizer contribution significantly influenced NO3--N concentration in the karst cascade reservoirs. The research highlighted that the notable N increase in karst cascade reservoirs could influence the aquatic health in the region and further investigations were required.

In order to advance our understanding of the driving factors controlling carbon dynamics and evolution of water quality in river-reservoir systems for the generation of hydropower, we conducted a study in a karst deep-water reservoir (Wujiangdu Reservoir), southwest China. Water samples were collected from the inflow/outflow of the Wujiangdu Reservoir, four vertical columns along the reservoir, and from three tributaries to the reservoir in January, April, July, and October 2017. The dissolved inorganic carbon (DIC) concentrations and carbon isotope composition (δ13CDIC) varied greatly (1.99–3.45 mmol/L and −10.7‰ to −6.0‰, respectively) and were controlled by multiple processes including CO2 outgassing, primary production, and organic matter degradation. In the four vertical profiles, the difference between the values of samples with those at 15 m of Δ[DIC] and Δ[δ13CDIC], Δ[DIC] and ΔCa2+ in the same water column had positive correlations, and the variation in dissolved O2, partial pressure of CO2, and flux of CO2 suggested that primary production dominated above the epilimnion and degradation of OM dominated below the epilimnion during the warm season. Continuous CO2 outgassing was found from riverine water to surface water of the reservoir before the dam based on carbon isotopic compositions and water chemistry. These processes would cause isotopic fractionation between the residual DIC and CO2 (aqueous), the degree of which varied among different seasons (July > April > October > January). Carbon transport and biogeochemical processes were highly controlled by the hydraulic retention time (HRT) in the river-reservoir system. It was estimated that approximately 71.5% of the annual DIC flux was produced in the water column below 15 m depth during the study year, suggesting that the processes of DIC generation and consumption occurred at the same time. The results highlight that carbon behavior in the impounded rivers is influenced by multiple processes. The carbon transformation processes should be taken into account for improving the estimation accuracy of the carbon budget calculations and the management of water quality for river-reservoir systems.

Reservoirs are widely established worldwide with considerable environmental impacts, especially on the riverine carbon cycle. However, the influence of reservoirs on the cycling of different forms of carbon and its regulation factors (e.g., seasonal variations versus hydrological management) have not been simultaneously studied. To fill this knowledge gap, seasonal water samples from the deep subtropical reservoir (Longtan reservoir) in the Pearl River were collected, and the concentrations and stable carbon isotopes of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC) were determined. The variations in stable carbon isotopes of DIC (-11.4‰ to -5.2‰), DOC (-32.2‰ to -26.2‰), and POC (-38.9‰ to -25.3‰) in the river-reservoir system indicated active production and degradation processes in different layers. We estimated that up to 23.0% of DIC, 20.5% of DOC, and most POC were intercepted or degraded within the reservoir. Our results further illustrated that hydrological management (water storage regulation) and seasonal variations from different perspectives controlled the cycling of different forms of carbon in the reservoir. In addition, with the gradual increase in the number of reservoirs, hydrological management can be considered as a potentially effective strategy to adjust the carbon biogeochemical cycling of reservoirs in the future.

The increase of affected river reaches by reservoirs has drastically disturbed the original hydrological conditions, and subsequently influenced the nutrient biogeochemistry in the aquatic system, particularly in the cascade reservoir system. To understand the seasonal variation of nitrogen (N) behaviors in cascade reservoirs, hydrochemistry and nitrate dual isotopes (δ15N-NO3− andδ18O-NO3−) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO3−–N accounted for almost 90% of the total dissolved nitrogen (TDN) concentration with high average concentration 3.8 ± 0.4 mg/L among four cascade reservoirs. Higher N concentration (4.0 ± 0.8 mg/L) and larger longitudinal variation were observed in summer than in other seasons. The relationship between the variation of NO3−–N and dual isotopes in the profiles demonstrated that nitrification was dominated transformation, while assimilation contributed significantly in the epilimnion during spring and summer. The high dissolved oxygen concentration in the present cascade reservoirs system prevented the occurrence of N depletion processes in most of the reservoirs. Denitrification occurred in the oldest reservoir during winter with a rate ranging from 18 % to 28 %. The long-term record of surface water TDN concentration in reservoirs demonstrated an increase from 2.0 to 3.6 mg/L during the past two decades (~ 0.1 mg/L per year). The seasonal nitrate isotopic signature and continuously increased fertilizer application demonstrated that chemical fertilizer contribution significantly influenced NO3−–N concentration in the karst cascade reservoirs. The research highlighted that the notable N increase in karst cascade reservoirs could influence the aquatic health in the region and further investigations were required.

Riverine transport of carbon from the land to the oceans plays a significant role in global carbon cycle. However, multiple processes can affect aquatic carbon cycling, and the carbon sources and processing in river systems are still elusive. Here, we analysed the water chemistry and dual carbon isotopes (δ

Due to the rapid urbanization process, the consumption of trace and rare earth elements has dramatically increased. Although some elements have been extensively studied due to their high biological toxicity, most elements are ignored and taken seriously in recent years. Here, we investigated the urban geochemistry, source, and anthropogenic responding factor for 15 trace elements (Cd, Pb, Co, Sn, Cu, Ni, V, As, Mo, Sb, Al, Li, Fe, Zn, and Sr) and rare earth elements in surface water of the Suzhou city. The percentage of anthropogenic gadolinium vary from 46.9% (YCH-2) to 92.8% (WS-2), while the analysis of variance shows that human activities may affect the distribution of Cd, Co, Sn, Ni, As, Li, Fe, and Sr. Three clusters are obtained from the correlation and cluster analysis. The Cluster 1 with a significant positive correlation of Pb, Cd, Gd, Li, Sr, Co, Fe, Ni, and Sn reflecting these elements are dominantly influenced by urban sewage and industrial activities. The Cluster 2 (Zn, Cu, and Al) can be attributed to geologic sources, while the Cluster 3 (V, Mo, As, and Sb) indicate the combined action of agricultural and urban activities. The Gd versus Li plot showed a significant positive correlation, which can be used as a new indicator to trace the anthropogenic impact on urban waters. Overall, this study provides clear evidence that the content and distribution of Gd and Li are deeply affected by human activities in a high-tech industrial city (Suzhou), which can be regarded as emerging elements contaminations.

Riverine transport of carbon from the land to the oceans plays a significant role in global carbon cycle. However, multiple processes can affect aquatic carbon cycling, and the carbon sources and processing in river systems are still elusive. Here, we analysed the water chemistry and dual carbon isotopes (δ13C and Δ14C) of dissolved inorganic carbon (DIC) and particulate organic carbon (POC) from mixed karst and non-karst subtropical monsoonal catchments, southwest China. The water chemistry of the river water showed that DIC concentrations were mainly controlled by carbonate weathering and modulated by agricultural activities and geomorphic characteristics (i.e. elevation and slope), but the stable isotope of DIC (δ13CDIC) was highly affected by CO2 outgassing and in-stream photosynthesis. The C/N ratios and stable isotope of POC (δ13CPOC) indicated that the composition of riverine POC derived from a mixture of terrestrial sources and algae/microbial sources. Based on the δ13C and Δ14C of POC, we used a Bayesian mixing model to constrain the POC sources, which showed that aquatic photosynthesis was the main source for POC. Our findings suggest that carbon dynamics in subtropical rivers are highly affected by aquatic photosynthesis, which has significant implications on carbon cycling within river systems.

Rivers play an important role in global water and carbon cycling, but there are still large uncertainties concerning evaporation and aquatic photosynthesis. Here we combined measurements of water chemistry, isotopic compositions (i.e., δ D w , δ 18 O w , δ 13 C DIC and ▵ 14 C DIC ) and geographic characteristics (i.e., river width) to elucidate in-stream hydrological and biogeochemical processes across rivers in Hainan Island, China. The results showed that dissolved inorganic carbon (DIC) in river waters was largely of modern origin, with about 95% from contemporary biogenic sources based on an isotopic mass balance of ▵ 14 C DIC . Significant evaporation and aquatic primary production co-occurred in these tropical rivers with large amounts of water and DIC being rapidly turned over in the water column, altering the water cycle and the carbon balance. High rates of evaporation and aquatic primary production were observed in the headwater segments, with narrow river width but broad available reactive surface area at the air-water interface. The asymmetric aquatic photosynthesis at different river segments caused the spatial heterogeneities of dissolved solutes. The results suggest that the available reactive area at the water-air interface is responsible for synchronous water loss and dissolved carbon evolution in flat tropical rivers. This study provides evidence that intense evaporation and aquatic photosynthesis mainly occurred in headwater segments, which has implications for understanding global carbon cycling.

Cascade damming has seriously affected the hydrologic regimes and solutes transport of the river systems. However, the effect of damming on carbon cycle and the underlying mechanisms are still unclear. In this study, to reveal the effects of damming on riverine dissolved inorganic carbon (DIC) transport, the water chemistry and stable isotopic signature of dissolved inorganic carbon (δ13CDIC) were analyzed in the tributaries and four subtropical karst reservoirs on the Pearl River in southern China. From reservoir epilimnion to hypolimnion, DIC concentrations increased from 183.2 to 229.4 mg L−1, 167.6 to 192.0 mg L−1, 179.1 to 181.5 mg L−1, and 182.4 to 183.8 mg L−1 while δ13CDIC decreased from −6.6‰ to −9.7‰, −7.8‰ to −10.4‰, −9.7‰ to −10.0‰ and −9.9‰ to −10.0‰ in the Chaishitan (CST), Longtan (LT), Yantan (YT), and Dahua (DH) reservoirs, respectively. Our analyses suggest that primary production and organic matter degradation were two dominant processes in carbon evolution in the epilimnion and hypolimnion, respectively. However, there was no obvious spatial heterogeneity of solutes’ concentration in the DH reservoir. Our results also suggested that cascade reservoirs had a regional damming effect and could be significantly weakened without damming or in daily regulated reservoirs. Furthermore, δ13CDIC had a significant positive relationship with the reservoir effect index (Ri), which was affected by hydraulic retention time and water temperature in the water column of reservoirs and is useful to evaluate the thermal stratification strength. This study highlights that dam constructions affected carbon transport and transformation significantly, and the degree of the damming effect on the carbon cycle can be expressed by Ri.

The effect of dams on dissolved inorganic carbon (DIC) transport and riverine ecosystems is unclear in karst cascade reservoirs. Here, we analyzed water samples from a karst river system with seven cascade reservoirs along the Wujiang River, southwestern China, during one hydrological year. From upstream to downstream, the average concentration of DIC increased from 2.2 to 2.6 mmol/L and its carbon isotope composition (δ13CDIC) decreased from −8.0 to −10‰. Meanwhile, the air temperature (Ta) increased from 20.3 °C to 26.7 °C and 10 °C to 13.7 °C in the warm and cold seasons, respectively. The results suggest that a cascade of dams has a stronger effect on DIC dynamics and retention than a single dam. The good correlation between Ta/HRT (hydraulic retention time) and Δ[DIC] as well as Δ[δ13CDIC] mean that Ta and HRT affected the magnitude of the damming effect by altering changes in concentration of DIC and δ13CDIC in the reservoir compared to the inflowing water. In particular, daily regulated reservoirs with short retention times acted more like river corridors and had a smaller effect on carbon dynamics, so modulating retention time might be used reduce the effect of dams on the riverine ecosystem.