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The impact of the acidic polysaccharides pool on CO2 concentrations and fluxes: A case study on urban waters of China
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Urban rivers are significant hotspots of CO2 emissions into the atmosphere, playing important roles in global carbon emission inventories. However, little is known about the effect of ecological restoration on CO2 dynamics in severely polluted urban rivers, and this strongly hindering our understanding of the positive effects of human activities on CO2 emissions in urbanized aquatic ecosystems. We measured CO2 partial pressure (pCO2) and fluxes in nine severely polluted urban rivers with varying degrees of ecological restoration, and assessed the relationships with water physicochemical parameters, pollution levels and basin environmental investment. Our results indicate that urban rivers with high pollution loadings were indeed hotspots of CO2 evasion. However, fully restored rivers had generally lower pCO2 and CO2 emissions than partially restored rivers and were observably lower than unrestored rivers, suggesting that watershed eco‐restoration could effectively reduce CO2 evasion. In particular, environmental investment per unit basin area had a significant negative correlation with CO2 evasion and could explain 51%–85% and 51% of total variability in pCO2 and CO2 fluxes among nine urban rivers respectively, emphasizing the potential benefits of carbon emission regulation resulting from positive environmental management. Nutrient removal and sewage interception during watershed eco‐restoration were key processes reducing pCO2 and CO2 fluxes in polluted urban rivers. pH alteration kept close correlations with pCO2 and acted as a sensitive regulator of CO2 evasion in the nine rivers. We highlight the importance of considering the coupling effects of pollution and restoration on the spatiotemporal variability of CO2 evasion and the uncertainty of related monitoring methods in urban watersheds.
Plain Language Summary
Despite occupying only 1% of the Earth's surface, inland waters (rivers, lakes, and reservoirs) play a critical role in global carbon (C) cycling by linking two of the Earth's largest C pools, terrestrial and marine ecosystems, as well as by exchanging CO2 with the atmosphere. Inland waters emit and bury C before it reaches the oceans, with important implications for the global C budget. Although global estimates of lateral C fluxes have been made previously, much uncertainty exists in their magnitudes, spatiotemporal patterns, and underlying controls (anthropogenic vs. natural processes). By improving a coupled terrestrial–aquatic model, we assess how climate, land use, atmospheric CO2, and nitrogen enrichment affected global CO2 evasion and riverine C export along the terrestrial–aquatic continuum since the 1800s. We estimate a 25% increase in terrestrial C loading since the 1800s, of which 59% was emitted to the atmosphere and 23% was exported to the ocean. The increased riverine C exports were primarily due to increasing atmospheric CO2 level and nitrogen inputs; additionally, climate and land conversion dominated interannual and decadal variations in CO2 evasion. Our findings indicate that anthropogenic‐induced climate change and multiple environmental stresses since the preindustrial era have resulted in significant increases in terrestrial C export to oceans and CO2 evasion. Global inland water recycles and exports nearly half of the net land C sink into the atmosphere and ocean, underscoring the importance of inland waters for closing the global carbon budget.
Many recent studies have revealed that polluted rivers are emerging as potent emitters of greenhouse gases (GHGs). However, the scientific understanding of GHG emissions is still limited. The present review aims to provide a systematic analysis of GHG emissions from polluted rivers in various eco-regions. The analysis highlighted that high nutrient loadings such as total nitrogen (TN), organic carbon (OC), total phosphorous (TP), and the anoxic condition of rivers are the major influencing factors for CO2, N2O, and CH4 emissions. Moreover, a strong positive correlation of trophic level index (TLI) with CH4 and CO2 fluxes was observed. The state-of-art suggested
that based on land use and water quality, GHG fluxes from different rivers vary considerably. For instance, the reported N2O fluxes ranged between 0.007 and 3.721 mmol m-2d-1 for urban-impacted rivers and 0.009–0.049 mmol m-2d-1 for agricultural rivers. The CH4 fluxes ranged between 0.001 and 458 mmol m-2d-1, 0.05–1.02 mmol m-2d-1, and 1.40–14.30 mmol m-2d-1 for urban impacted, agricultural and natural rivers, respectively. Further reported CO2 fluxes range between 19.94 and 895 mmol m-2d-1 for urban-impacted rivers. Overall the in-depth analysis of the existing state of the art revealed that water pollution (i.e., eutrophication) enhances the GHG emissions from rivers; therefore, incorporating effective restoration measures can mitigate the intensity of GHG emissions from rivers and assist in achieving sustainable development goals (SDGs).
Inland waters play a critical role in the carbon cycle by emitting significant amounts of land‐exported carbon to the atmosphere. While carbon gas emissions from individual aquatic systems have been extensively studied, how networks of connected streams and lakes regulate integrated fluxes of organic and inorganic forms remain poorly understood. Here, we develop a process‐based model to simulate the fate of terrestrial dissolved organic carbon (DOC) and carbon dioxide (CO2) in artificial inland water networks with variable topology, hydrology, and DOC reactivity. While the role of lakes is highly dependent on DOC reactivity, we find that the mineralization of terrestrial DOC is more efficient in lake‐rich networks. Regardless of typology and hydrology, terrestrial CO2 is emitted almost entirely within the network boundary. Consequently, the proportion of exported terrestrial carbon emitted from inland water networks increases with the CO2 versus DOC export ratio. Overall, our results suggest that CO2 emissions from inland waters are governed by interactions between the relative amount and reactivity of terrestrial DOC and CO2 inputs and the network configuration of recipient lakes and streams.
Transparent exopolymer particles (TEP) are essential for the carbon cycle in aquatic environments. However, the distribution of TEP, its precursor, and the contribution to the organic carbon pool in the large freshwater lake remain inadequately understood. Here, we focused on the spatial distribution of TEP and dissolved acidic polysaccharides (dAPS) in four typical seasons in Lake Taihu, the third-largest lake in China. TEP concentrations in Lake Taihu ranged from 0.05 to 5.19 mg Xeq/L, with a mean value of 1.31 ± 1.08 mg Xeq/L. The concentration of dAPS in the lake averaged 3.7 ± 2.19 mg Xeq/L (range, 0.19–13.12 mg Xeq/L). Higher content of TEP and dAPS was found in summer, and their distributions appeared to be influenced by the cyanobacterial blooms, as they showed significant correlations with chlorophyll-a content in Lake Taihu in summer. In addition, TEP accounted for an average of 24.3% of particulate organic carbon, and dAPS represented 25.9% of dissolved organic carbon. These results provide valuable insights into the contribution of TEP to organic carbon pools in inland waters and highlight the role of TEP in the carbon cycling of large freshwater lakes.
In the modern era, urban freshwater bodies play a significant role in global carbon (C) budgeting, therefore, impacting climate change under rising global mean temperature. However, the trend, magnitude, and drivers of Greenhouse gas (GHG) emissions in these urban freshwater bodies of China remain uncertain. Present study investigated temporal changes in GHG emissions in urban water bodies, including artificial lakes, reservoirs, aquaculture ponds, and rivers, within a year in Nanjing city in the areas of the Yangtze River delta of China. In addition, meteorological and hydrochemical parameters were measured to elucidate the key drivers of GHG emissions. The results showed that the average annual flux of carbon dioxide (CO2) and methane (CH4) in aquaculture ponds was estimated to be 1355.6 mg CO2 m‐2 day‐1 and 116.6 mg CH4 m‐2 day‐1, followed by that of artificial lakes were 1172.2 mg CO2 m‐2 day‐1 and 44.1 mg CH4 m‐2 day‐1, rivers reached 775.9 mg CO2 m‐2 day‐1 and 23.9 mg CH4 m‐2 day‐1 and reservoirs were 170.1 mg CO2 m‐2 day‐1 and 7.2 mg CH4 m‐2 day‐1, respectively. The results further suggest that although artificial lakes and aquaculture ponds occupied only 23% of the cumulative area under lakes and ponds in the Yangtze River delta, contribute approximately 43%, about 27.5 Gg C, of total fluxes. Furthermore, high concentrations of dissolved organic carbon (DOC) and low dissolved oxygen (DO) coincided with the high GHG emissions. The study suggests that DO, DOC, temperature, and wind speed are the key factors impacting the potential of GHG emissions in urban freshwater ecosystems. Strategic mitigation measures in the vicinity of the urban freshwater bodies could efficiently reduce carbon emissions in the future.
Inland waters are important global sources, and occasional sinks, of CO2, CH4, and N2O to the atmosphere, but relatively little is known about the contribution of GHGs of constructed waterbodies, particularly small sites in agricultural regions that receive large amounts of nutrients (carbon, nitrogen, phosphorus). Here, we quantify the magnitude and controls of diffusive CO2, CH4, and N2O fluxes from 20 agricultural reservoirs on seasonal and diel timescales. All gases exhibited consistent seasonal trends, with CO2 concentrations highest in spring and fall and lowest in mid-summer, CH4 highest in mid-summer, and N2O elevated in spring following ice-off. No discernible diel trends were observed for GHG content. Analyses of GHG covariance with potential regulatory factors were conducted using generalized additive models (GAMs) that revealed CO2 concentrations were affected primarily by factors related to benthic respiration, including dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), dissolved organic carbon (DOC), stratification strength, and water source (as δ¹⁸Owater). In contrast, variation in CH4 content was correlated positively with factors that favoured methanogenesis, and so varied inversely with DO, soluble reactive phosphorus (SRP), and conductivity (a proxy for sulfate content), and positively with DIN, DOC, and temperature. Finally, N2O concentrations were driven mainly by variation in reservoir mixing (as buoyancy frequency), and were correlated positively with DO, SRP, and DIN levels and negatively with pH and stratification strength. Estimates of mean CO2-eq flux during the open-water period ranged from 5,520 mmol m⁻² year¹ (using GAM-predictions) to 10,445 mmol m⁻² year⁻¹ (using interpolations of seasonal data) reflecting how extreme values were extrapolated, with true annual flux rates likely falling between these two estimates.
In the modern era, due to urbanization, industrialization, and anthropogenic activities in the catchment, greenhouse gas (GHG; CO2, CH4, and N2O) emissions from freshwater ecosystems received scientific attention because of global warming and future climate impacts. A developing country like India contributes a huge share (4% of global) of GHGs from its freshwater ecosystems (e.g., rivers, lakes, reservoirs) to the atmosphere. This is the first comprehensive review dealing with the GHG emissions from Indian freshwater bodies. Literature reveals that the majority of GHG from India is emitted from its inland water with 19% of CH4 flux and 56% of CO2 flux. A large part of India’s Gross domestic product (GDP) is manipulated by its rivers. As a matter of fact, 117.8 Tg CO2 yr-1 of CO2 is released from its major riverine waters. The potential of GHG emission from Hydropower reservoirs varies between 11-52.9% (mainly CH4 and CO2) because of spatio-temporal variability in the GHG emissions. A significant contribution was also reported from urban lakes, wetlands and other Inland waters. Being a subtropical country, India is one of the GHG hotspots around the world having the highest ratio (GHG: GDP) of 1301.79. Although, a large portion of India’s freshwater has not been considered yet and needs to be accounted for précised regional carbon budgets. Therefore, in this review, GHG emissions from India’s freshwater bodies, drivers behind GHG emissions (e.g., pH, mean depth, dissolved oxygen, and nutrients), and long-term climatic risks are thoroughly reviewed. Besides, research gaps, future directions as well as mitigation measures are being suggested to provide useful insight into the carbon dynamics (sink/source) and control of GHG emissions.
Reservoirs are essential for human populations, but their global carbon footprint is substantial (0.73–2.41 PgCO2-equivalent yr−1). Yet the temporal evolution of reservoir carbon emissions and their contribution to anthropogenic radiative forcing remains unresolved. Here we quantify the long-term historical and future evolution (1900–2060) of cumulative global reservoir area, carbon dioxide and methane emissions and the resulting radiative forcing. We show that global reservoir carbon emissions peaked in 1987 (4.4 TmolC yr−1) and have been declining since, due largely to decreasing carbon dioxide emissions as reservoirs age. However, reservoir-induced radiative forcing continues to rise due to ongoing increases in reservoir methane emissions, which accounted for 5.2% of global anthropogenic methane emissions in 2020. We estimate that, in the future, methane ebullition and degassing flux will make up >75% of the reservoir-induced radiative forcing, making these flux pathways key targets for improved understanding and mitigation. Reservoir-induced radiative forcing is increasing globally due to rising methane emissions outweighing declining carbon dioxide emissions, according to modelling based on reservoir surface area observations.
Transparent exopolymer particles (TEPs) have drawn extensive attention in recent decades due to their crucial role in the biogeochemical and ecological processes of the ocean. However, TEP distribution and fluxes are relatively less addressed in the shelf-seas, where its variability can be affected by not only biology but also complex physical dynamics. Here, we present a comprehensive study of TEP from the coast to the basin (12 sampling sites) of the northern South China Sea (NSCS). We found a large TEP variability from 0.6 to 78.6 μg Xeq. L −1 with higher levels in the coastal waters than the offshore epipelagic waters and the deep waters. In addition, the spatial distribution of TEP was significantly correlated to the cross-shelf change of temperature, salinity, and chlorophyll a , revealing the complex physical-biogeochemical controls on TEP variability. We found the TEP dynamics nearshore largely influenced by the sedimentation and transportation of TEP-rich aggregates from the river plume. The contribution of TEP to particulate organic carbon (POC) increased gradually when approaching the shore from the sea, suggesting an elevated role of TEP in the coastal carbon cycle. Finally, a good correlation of particle-attached bacteria (PAB) with TEP but not POC revealed a preferential utilization of TEP by PAB. Thus, TEP may play an essential role in the recycling of carbon and nitrogen in the shelf-sea. These findings are crucial for understanding of the TEP dynamics under a changing environment and the associated impacts on the oceanic carbon cycle.
The pollution of water bodies by nutrients and heavy metals can lead to a loss of biodiversity, environmental degradation, and harm to human health. During the two-month monitoring period (e.g., December 2019 to January 2020), variables such as trace metals (e.g., Cu, Zn, As, and Cr), nutrients (e.g., NH4+-N, TN, and TP), water temperature, pH value, dissolved oxygen (DO), chemical oxygen demand (COD) and five-day biochemical oxygen demand (BOD5) were measured at 102 monitoring points in the main stream and tributaries of the Ganjiang River in the Poyang Lake Basin. A variety of multivariate statistical techniques, including cluster analysis (CA), principal component analysis (PCA), and correlation analysis, were used to conduct risk assessments and source analyses of the nutrient elements and heavy metals in the Ganjiang River system. The results show that although the Ganjiang River Basin is polluted by human activities, its water chemistry characteristics and trace metal and nutrient elements concentrations were better than the national standards. Through principal component analysis, the water pollution sources could be divided into urban sewage, agricultural activities, industrial activities, and the sources of industrial activities and transportation activities. The comprehensive risks of noncarcinogens (Hc) and comprehensive risks of carcinogens (Rc) for adults and children due to drinking water indicated that the risk from drinking water for the children in the basin was greater than that for adults, and that the Hc for adults and children was acceptable. However, the Rc for adults and children was slightly higher than the acceptable values. This study provides a reference for the fine control of the environmental water pollution sources in the Ganjiang river basin and health risk assessments in the basin, which are of great significance for improving the environmental water quality standards in the river basin and for reducing the risk of carcinogenesis.
Fluxes of carbon dioxide (CO2) and methane (CH4) in shallow lakes are strongly affected by dominant primary producers which mostly has been studied in temperate and boreal regions. We compared summer CO2 and CH4 fluxes (diffusion and ebullition) in littoral and pelagic zones of three subtropical shallow lakes with contrasting regimes: clear-vegetated, phytoplankton-turbid, and sediment-turbid, and assessed fluxes in different seasons in the clear-vegetated system. Significant differences among the lakes occurred only for CH4 fluxes. In the sediment-turbid lake we found undersaturated CH4 concentrations were below atmospheric equilibrium, implying CH4 uptake (< 0 mg m−2 day−1), likely due to low availability of organic matter. Differences between zones occurred in the clear-vegetated and phytoplankton-turbid lakes, with higher total CH4 emissions in the littoral than in the pelagic zones (mean: 4342 ± 895 and 983 ± 801 mg m−2 day−1, respectively). CO2 uptake (< < 0 mg m−2 day−1) occurred in the littoral of the phytoplankton-turbid lake (in summer), and in the pelagic of the clear-vegetated lake even in winter, likely associated with submerged macrophytes dominance. Our work highlights the key role of different primary producers regulating carbon fluxes in shallow lakes and points out that, also in the subtropics, submerged macrophyte dominance may decrease carbon emissions to the atmosphere.
Urban lakes in developing economies face tremendous anthropogenic pressure which tends to modify their role in the regional carbon cycle. This makes it imperative to quantify
their carbon budget and identify the underlying factors and processes. Present study aims
to understand the seasonal carbon dioxide (CO2) dynamics of an urban lake in a semiarid
subtropical region and identify major controls operating on it. Systematic sampling of Bhalswa lake waters was undertaken in winter and summer of 2017–2018. The hydrochemical data generated were used to determine partial pressure and evasion flux of CO2 using pH and total alkalinity couple. The lake waters show CO2 supersaturation with respect to atmospheric equilibrium and act as a source of CO2 to the atmosphere in both seasons. The average partial pressure of carbon dioxide (pCO2) and CO2 evasion flux observed is 1033.73 ± 229.07 μatm and 6.33 ± 2.23 mmol m−2 d−1, and 1034.99 ± 187.37 μatm and 11.65 ± 3.42 mmol m−2 d−1 during winter and summer, respectively. Neither pCO2 nor CO2 evasion flux shows any significant seasonal difference. For yearly dynamics, dissolved organic carbon, dissolved inorganic carbon and dissolved oxygen act as strong controls on lake water pCO2. While for individual seasons, pH and water temperature act as significant controls. Among various pollution sources, untreated sewage and dairy waste, seepage of polluted groundwater and atmospheric dust impact the lake’s carbon dynamics. The present study will help better understand the role of freshwater wetlands of ever-expanding urban areas in the regional carbon cycle of developing economies.
Rapid urbanization has been reported to affect carbon biogeochemical cycle of waterways, contributing to even higher carbon dioxide (CO2) outgassing from rivers to the atmosphere. However, knowledge on the magnitude and extent of the urbanization influence on riverine CO2 dynamics is still limited. In this study, we investigated partial pressure of CO2 (pCO2) and CO2 degassing rate in the surface water of three rivers that drain land areas with varied urban coverages. Field sampling and measurements were conducted in the Taohua, Nan and Puli Rivers in China’s mountainous Three Gorges Reservoir (TGR) area during winter 2018 and summer 2019 to determine the effect of urbanization intensity on riverine CO2 degassing. We found that pCO2 level was significantly higher in the river with increased proportion of urban land. Both pCO2 level and CO2 flux rate of the Taohua River (3872 μatm and 574 mmol m⁻² d⁻¹) with the highest urban land coverage were significantly higher than those of the Nan River (1737 μatm and 218 mmol m⁻² d⁻¹) and Puli River (1218 μatm and 130 mmol m⁻² d⁻¹) that drain less urbanized land areas. No significant seasonal difference in pCO2 was found in these subtropical rivers (2402 ± 1421 vs 2112 ± 1254 μatm in the summer and the winter). Overall, pCO2 was positively correlated with the concentration of chlorophyll-a (Chl-a), nutrients (i.e., TDN and TDP), dissolved organic carbon (DOC) and colony-forming units (CFU), and was negatively correlated with pH and dissolved oxygen (DO). We found that pH and DOC loading were the better parameters for predicting pCO2 in the river draining more urbanized land area, and pH and Chl-a were the better parameters for predicting pCO2 in the river draining less urbanized area. These findings highlight the role that urbanization plays in increasing riverine pCO2 through increasing nutrient and DOC inputs from the drainage basin under urban development.
Industrial activities, dam construction, and mining are three human activities important for societal and economic development. However, the effects of these activities on phytoplankton communities have been less quantitatively assessed than those on other groups, such as macroinvertebrates, fish, and periphytic algae. In the present study, we selected the Ganjiang River basin, a tributary of the Yangtze River as the representative area to develop a feasible phytoplankton index of biotic integrity (Phyto-IBI) to evaluate the effects of industrial activities , dam construction, and mining on the biotic integrity of riverine ecosystems. The results showed that the three activities greatly altered the abundance and composition of phytoplankton, with a reduction in phyto-plankton species quantity and diversity and an increase in abundance. The health status of the Ganjiang River was fair, and the health statuses of industrial areas, dam areas, mining areas, and reference points were poor, poor, fair, and good, respectively. The three activities damaged the biotic integrity of the aquatic system. Moreover, compared to industrial activities and mining, dam construction is more harmful to aquatic systems in the Ganjiang River. The locally weighted regression scatter plot smoother (LOWESS) method showed that an ammonium nitrogen (NH 3-N) concentration of 0.65 mg L −1 is the environmental protection threshold for planktonic biotic integrity in the Ganjiang River. This study not only quantitatively assesses phytoplankton responses to industrial activities, dam construction, and mining but also provides guidance regarding the ecological monitoring, assessment and protection of riverine ecosystems.
Transparent exopolymer particles (TEP) are ubiquitous in aquatic ecosystems and contribute, for example, to sedimentation of organic matter in oceans and freshwaters. Earlier studies indicate that the formation of TEP is related to the in situ activity of phytoplankton or bacteria. However, terrestrial sources of TEP and TEP precursors are usually not considered. We investigated TEP concentration and its driving factors in boreal freshwaters, hypothesizing that TEP and TEP precursors can enter freshwaters via terrestrial inputs. In a field survey, we measured TEP concentrations and other environmental factors across 30 aquatic ecosystems in Sweden. In a mesocosm experiment, we further investigated TEP dynamics over time after manipulating terrestrial organic matter input and light conditions. The TEP concentrations in boreal freshwaters ranged from 83 to 4940 μg Gum Xanthan equivalent L−1, which is comparable to other studies in freshwaters. The carbon fraction in TEP in the sampled boreal freshwaters is much higher than the phytoplanktonic carbon, in contrast to previous studies in northern temperate and Mediterranean regions. Boreal TEP concentrations were mostly related to particulate organic carbon, dissolved organic carbon, and optical indices of terrestrial influence but less influenced by bacterial abundance, bacterial production, and chlorophyll a. Hence, our results do not support a major role of the phytoplankton community or aquatic bacteria on TEP concentrations and dynamics. This suggests a strong external control of TEP concentrations in boreal freshwaters, which can in turn affect particle dynamics and sedimentation in the recipient aquatic ecosystem.
Evasion of carbon dioxide (CO2) from lakes is a significant component in the continental carbon balance, but most current CO2 evasion estimates ignore daily CO2 fluctuations. To test the hypothesis that partial pressure of CO2 (pCO2) and CO2 evasion vary throughout a day due to biological processes driven by solar radiation, we conducted in-situ pCO2 and ambient water measurements over eleven 10-hour periods in a subtropical, eutrophic shallow lake from November 2017 to May 2018. In-situ measurements were performed at 7:00, 10:00, 14:00, and 17:00 Central Standard Time of the United States (CST), and CO2 evasion rates were estimated based on the field pCO2 records. Strong daily declining trends of pCO2 and CO2 flux were found throughout the seasons except for one winter day with unusually low temperatures. At 7:00, 10:00, 14:00, and 17:00 CST of a day, average pCO2 were 1131, 839, 345 and 205 µatm, respectively, while average CO2 fluxes were 80, 67, -10, and -34 mmol m2 h-1. Significant differences were found in average pCO2 between any two measured time points in a day, while significant reductions in CO2 flux were observed between 10:00 and 14:00 CST and between 14:00 and 17:00 CST. pCO2 and CO2 flux dynamics were most likely driven by the air-water exchanges during nighttime hours and mainly driven by aquatic metabolism in the daytime. These findings suggest possible large uncertainties in the estimation of carbon emitted from trophic lakes, highlighting the need for further research on diurnal pCO2 fluctuation from different aquatic ecosystems to improve CO2 evasion estimation. [The article can be read online at https://authors.elsevier.com/a/1YSij52cuNBxt]
Streams are major sources of carbon dioxide (CO 2) and methane (CH 4) to the atmosphere, but current large-scale estimates are associated with high uncertainties because knowledge concerning the spatiotemporal control on stream emissions is limited. One of the largest uncertainties derives from the choice of gas transfer velocity (k 600), which describes the physical efficiency of gas exchange across the water-atmosphere interface. This study therefore explored the variability in k 600 and subsequent CO 2 and CH 4 emission rates within and across streams of different stream order (SO). We conducted, for the first time in streams, direct turbulence measurements using an acoustic Doppler velocimeter (ADV) to determine the spatial variability in k 600 across a variety of scales with a consistent methodology. The results show high spatial variability in k 600 and corresponding CO 2 and CH 4 emissions at small spatial scales, both within stream reaches and across SO, especially during high discharge. The k 600 was positively related to current velocity and Reynolds number. By contrast, no clear relationship was found between k 600 and specific stream characteristics such as width and depth, which are parameters often used in empirical models of k 600. Improved understanding of the small-scale variability in the physical properties along streams, especially during high discharge, is therefore an important step to reduce the uncertainty in existing gas transfer models and emissions for stream systems. The ADV method was a useful tool for revealing spatial variability in this work, but it needs further development. We recommend that future studies conduct measurements over shorter time periods (e.g., 10-15 min instead of 40 min) and at more sites across the reach of interest, and thereby derive more reliable mean-reach k 600 as well as more information about controls on the spatial variability in k 600. ARTICLE HISTORY
Ocean CO2 uptake accounts for 20–40% of the post-industrial sink for anthropogenic CO2. The uptake rate is the product of the CO2 interfacial concentration gradient and its transfer velocity, which is controlled by spatial and temporal variability in near-surface turbulence. This variability complicates CO2 flux estimates and in large part reflects variable sea surface microlayer enrichments in biologically derived surfactants that cause turbulence suppression. Here we present a direct estimate of this surfactant effect on CO2 exchange at the ocean basin scale, with derived relationships between its transfer velocity determined experimentally and total surfactant activity for Atlantic Ocean surface seawaters. We found up to 32% reduction in CO2 exchange relative to surfactant-free water. Applying a relationship between sea surface temperature and total surfactant activity to our results gives monthly estimates of spatially resolved ‘surfactant suppression’ of CO2 exchange. Large areas of reduced CO2 uptake resulted, notably around 20° N, and the magnitude of the Atlantic Ocean CO2 sink for 2014 was decreased by 9%. This direct quantification of the surfactant effect on CO2 uptake at the ocean basin scale offers a framework for further refining estimates of air–sea gas exchange up to the global scale.
CO2 emissions from inland surface waters to the atmosphere are almost as large as the net carbon transfer from the atmosphere to Earth's land surface. This large flux is supported by dissolved organic matter (DOM) from land and its complete oxidation to CO2 in freshwaters. A critical nexus in the global carbon cycle is the fate of DOM, either complete or partial oxidation. Interactions between sunlight and microbes control DOM degradation, but the relative importance of photodegradation vs. degradation by microbes is poorly known. The knowledge gaps required to advance understanding of key interactions between photochemistry and biology influencing DOM degradation include: (1) the efficiencies and products of DOM photodegradation, (2) how do photo-products control microbial metabolism of photo-altered DOM and on what time scales, and (3) how do water and DOM residence times and light exposure interact to determine the fate of DOM moving across the landscape to oceans?
Turbulent mixing is enhanced in shallow lakes. As a result, exchanges across the air-water and sediment-water interfaces are increased, causing these systems to be large sources of greenhouse gases. This study investigated the effects of turbulence on carbon dioxide (CO2) and methane (CH4) emissions in shallow lakes using simulated mesocosm experiments. Results demonstrated that turbulence increased CO2 emissions, while simultaneously decreasing CH4 emissions by altering microbial processes. Under turbulent conditions, a greater fraction of organic carbon was recycled as CO2 instead of CH4, potentially reducing the net global warming effect because of the lower global warming potential of CO2 relative to CH4. The CH4/CO2 flux ratio was approximately 0.006 under turbulent conditions, but reached 0.078 in the control. The real-time quantitative PCR analysis indicated that methanogen abundance decreased and methanotroph abundance increased under turbulent conditions, inhibiting CH4 production and favoring the oxidation of CH4 to CO2. These findings suggest that turbulence may play an important role in the global carbon cycle by limiting CH4 emissions, thereby reducing the net global warming effect of shallow lakes.
Most water sources are full of microscopic transparent exopolymer particles (TEP), which are currently regarded as a major initiator of biofilm formation. This study developed and applied an auto-imaging FlowCAM-based method for online observation and quantification of TEP in freshwater. Samples from reservoirs in Taiwan with a wide range of water quality were directly used to develop this methodology. Factors that potentially affect the measurement were tested. The results showed that characteristics of the particles measured instantaneously after staining samples with Alcian blue differed significantly from those measured at steady states, as a result of particle aggregation. Compared to traditional microscopic methods, this proposed method provides a simple, rapid, and less labor-intensive analysis with particle morphological conservation and a large number of particle attributes. By overcoming the limitations from the former, this technique would offer routine monitoring of these transparent particles from various freshwater sources and feed water in membrane filtration, hence facilitating the use of TEP as a critical parameter for biofouling investigation in water treatment. Application of the method for Taiwan reservoirs showed a wide variety of morphological forms of TEP and its abundance, up to 25,000 ppm.
Accurately quantifying the carbon dioxide (CO2) emissions from lakes, especially in urban areas, remains challenging due to constrained temporal resolution in field monitoring. Current lake CO2 flux estimates primarily rely on daylight measurements, yet nighttime emissions is normally overlooked. In this study, a non-dispersive infrared CO2 sensor was applied to measure dissolved CO2 concentrations over a 24-hour period in a largest urban lake (Tangxun Lake) in Wuhan City, Central China, yielding extensive data on diel variability of CO2 concentrations and emissions. We showed the practicality and efficiency of the sensor for real-time continuous measurements in lakes. Our findings revealed distinct diurnal variations in CO2 concentrations (Day: 38.58 ± 23.8 μmol L-1; Night: 42.01 ± 20.2 μmol L-1) and fluxes (Day: 7.68 ± 10.34 mmol m-2 d-1; Night: 9.68 ± 9.19 mmol m-2 d-1) in the Tangxun Lake. The balance of photosynthesis and respiration is of utmost importance in modulating diurnal CO2 dynamics and can be influenced by nutrient loadings and temperature. A diel variability correction factor of 1.14 was proposed, suggesting that daytime-only measurements could underestimate CO2 emissions in urban lakes. Our data suggested that samplings between 11:00 and 12:00 could better represent the average diel CO2 fluxes. This study offered valuable insights on the diel variability of CO2 fluxes, emphasizing the importance of in situ continuous measurements to accurately quantify CO2 emissions, facilitating selections of sampling strategies and formulation of management strategies for urban 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.
Constituents and functionality of urban inland waters are significantly perturbed by municipal sewage inputs and tailwater discharge from wastewater treatment plants. However, large knowledge gaps persist in understanding greenhouse gas dynamics in urban inland waters due to a lack of in situ measurements. Herein, via a 3-year field campaign (2018-2020), we report river and lake CO2 emission and related aquatic factors regulating the emission in the municipality of Beijing. Mean pCO2 (546 ± 481 μatm) in the two urban lakes was lower than global non-tropical freshwater lakes and CO2 flux in 47% of the lake observations was negative. Though average pCO2 in urban rivers (3124 ± 3846 μatm) was among the higher range of global rivers (1300-4300 μatm), average CO2 flux was much lower than the global river average (99.7 ± 147.5 versus 358.4 mmol m-2 d-1). The high pCO2 cannot release to the atmosphere due to the low gas exchange rate in urban rivers (average k600 of 1.3 ± 1.3 m d-1), resulting in low CO2 flux in urban rivers. Additionally, eutrophication promotes photosynthetic uptake and aquatic organic substrate production, leading to no clear relationships observed between pCO2 and phytoplankton photosynthesis or dissolved organic carbon. In consistence with the findings, CO2 emission accounted for only 32% of the total greenhouse gas (GHG) emission equivalence (CO2, CH4 and N2O) in Beijing waters, in contrast to a major role of anthropogenic CO2 to anthropogenic GHG in the atmosphere in terms of radiative forcing (66%). These results pointed to unique GHG emission profiles and the need for a special account of urban inland waters in terms of aquatic GHG emissions.
River networks represent the largest biogeochemical nexus between the continents, ocean and atmosphere. Our current understanding of the role of rivers in the global carbon cycle remains limited, which makes it difficult to predict how global change may alter the timing and spatial distribution of riverine carbon sequestration and greenhouse gas emissions. Here we review the state of river ecosystem metabolism research and synthesize the current best available estimates of river ecosystem metabolism. We quantify the organic and inorganic carbon flux from land to global rivers and show that their net ecosystem production and carbon dioxide emissions shift the organic to inorganic carbon balance en route from land to the coastal ocean. Furthermore, we discuss how global change may affect river ecosystem metabolism and related carbon fluxes and identify research directions that can help to develop better predictions of the effects of global change on riverine ecosystem processes. We argue that a global river observing system will play a key role in understanding river networks and their future evolution in the context of the global carbon budget. A review of current river ecosystem metabolism research quantifies the organic and inorganic carbon flux from land to global rivers and demonstrates that the carbon balance can be influenced by a changing world.
Reservoir building is one of mankind's most significant infrastructure undertakings, resulting in the retention of massive volumes of water on continents. The reservoirs slow down the flow of terrestrial carbon through inland water bodies by providing a longer hydraulic retention time, facilitating a hotspot of biogeochemical reactions related to retention and transport of carbon. The major sources of carbon to the reservoirs include terrestrial carbon imports from inflowing rivers, algal production, and plant regeneration along shorelines during drawdown periods. The unique hydrological regime of reservoirs plays a vital role in regulating the deposition and decomposition rate of carbon. Reservoirs are carbon cycling hotspots and critical participants in the global carbon cycle. Recent studies have discovered that the rate of carbon deposition in inland reservoirs is much higher than in the oceans. However, studies also show that oxic and anoxic C decomposition conditions produce substantial fluxes of greenhouse gases (GHGs) at the surface waters and bottom sediments. The estimated GHGs emission from reservoir water surfaces account for ~1.3% of all anthropogenic GHG emissions, with the majority (79%) of this forcing by CH 4 . The carbon dynamics in reservoirs is highly affected by the influx of nutrients, biotic, abiotic, and climatic factors; thus, carbon stocks in reservoir are highly sensitive in response to future climate change. This chapter aims to explore the role of freshwater reservoirs in the global carbon cycle. We will also discuss the processes involved in accumulation, transformation, and transportation of the C in reservoir system.
Despite the importance of small and medium size rivers of Siberian boreal zone in greenhouse gases (GHG) emission, major knowledge gaps exist regarding its temporal variability and controlling mechanisms. Here we sampled 11 pristine rivers of the southern taiga biome (western Siberia Lowland, WSL), ranging in watershed area from 0.8 to 119,000 km², to reveal temporal pattern and examine main environmental controllers of GHG emissions from the river water surfaces. Floating chamber measurements demonstrated that CO2 emissions from water surface decreased by 2 to 4-folds from spring to summer and autumn, were independent of the size of the watershed and stream order and did not exhibit sizable (>30 %, regardless of season) variations between day and night. The CH4 concentrations and fluxes increased in the order “spring ≤ summer < autumn” and ranged from 1 to 15 μmol L⁻¹ and 5 to 100 mmol m⁻² d⁻¹, respectively. The CO2 concentrations and fluxes (range from 100 to 400 μmol L⁻¹ and 1 to 4 g C m⁻² d⁻¹, respectively) were positively correlated with dissolved and particulate organic carbon, total nitrogen and bacterial number of the water column. The CH4 concentrations and fluxes were positively correlated with phosphate and ammonia concentrations. Of the landscape parameters, positive correlations were detected between riparian vegetation biomass and CO2 and CH4 concentrations. Over the six-month open-water period, areal emissions of C (>99.5 % CO2; <0.5 % CH4) from the watersheds of 11 rivers were equal to the total downstream C export in this part of the WSL. Based on correlations between environmental controllers (watershed land cover and the water column parameters), we hypothesize that the fluxes are largely driven by riverine mineralization of terrestrial dissolved and particulate OC, coupled with respiration at the river bottom and riparian sediments. It follows that, under climate warming scenario, most significant changes in GHG regimes of western Siberian rivers located in permafrost-free zone may occur due to changes in the riparian zone vegetation and water coverage of the floodplains.
Biodegradable dissolved organic carbon (BDOC) constitutes the most labile fraction of dissolved organic matter (DOM), which also functions as a source of CO2 emissions from inland waters. However, no systematic review is available on DOM indicators of BDOC and CO2 production potential. Optical and molecular indices can be used to track small changes in DOM composition during biodegradation. In this review, we identified four different methods for measuring BDOC together with their strengths and limitations. In addition, we discuss the potential of using documented optical indices based on absorption and fluorescence spectroscopy and molecular indices based on Fourier transform ion cyclotron mass spectrometry as proxies for estimating BDOC and biodegradation-induced CO2 production based on previously reported relationships in the literature. Many absorbance- and fluorescence-based indices showed inconsistent relationships with BDOC depending on watershed characteristics, hydrology, and anthropogenic impacts. Nevertheless, several indices, including specific UV absorbance at 254 nm (SUVA254), humification index (HIX), and terrestrial humic-like fluorescent DOM (FDOM) components, tended to have negative relationships with BDOC in tropical and temperate watersheds under baseflow or drought periods. Protein-like FDOM exhibited the strongest correlation with BDOC in different systems, except during storms and flood events. Despite the limited number of studies, DOM molecular indices exhibited consistent relationships with BDOC, suggesting that the relative abundance of aliphatic formulas and the molecular lability index could act as reliable proxies. The DOM optical indices explain up to 96% and 78% variability in BDOC and CO2, respectively; nonetheless, there were limited studies on molecular indices, which explains up to 74% variability in BDOC. Based on literature survey, we recommend several sensitive indices such as SUVA254, HIX, and terrestrial humic- and protein-like FDOM, which could be useful indicators of BDOC and dissolved CO2 in inland water. Future research should incorporate a wider range of geographic regions with various land use, hydrology, and anthropogenic disturbances to develop system- or condition-specific DOM optical or molecular proxies for better prediction of BDOC and CO2 emissions.
Carbon dioxide (CO2) emissions from inland waters to the atmosphere are a pivotal component of the global carbon budget. Anthropogenic land use can influence riverine CO2 emissions, but empirical data exploring cause-effect relationships remain limited. Here, we investigated CO2 partial pressures (pCO2) and degassing in a monsoonal river (Yue River) within the Han River draining to the Yangtze in China. Almost 90% of river samples were supersaturated in CO2 with a mean ± standard deviation of 1474 ± 1614 µatm, leading to emissions of 557 - 971 mmol/m²/day from river water to the atmosphere. Annual CO2 emissions were 1.6 - 2.8 times greater than the longitudinal exports of riverine dissolved inorganic and organic carbon. pCO2 was positively correlated to anthropogenic land use (urban and farmland), and negatively correlated to forest cover. pCO2 also had significant and positive relationships with total dissolved nitrogen and total dissolved phosphorus. Stepwise multiple regression models were developed to predict pCO2. Farmland and urban land released nutrients and organic matter to the river system, driving riverine pCO2 enrichment due to enhanced respiration in these heterotrophic rivers. Overall, we show the crucial role of land use driving riverine pCO2, which should be considered in future large-scale estimates of CO2 emissions from streams. Land use change can thus modify the carbon balance of urban-river systems by enhancing river emissions, and reforestation helps carbon neutral in rivers.
Hydropower is one of the widely used renewable energy sources and contributing a huge share in overall electricity generation in developing countries like India. A few decades ago, hydropower reservoirs were found to be a clean source of renewable energy, but after the pre-industrial era (1990s), these reservoirs were emitting a significant amount of greenhouse gases (GHG). The GHG are contributing a huge share to the regional/global carbon budget. The current study represents the first regional review and focuses on: present scenarios of Indian hydropower, environmental issues, constraints in dam construction, identified key predictors (i.e., age, depth of reservoir) impacting GHG potential, GHG risk to reservoirs, and suggested mitigation measures to the way forward for the sustainable planning of hydropower development. The review reveals that most Indian hydropower reservoirs are coming under the low to medium GHG risk of reservoirs. Small hydropower projects (SHPs) are deliberated as a worthy alternative for the sustainable management of hydropower projects. There are still many issues associated with SHPs, which cannot be neglected completely. This study provides baseline information to the hydropower industries, policy-makers, environmentalists, and water managers to take corrective measures before impoundment of a reservoir at the regional and global scale.
Freshwaters are receiving growing concerns on atmospheric carbon dioxide (CO 2) and methane (CH 4) budget; however, little is known about the anthropogenic sources of CO 2 and CH 4 from river network in agricultural-dominated watersheds. Here, we chose such a typical watershed and measured surface dissolved CO 2 and CH 4 concentrations over two years (2015-2017) in Jurong Reservoir watershed for different freshwater types (river network, ponds, reservoir, and ditches), which located in Eastern China and were impacted by agriculture with high fertilizer N application. Results showed that significantly higher gas concentrations occurred in river network (CO 2 : 112 ± 36 μmol L-1 ; CH 4 : 509 ± 341 nmol L-1) with high nutrient concentrations. Dissolved CO 2 and CH 4 concentrations were supersaturated in all of the freshwater types with peak saturation ratios generally occurring in river network. Temporal variations in the gas saturations were positively correlated with water temperature. The saturations of CO 2 and CH 4 were positively correlated with each other in river network, and both of these saturations were also positively correlated with nutrient loadings, and negatively correlated with dissolved oxygen concentration. The highly agricultural river network acted as significant CO 2 and CH 4 sources with estimated emission fluxes of 409 ± 369 mmol m-2 d-1 for CO 2 and 1.6 ± 1.2 mmol m-2 d-1 for CH 4 , and made a disproportionately large, relative to the area, contribution to the total aquatic carbon emission of the watershed. Our results suggested the aquatic carbon emissions accounted for 6% of the watershed carbon budget, and fertilizer N and watersheds land use played a large role in the aquatic carbon emission.
Growing evidence shows that riverine networks surrounding urban landscapes may be hotspots of riverine greenhouse gas (GHG) emissions. This study strengthens the evidence by investigating the spatial variability of diffusive GHG (N2O, CH4, CO2) emissions from river reaches that drain from different types of landscapes (i.e., urban, agricultural, mixed, and forest landscapes), in the Chaohu Lake basin of eastern China. Our results showed that almost all the rivers were oversaturated with dissolved GHGs. Urban rivers were identified as emission hotspots, with mean fluxes of 470 μmol m-2 d-1 for N2O, 7 mmol m-2 d-1 for CH4, and 900 mmol m-2 d-1 for CO2, corresponding to ~14, seven, and two times of those from the non-urban rivers in the Chaohu Lake basin, respectively. Factors related to the high N2O and CH4 emissions in urban rivers included large nutrient supply and hypoxic environments. The factors affecting CO2 were similar in all the rivers, which were temperature-dependent with suitable environments that allowed rapid decomposition of organic matter. Overall, this study highlights that better recognition of the influence that river networks have on global warming is required—particularly when it comes to urban rivers, as urban land cover and populations will continue to expand in the future. Management measures should incorporate regional hotspots to more efficiently mitigate GHG emissions.
Lakes play an important role in the global carbon cycle; however, there are still large uncertainties in the estimation of global lake carbon emission due to the limitations in conducting field surveys at large geographic scales. Using long-term Moderate-Resolution Imaging Spectroradiometer (MODIS) imagery and field observation data in eutrophic Lake Taihu, we developed a novel approach to estimate the concentration of dissolved carbon dioxide (cCO2) in lakes. Based on the MODIS-derived chlorophyll-a concentration, lake surface temperature, diffuse attenuation coefficient of photosynthetically active radiation, and photosynthetically active radiation, a spatially explicit cCO2 model was developed using multivariate quadratic polynomial regression (coefficient of determination (R2) = 0.84, root-mean-square error (RMSE) = 11.81 μmol L−1, unbiased percent difference (UPD) = 22.46%). Monte Carlo simulations indicated that the model is stable with relatively small deviations in cCO2 estimates caused by input variables (UPD = 26.14%). MODIS data from 2003 to 2018 showed a significant declining trend (0.42 μmol L−1 yr−1, p < 0.05) in the annual mean cCO2. This was associated with a complex balance between the increasing algae biomass and decreasing external inputs of inorganic carbon, nutrients, and organic matter. The high spatiotemporal variabilities in cCO2 were attributed to river inputs and seasonal changes in temperature and algae biomass. The study shows that satellite remote sensing can play an important role in the field of inland water carbon cycling, providing timely much-needed insights into the drivers of the spatial and temporal changes in dissolved CO2 concentrations in inland waters.
Establishing ecological security patterns provides new ideas for maintaining regional ecological security. Methods for establishing these patterns have been extensively investigated in several studies, but the ecological protection effects of these patterns need further examination. Nanchang is the capital city of Jiangxi province and a typical representative of rapidly developing cities. With the proposal of an ecological environment protection plan for Nanchang metropolitan area, the coordinated development of ecology, economy and society has become the local development goal. This study used Nanchang City as an example for the establishment of an ecological security pattern through the circuit theory. The ecological sources of a 1068.56 km² location and 20 ecological corridors with a total area of 957.39 km² were identified. Three development scenarios in 2015–2040 were set up, namely, unrestricted development (UD), core area protection (CP) and ecological security pattern restriction (ESPR) scenarios. The UD scenario followed the land expansion rate from 2010 to 2015. The CP scenario used a nature reserve as a forbidden conversion area. Under the ESPR scenario, ecological security pattern was regarded as a prohibited conversion area. The CLUMondo model was used in simulating land use and evaluating the ecological protection effects of the scenarios. Through comparison, we determined that the ecological security indices under UD, CP and ESPR were 0.230, 0.242 and 0.249, respectively, from the perspective of the overall ecological security of the region. In the evaluation of the landscape characteristics of EL, under ESPR, the landscape connectivity was the best. The detailed analysis results showed that the ecological security pattern not only could protect the regional ecological security on the regional scale but also had an outstanding protection effect on the local scale. In summary, compared with the UD and CP scenarios, ecological security patterns had a better effect on regional ecological protection.
Transparent exopolymer particles (TEP) as gel-like particulate acidic polysaccharide have been commonly found in marine, surface water and wastewater. Currently, increasing interest has been devoted to TEP-associated membrane fouling in different membrane systems for water and wastewater treatment, thus this review attempts to provide a holistic view and critical analysis with regard to the definition, formation, detection and properties of TEP, which could ultimately determine its fouling potential. It appears that there is not a common consensus on the actual role of TEP in membrane fouling development due to the subjective definition and highly debatable detection method of TEP. It was clearly demonstrated in this review that the formation of TEP was largely related to cations in water and wastewater which indeed determined the cross-linking degree of precursor materials (e.g. polysaccharides) via intermolecular interactions, and subsequently the quantity of TEP formed. The binding between cations ions (e.g. monovalent, divalent and trivalent cations) and polysaccharide not only depends on the functional groups of polysaccharide, but also its spatial configuration. These in turn suggest that the formation, property and ultimate fouling potential of TEP would be closely related to the type and concentration of cations, while well explaining the controversial reports on TEP-associated fouling in the literature. In addition, the fouling mechanisms of TEP are also elucidated with details in this review, including (i) the formation of TEP-associated gel layer on membrane surface; (ii) carrying microorganisms to membrane surface via protobiofilm and (iii) trapping of deformable TEP in membrane pores. Consequently, it is apparent that TEP is an ignored determinant of membrane fouling, which has not yet been seriously addressed in the design and operation of membrane systems for water and wastewater treatment.
Inland waters emit large amounts of carbon dioxide (CO2) to the atmosphere, but emissions from urban lakes are poorly understood. This study investigated seasonal and interannual variations in the partial pressure of CO2 (pCO2) and CO2 flux from Lake Wuli, a small eutrophic urban lake in the heart of the Yangtze River Delta, China, based on a long-term (2000–2015) dataset. The results showed that the annual mean pCO2 was 1030 ± 281 μatm (mean ± standard deviation) with a mean CO2 flux of 1.1 ± 0.6 g m⁻² d⁻¹ during 2000–2015, suggesting that compared with other lakes globally, Lake Wuli was a significant source of atmospheric CO2. Substantial interannual variability was observed, and the annual pCO2 exhibited a decreasing trend due to improvements in water quality driven by environmental investment. Changes in ammonia nitrogen and total phosphorus concentrations together explained 90% of the observed interannual variability in pCO2 (R² = 0.90, p < 0.01). The lake was dominated by cyanobacterial blooms and showed nonseasonal variation in pCO2. This finding was different from those of other eutrophic lakes with seasonal variation in pCO2, mostly because the uptake of CO2 by algal-derived primary production was counterbalanced by the production of CO2 by algal-derived organic carbon decomposition. Our results suggested that anthropogenic activities strongly affect lake CO2 dynamics and that environmental investments, such as ecological restoration and reducing nutrient discharge, can significantly reduce CO2 emissions from inland lakes. This study provides valuable information on the reduction in carbon emissions from artificially controlled eutrophic lakes and an assessment of the impact of inland water on the global carbon cycle.
Gas exchange across the air–water boundary of streams and rivers is a globally large biogeochemical flux. Gas exchange depends on the solubility of the gas of interest, the gas concentrations of the air and water, and the gas exchange velocity ( k ), usually normalized to a Schmidt number of 600, referred to as k 600 . Gas exchange velocity is of intense research interest because it is problematic to estimate, is highly spatially variable, and has high prediction error. Theory dictates that molecular diffusivity and turbulence drives variation in k 600 in flowing waters. We measure k 600 via several methods from direct measures, gas tracer experiments, to modeling of diel changes in dissolved gas concentrations. Many estimates of k 600 show that surface turbulence explains variation in k 600 leading to predictive models based upon geomorphic and hydraulic variables. These variables include stream channel slope and stream flow velocity, the product of which, is proportional to the energy dissipation rate in streams and rivers. These empirical models provide understanding of the controls on k 600 , yet high residual variation in k 600 show that these simple models are insufficient for predicting individual locations. The most appropriate method to estimate gas exchange depends on the scientific question along with the characteristics of the study sites. We provide a decision tree for selecting the best method to estimate k 600 for individual river reaches to scaling to river networks.
This article is categorized under: Water and Life > Nature of Freshwater Ecosystems
Science of Water > Water Quality
Water and Life > Methods
Coastal megacities deposit significant amounts of carbon (C), nitrogen (N), and pollutants into surrounding waters. In urbanized estuaries, these inputs, including wastewater discharge and surface runoff, can affect biogeochemical cycles, microbial production, and greenhouse gas (GHG) efflux. To better understand estuarine GHG production and its connection to anthropogenic drivers, we quantified carbon dioxide (CO2) and methane (CH4) surface‐water concentrations and efflux in combination with a suite of biogeochemical parameters, including anthropogenic indicators, in the Hudson River Estuary (HRE) and adjacent waters surrounding New York, NY, over a 2‐yr period. The HRE was a source of both CO2 (33 ± 3 mmol CO2 m−2 d−1) and CH4 (177 ± 22 μmol CH4 m−2 d−1) under all measured conditions. Surface‐water salinity, oxygen saturation, fecal indicator bacteria, nitrate concentrations, and temperature best explained the variance in CO2 and CH4 concentrations in multiple regression analyses, producing robust predictive power for both GHGs. Our multifaceted data set demonstrated that CH4 and CO2 surface concentrations are explained in part by enterococci concentrations, a widely used wastewater biological indicator, explicitly linking wastewater inputs to GHG surface concentrations in the HRE. The greatest CO2 and CH4 surface‐water concentrations were found in urban tributaries and embayments, primary wastewater delivery areas throughout the HRE. Estuarine tributaries and embayments have historically received less research attention than midchannel sites, but since these shallow sites may contribute to increased GHG efflux in anthropogenically impacted estuaries, further study is warranted.
Freshwaters are important sources of greenhouse gases (GHGs) to the atmosphere that may partially offset the terrestrial carbon sink. However, current emission estimates from inland waters remain uncertain due to data paucity in key regions with a large freshwater surface area, such as China. Here, we show that the areal fluxes of GHGs (carbon dioxide, methane, and nitrous oxide) from lakes and reservoirs in China are much larger than previous estimates. Our work summarized data from 310 lakes and 153 reservoirs, and revealed diffusive emissions of 1.56 (95% confidence interval: 1.12–2.00) Tg C-CH4/y and 25.2 (20.8–29.5) Tg C-CO2/y from reservoirs and lakes. Chinese lakes and reservoirs emit 175.0 (134.7–215.3) Tg CO2 equivalent, with 73.4% of this forcing contributed by lakes. These aquatic sources are equivalent to 14.1%–22.6% of China's estimated terrestrial carbon sink. Our results suggest a disproportionally high contribution of China's reservoirs and lakes to national and global GHGs emissions, highlighting major data gaps and the need of including more artificial and natural lakes data from developing countries like China in global GHGs budgets.
Water exchange in shallow lakes alongside Poyang Lake is weak because of sluggish water flow, and thus these lakes are susceptible to the effects of human activities and nutrient oversupply. Therefore, it is meaningful to study the sources of organic matter in these lakes, which is supposed to provide theoretical basis for the control of eutrophication of the lakes. The sources of organic matter in surface sediment in shallow lakes (Bang, Sha, Dahuchi, and Zhu lakes) alongside Poyang Lake and urban lakes (Qingshan and Xiang lakes) in Nanchang City were investigated by measuring the δ¹⁵N and δ¹³C values, total organic carbon and total organic nitrogen contents, and C/N ratios of sedimented organic matter (SOM). The δ¹⁵N, δ¹³C, and C/N ratios indicated that SOM in Sha and Dahuchi lakes mostly originated in phytoplankton, SOM in Bang and Zhu lakes was supplied by phytoplankton and soil organic matter, SOM in Qingshan Lake was strongly affected by sewage organic matter, and SOM in Xiang Lake mainly came from aquatic macrophytes. The results demonstrate that stable isotopes in SOM in lakes can be used to discriminate between different sources of organic material.
The evasion of greenhouse gases (including CO2, CH4, and N2O) from streams and rivers to the atmosphere is an important process in global biogeochemical cycles, but our understanding of gas transfer in steep (>10%) streams, and under varying flows, is limited. We investigated gas transfer using combined tracer injections of SF6 and salt. We used a novel experimental design in which we compared four very steep (18.4–29.4%) and four moderately steep (3.7–7.6%) streams and conducted tests in each stream under low flow conditions and during a high-discharge event. Most dissolved gas evaded over short distances (~100 and ~200–400 m, respectively), so accurate estimates of evasion fluxes will require sampling of dissolved gases at these scales to account for local sources. We calculated CO2 gas transfer coefficients (KCO2) and found statistically significant differences between larger KCO2 values for steeper (mean 0.465 min−1) streams compared to those with shallower slopes (mean 0.109 min−1). Variations in flow had an even greater influence. KCO2 was substantially larger under high (mean 0.497 min−1) compared to low flow conditions (mean 0.077 min−1). We developed a statistical model to predict KCO2 using values of streambed slope × discharge which accounted for 94% of the variation. We show that two models using slope and velocity developed by Raymond et al. (2012) for streams and rivers with shallower slopes also provide reasonable estimates of our CO2 gas transfer velocities (kCO2; m d−1). We developed a robust field protocol which could be applied in future studies.
The abundance of Transparent Exopolymer Particles (TEP) was determined on a seasonal basis (autumn, spring and summer) along a north-south transect in the NE Aegean Sea and the vicinity of the Dardanelles Straits. Their distribution patterns were studied in respect to hydrographic conditions and water mass characteristics in the area, as well as particulate organic carbon (POC) concentrations, changes in standing stocks of chlorophyll-α and bacterial production. TEP concentrations ranged from 15.4 to 188 µg GX eq L⁻¹. Their spatial distribution patterns within the euphotic zone displayed significant seasonal variability, which appears to closely reflect the temporal variation of the water column structure, resulting from the encounter and interplay of the Black Sea and Levantine Water masses, and the associated biogeochemical processes. Minimum TEP concentrations during autumn could be likely attributed to a minor quantity of TEP and/or its dissolved precursors exuded by phytoplankton and their enhanced degradation due to their long residence time in the water column. During spring, high TEP production was mediated by actively growing phytoplankton, while during summer a positive link to the intense stratification of the water column and the enhanced bacterial growth within the Black Sea Water layer was observed. The results reported in this study highlight the fact that TEP carbon represents a significant fraction of the POC pool. Moreover, TEP production is critical in promoting particle coagulation rates, playing an important role in carbon cycling/transportation out of the euphotic zone.
Freshwater ecosystems constitute a small fraction of our planet but play a disproportionately large and critical role in the global carbon cycle.
1. Lakes word-wide are in rapid change due to direct or indirect climate impacts. In boreal areas, the increased concentrations of dissolved organic matter (DOM) are profoundly affecting light climate and productivity in multiple ways. Photochemical and microbial mineralization of DOM are major sources of CO2 in these lakes. It has been suggested that this CO2 could potentially promote primary production and offset negative impacts of increased light attenuation.
2. A synoptic survey of 76 Scandinavian lakes along gradients of DOM and total phosphorus (TP) revealed a major negative impact of DOM on primary production and algal biomass primarily related to light attenuation, while a strong positive impact of TP. The negative impact of DOM on lake productivity is partly thus partly offset by DOM-associated P,
3. Concentrations of CO2 did not relate significantly to neither production, biomass nor seston stoichiometry, and thus while elevated CO2 may promote primary production in more productive lakes, it does not in these CO2-supersaturated boreal lakes.
4. Zooplankton biomass correlated strongly with TP and total algal biomass, less so with primary production, and was negatively related to DOM, likely reflecting the autotroph responses to DOM.
Transparent Exopolymer Particles (TEP) have received considerable attention since they were first described in the ocean more than 20 years ago. This is because of their carbon-rich composition, their high concentrations in ocean’s surface waters, and especially because of their ability to promote aggregation due to their high stickiness (i.e. biological glue). As large aggregates contribute significantly to vertical carbon flux, TEP are commonly seen as a key factor that drives the downward flux of particulate organic carbon (POC). However, the density of TEP is lower than that of seawater, which causes them to remain in surface waters and even move upwards if not ballasted by other particles, which often leads to their accumulation in the sea surface microlayer. Hence we question here the generally accepted view that TEP always increase the downward flux of POC via gravitational settling. In the present reassessment of the role of TEP, we examine how the presence of a pool of non-sinking carbon-rich particulate organic matter in surface waters influences the cycling of organic carbon in the upper ocean at daily to decadal time scales. In particular, we focus on the role of TEP in the retention of organic carbon in surface waters versus downward export, and discuss the potential consequences of climate change on this process and on the efficiency of the biological carbon pump. We show that TEP sink only when ballasted with enough high-density particles to compensate their low density, and hence that their role in vertical POC export is not solely linked to their ability to promote aggregation, but also to their contribution to the buoyancy of POC. It follows that the TEP fraction of POC determines the degree of retention and remineralization of POC in surface waters versus its downward export. A high TEP concentration may temporally decouple primary production and downward export. We identify two main parameters that affect the contribution of TEP to POC cycling; TEP stickiness, and the balance between TEP production and degradation rates. Because stickiness, production and degradation of TEP vary with environmental conditions, the role of TEP in controlling the balance between retention versus export, and hence the drawdown of atmospheric CO2 by the biological carbon pump, can be highly variable, and is likely to be affected by climate change.
