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The export and transformation of riverine dissolved organic matter (DOM) at the river-sea interface are critical to understanding carbon budgets in tropical regions, yet remain underexplored in small watersheds. This study analyzed dissolved organic carbon (DOC), chromophoric (CDOM), and fluorescent DOM (FDOM) over five cruises from summer 2021 to summer 2022 in the Suixi Estuary-Zhanjiang Bay continuum, a eutrophic tropical transition zone in the northwestern South China Sea. Seasonal variations were pronounced for DOM in the continuum and two endmembers. Freshwater DOM, dominated by terrestrial organic materials, was closely tied to soil leaching and erosion, correlating with precipitation patterns. Conversely, seawater DOM, enriched in protein-like FDOM, varied with the West-Guangdong Coastal Current and its associated primary production. Linear decreases in DOM with increasing salinity indicate that conservative mixing largely shapes DOM distributions in the continuum, especially for humic-like FDOM. Non-conservative processes, including flocculation, phytoplankton production, and microbial transformation, variably alter DOM components across seasons. The Suixi River in summer contributed considerably but disproportionately to the annual DOM flux (47-59%), due to intense soil leaching processes that mobilize refractory terrigenous DOM. The deviation observed in the Suixi River's DOC-CDOM relationship from that of global rivers, coupled with a distinctive single-peak pattern between the carbon-specific absorption coefficient and water yield in global rivers, highlights the critical role of watershed nature in influencing river-exported DOM composition. The notably high DOC and CDOM yields for the Suixi River further emphasize the importance of small, tropical rivers in shaping the estuarine and coastal carbon budget. [ABSTRACT FROM AUTHOR]

Ecosystems are multifaceted and complex systems and understanding their composition is crucial for the implementation of efficient conservation and management. Conventional approaches to biodiversity surveys can have limitations in detecting the complete range of species present. In contrast, the study of environmental RNA (eRNA) offers a non-invasive and comprehensive method for monitoring and evaluating biodiversity across different ecosystems. Similar to eDNA, the examination of genetic material found in environmental samples can identify and measure many species, including ones that pose challenges to traditional methods. However, eRNA is degraded quickly and therefore shows promise in detection of living organisms closer to their actual location than eDNA methods. This method provides a comprehensive perspective on the well-being of ecosystems, facilitating the development of focused conservation approaches to save at-risk species and uphold ecological equilibrium. Furthermore, eRNA has been recognized as a valuable method for the identification of COVID-19 in the environment, besides its established uses in biodiversity protection. The SARS-CoV-2 virus, which is accountable for the worldwide epidemic, releases RNA particles into the surrounding environment via human waste, providing insights into the feasibility of detecting it in wastewater and other samples taken from the environment. In this article, we critically reviewed the recent research activities that use the eRNA method, including its utilization in biodiversity conservation, ecological surveillance, and ecotoxicological monitoring as well as its innovative potential in identifying COVID-19. Through this review, the reader can understand the recent developments, prospects, and challenges of eRNA research in ecosystem management and biodiversity conservation. [ABSTRACT FROM AUTHOR]

This study quantified the seasonal and spatial variability of partial pressure of CO2 (pCO2) and water‐atmosphere CO2 fluxes in the Parnaíba River Delta, the largest delta in the Americas. It is a pristine equatorial, mangrove‐dominated environment located in a transitional between humid and semi‐arid climates, with marked seasonality in rainfall and river discharge. Major channels and bays were sampled during dry and wet seasons, with continuous measurements of pCO2, temperature, salinity, and wind velocity. Subsurface water samples were collected in discrete stations for pH, total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved oxygen and chlorophyll a quantification. A significant positive correlation between carbonate system parameters with salinity was found in both periods, with salinity significantly higher in the dry season. Strong deviations of pCO2, TA, and DIC from two endmembers conservative mixing were found, particularly in mangrove‐dominated waters, due to organic matter degradation. The Delta showed high spatial variability of pCO2, with the highest values in mangrove‐dominated waters, moderate in the river‐dominated regions, and lowest in the high salinity areas, suggesting that pCO2 variability is likely controlled by a combination of river‐ocean mixing and biological processes (respiration and photosynthesis). The Delta outgasses about 20 times less CO2 in the dry season (9.06 ± 11.09 mmol m−2.d−1) than in the rainy season (209.68 ± 250.87 mmol m−2 d−1). Our results indicate this large mangrove‐dominated tropical delta is an important source of CO2 to the atmosphere, but a sharp decrease was observed during dry periods. [ABSTRACT FROM AUTHOR]

Floodplain lakes are widespread and ecologically important throughout tropical river systems, however data are rare that describe how temporal variations in hydrological, meteorological and optical conditions moderate stratification and mixing in these shallow lakes. Using time series measurements of meteorology and water‐column temperatures from 17 several day campaigns spanning two hydrological years in a representative Amazon floodplain lake, we calculated surface energy fluxes and thermal stratification, and applied and evaluated a 3‐dimensional hydrodynamic model. The model successfully simulated diel cycles in thermal structure characterized by buoyancy frequency, depth of the actively mixing layer, and other terms associated with the surface energy budget. Diurnal heating with strong stratification and nocturnal mixing were common; despite considerable heat loss at night, the strong stratification during the day meant that mixing only infrequently extended to the bottom at night. Simulations indicated that the diurnal thermocline up and downwelled creating lake‐wide differences in near‐surface temperatures and mixing depths. Infrequent full mixing creates conditions conducive to anoxia in these shallow lakes given their warm temperatures. Key Points: Diel thermal structure in a tropical floodplain lake was characterized by high‐resolution field measurements over two hydrological yearsState and process evaluation show that diel and seasonal stratification and mixing were simulated well by a 3‐D hydrodynamic modelDiurnal heating with strong stratification and nocturnal mixing were common while mixing to the bottom was intermittent [ABSTRACT FROM AUTHOR]

Plankton are promising ecological monitoring tool that responds quickly to any sort of aquatic ecological alteration, of which many of them are much susceptible to ecological variations. Therefore, monitoring shifts in plankton composition can indicate changes in water quality and aid to identify potential pollution sources. In the present study, the variation in plankton dynamics in relation to ecological variables were monitored in the freshwater zone of the Hooghly estuary from May 2020 to April 2021. The study was conducted in the interval of every six hours. i.e., at 6 A.M., 12 P.M., 6 P.M., and 12 A.M. The present finding revealed the occurrence of 54 phytoplankton and 20 zooplankton taxa/species. Diel variation revealed that among different time intervals, the highest abundance of phytoplankton was recorded 28,307 cells l−1 at 12 P.M, while the lowest was recorded 10,632 cells l−1 at 6 A.M. However, the highest zooplankton abundance was observed 804 ind l−1 at 6 A.M., and the lowest was recorded 156 ind l−1 at 6 P.M. The ANOVA (p < 0.05) analysis indicated significant diel variation for many planktonic genera. The CCA exhibited that most of the phytoplankton were influenced by multiple water quality variables such as temperature, turbidity, calcium, pH, salinity, DO, and nutrients. However, the majority of the zooplankton were affected by turbidity, total phosphorus, sulphate, calcium and available nitrogen. Significant seasonal variation in plankton composition has also been observed. The present study will help to determine the varying diel pattern of planktons in retort to alterations in the water quality parameters and varying ecological niches. [ABSTRACT FROM AUTHOR]

Floodplains lakes are abundant in the Amazon basin and are important methane sources to the atmosphere. Existing biogeochemical models require modifications and inclusion of hydrodynamic processes operative in shallow, warm waters to be applied to these aquatic ecosystems. We modified a 1‐dimensional process‐based, lake biogeochemical model and combined a 3‐dimensional hydrodynamic model to suit Amazon floodplains. We evaluated the combined model's performance simulating methane concentrations and fluxes and several related processes in the open lake and an embayment of a well‐studied Amazon lake. Parameters for calibration were selected through sensitivity tests using a machine learning‐based algorithm, classification, and regression trees. Comparison between simulated and measured fluxes indicate generally good agreement in seasonal patterns and magnitudes. Comparisons of near‐surface concentrations varied with no clear patterns. Simulations of methane concentrations at near‐surface and near‐bottom, and diffusive emissions are most sensitive to carbon mineralization rate, Q10 factors for methanogenesis and oxidation, and methane oxidation potential. Modeled rates of planktonic photosynthesis were generally lower than measurements, though simulated planktonic respiration was often similar to measurements. Simulated rates of methane oxidation were considerably lower, with a few exceptions, than measurements of methane oxidation in oxic water of the lake. Improvements of results of the linked hydrodynamic‐biogeochemical model will result from inclusion of advective transport, use of parameter values appropriate for tropical waters, especially for methane oxidation and photosynthesis, and addition of changes in hydrostatic pressure to model of ebullition. Plain Language Summary: Methane emissions from lakes are large, highly variable and without adequate measurements, especially in tropical regions. The combination of mechanistic models with results from intensive field data provides one way to improve understanding of the processes and sources of variability. Riverine floodplain lakes, as occur in the Amazon basin, constitute extensive aquatic ecosystems and are important methane sources to the atmosphere. Though tropical lakes typically experience muted seasonal variations in climate, strong diel changes occur, and floodplain lakes have additional biogeochemical and ecological variability caused by changes in water level, connectivity to rivers and optical properties. We modified a one‐dimensional process‐based, lake biogeochemical model and combined a 3‐dimensional hydrodynamic model to be applicable to Amazon floodplains and similar shallow, warm waters. The combined model simulated well methane processes and fluxes based on evaluation using observations in a representative central Amazon floodplain lake with measurements of meteorological variables, water temperatures and ecological conditions, and methane emissions over the seasonal hydrological phases with large differences in water levels. Key Points: Methane fluxes from Amazon floodplains lakes simulated with biogeochemical model combined with hydrodynamic modelSeasonal patterns and magnitudes of simulated and measured fluxes are generally in good agreementMethane concentrations and diffusive emissions are sensitive to carbon mineralization and methane production and oxidation [ABSTRACT FROM AUTHOR]

Phytoplankton play a key role in marine ecosystems, making the accurate quantification of functional groups/species, using standardized microscopy techniques, essential in many research efforts. In this study we aimed to evaluate the effects of (a) storage time on the abundance of microphytoplankton in samples fixed with acid Lugol's solution; (b) storage time on the abundance of pico- and nanophytoplankton in frozen microscopy slides; (c) storage time on the abundance of pico- and nanophytoplankton in fixed refrigerated samples and (d) storage protocol (refrigerated versus frozen) on the abundance of pico- and nanophytoplankton. Microphytoplankton were analyzed using inverted microscopy, and pico- and nanophytoplankton using epifluorescence microscopy. Results indicate storage time negatively impacted the abundances of all phytoplankton size classes; however, effects were group-specific. For accurate abundance estimates, we suggest that samples fixed with acid Lugol's should be analyzed within 30 and 180 days, for dinoflagellates/total microphytoplankton and diatoms, respectively. For picoeukaryotes, glutaraldehyde fixed samples should be kept refrigerated for up to 4 days, and slides should be prepared immediately before observation. It is recommended that authors specify the exact lag times between sample collection, fixation and analysis, to allow the comparability of phytoplankton datasets across different studies and/or monitoring programs. [ABSTRACT FROM AUTHOR]

Climate change is causing seasonally ice‐covered lakes of the boreal region to undergo changes in their winter regime by altering patterns of precipitation and temperature, often reflected as reduced snow and ice cover duration. The duration, extent and quality of ice, and snow cover have a pivotal role for production and carbon cycling in lakes in winter, with potentially cascading effects for the following open water period. We investigated under‐ice carbon cycling by assessing bacterial growth (including bacterial production, bacterial respiration, and bacterial growth efficiency) and primary production at five water depths during early winter, midwinter, late winter and melting season in a boreal lake, and report significantly different temporal patterns. Bacterial respiration was dominant in early and midwinter, whereas the late winter and melting season were dominated by bacterial production. Multiple linear regression models indicated that high early winter bacterial respiration was associated with senescing phytoplankton, whereas bacterial production was promoted by the onset of spring processes. Collectively, bacterial growth indices were inherently linked with bacterioplankton community composition and specific biomarker taxa. Primary production under ice increased in late winter when light‐blocking snow cover melted, and primary production measured from the lake ice exceeded that of the water column at the melting season. Ice samples hosted diverse eukaryotic communities including photoautotrophs, suggesting that the habitat potential of the understudied lake ice and the role of ice for ecological processes at ice melt should be further explored. [ABSTRACT FROM AUTHOR]

Despite their small spatial extent, fluvial ecosystems play a significant role in processing and transporting carbon in aquatic networks, which results in substantial emission of methane (CH4) into the atmosphere. For this reason, considerable effort has been put into identifying patterns and drivers of CH4 concentrations in streams and rivers and estimating fluxes to the atmosphere across broad spatial scales. However, progress toward these ends has been slow because of pronounced spatial and temporal variability of lotic CH4 concentrations and fluxes and by limited data availability across diverse habitats and physicochemical conditions. To address these challenges, we present a comprehensive database of CH4 concentrations and fluxes for fluvial ecosystems along with broadly relevant and concurrent physical and chemical data. The Global River Methane Database (GriMeDB; 10.6073/pasta/f48cdb77282598052349e969920356ef, Stanley et al., 2023) includes 24 024 records of CH4 concentration and 8205 flux measurements from 5029 unique sites derived from publications, reports, data repositories, unpublished data sets, and other outlets that became available between 1973 and 2021. Flux observations are reported as diffusive, ebullitive, and total CH4 fluxes, and GriMeDB also includes 17 655 and 8409 concurrent measurements of concentrations and 4444 and 1521 fluxes for carbon dioxide (CO2) and nitrous oxide (N2O), respectively. Most observations are date-specific (i.e., not site averages), and many are supported by data for 1 or more of 12 physicochemical variables and 6 site variables. Site variables include codes to characterize marginal channel types (e.g., springs, ditches) and/or the presence of human disturbance (e.g., point source inputs, upstream dams). Overall, observations in GRiMeDB encompass the broad range of the climatic, biological, and physical conditions that occur among world river basins, although some geographic gaps remain (arid regions, tropical regions, high-latitude and high-altitude systems). The global median CH4 concentration (0.20 µmolL-1) and diffusive flux (0.44 mmolm-2d-1) in GRiMeDB are lower than estimates from prior site-averaged compilations, although ranges (0 to 456 µmolL-1 and - 136 to 4057 mmolm-2d-1) and standard deviations (10.69 and 86.4) are greater for this larger and more temporally resolved database. Available flux data are dominated by diffusive measurements despite the recognized importance of ebullitive and plant-mediated CH4 fluxes. Nonetheless, GriMeDB provides a comprehensive and cohesive resource for examining relationships between CH4 and environmental drivers, estimating the contribution of fluvial ecosystems to CH4 emissions, and contextualizing site-based investigations. [ABSTRACT FROM AUTHOR]

Tropical mangrove forests are important carbon sinks, the soil being the main carbon reservoir. Understanding the variability and the key factors that control fluxes is critical to accounting for greenhouse gas (GHG) emissions, particularly in the current scenario of global climate change. This study is the first to quantify carbon dioxide (CO 2) and methane (CH 4) emissions using a dynamic chamber in natural mangrove soil of the Amazon. The plots for the trace gases study were allocated at contrasting topographic heights. The results showed that the mangrove soil of the Amazon estuary is a source of CO 2 (6.66 g CO 2 m -2 d -1) and CH 4 (0.13 g CH 4 m -2 d -1) to the atmosphere. The CO 2 flux was higher in the high topography (7.86 g CO 2 m -2 d -1) than in the low topography (4.73 g CO 2 m -2 d -1) in the rainy season, and CH 4 was higher in the low topography (0.13 g CH 4 m -2 d -1) than in the high topography (0.01 g CH 4 m -2 d -1) in the dry season. However, in the dry period, the low topography soil produced more CH 4. Soil organic matter, carbon and nitrogen ratio (C/N), and redox potential influenced the annual and seasonal variation of CO 2 emissions; however, they did not affect CH 4 fluxes. The mangrove soil of the Amazon estuary produced 35.40 Mg CO 2eq. ha -1 yr -1. A total of 2.16 kg CO 2 m -2 yr -1 needs to be sequestered by the mangrove ecosystem to counterbalance CH 4 emissions. [ABSTRACT FROM AUTHOR]

Hydroelectric reservoirs can emit significant quantities of methane, particularly through degassing at turbine outlets. Improved understanding of processes affecting hydroelectric reservoir CH4 emissions is thus important as the world economy transitions to renewable forms of energy production. Here we develop and evaluate a new mechanistic model of CH4 emissions: ResME ([Res]ervoir [M]ethane [E]missions), which estimates carbon inputs and methanogenesis to predict CH4 release via ebullition and diffusion, plant emissions, and downstream emissions. ResME results demonstrate that the relative importance of allochthonous and autochthonous carbon input to methane emissions varies by latitude, with allochthonous carbon contributions typically being higher in tropical reservoirs. Results also demonstrate that total reservoir emissions are highly dependent on turbine intake depths, which are not typically reported. Potential maximum degassing emissions from existing hydroelectric reservoirs are estimated as 11 ± 4 Tg C/yr, if all reservoirs had deep turbine intakes and stratified for 5 months per year. In comparison, the estimated diffusive, ebullitive, and plant CH4 emissions are estimated to be 2.8 ± 0.2 Tg C/yr (where the true uncertainty is much higher than the model standard error). Future work should focus on improving estimates of reservoir carbon inputs and decomposition rates, as well as surveying turbine intake depths. Satellite measurements from missions such as TROPOMI may also help constrain hydropower methane emissions. Plain Language Summary: Methane is an important greenhouse gas that is naturally produced in lake and reservoir sediment, among other sources. Hydroelectric power reservoirs produce renewable energy, yet also emit methane at their surfaces, and from turbines and downstream reaches. To better understand drivers and pathways of methane emissions, we have developed a new mechanistic model for methane emissions as a function of carbon inputs, chemical decomposition, and physical processes. Results also show that downstream methane emissions have the potential to exceed surface emissions if turbines pull from stratified, anoxic waters. Large uncertainties remain in model inputs, and future work should focus on improved understanding of carbon loading to reservoirs, as well as decomposition rates and turbine intake depths. Key Points: Our mechanistic model (Reservoir Methane Emissions) illuminates the main drivers of hydropower methane emissionsEmissions from downstream degassing are comparable to surface emissions when turbines are parameterized with deep water intakesWe estimate global emissions from hydropower surfaces as 2.8 ± 0.2 Tg C/yr, plus 11 ± 4 Tg C/yr from downstream degassing [ABSTRACT FROM AUTHOR]

The Congo and Amazon are the two largest rivers on Earth and serve as major sources of dissolved organic carbon to the ocean. We compared the dissolved organic matter (DOM) composition of both rivers using Fourier‐transform ion cyclotron resonance mass spectrometry to investigate seasonal and regional differences in DOM composition exported to the ocean. We found that over a 15‐month observational period in the Congo River, molecular aromaticity and oxygenation between the wet and dry periods varied slightly, but most of the relative abundance of DOM formulae (∼90%) were present in all samples, suggesting that Congo River DOM quality is stable across different hydrological conditions. In contrast, the multi‐year DOM composition in the Amazon River was highly susceptible to changes in hydrology, with clear differences in molecular aromaticity, oxygenation, and heteroatom (N, S, P) content between the wet and dry seasons. Overall, the DOM composition of the Congo River was more terrestrial than Amazon River DOM, which was more characteristic of aquatic DOM. Finally, we compared the relative contribution of island of stability (IOS) formulae between the rivers and found that both rivers export similar amounts of these formulae annually, more than several major rivers combined, and that the Congo is more than twice as efficient in exporting these IOS formulae. With changing precipitation and land use, the quantity and composition of exported DOM will likely reflect the mobilization of additional terrestrial and anthropogenic sources that will also be subjected to downstream land‐to‐ocean cycling. Plain Language Summary: Almost 16% of the global riverine dissolved organic carbon (DOC) export is delivered by just two rivers: Amazon and Congo. Although both rivers drain immense tropical rainforests, differences in landscape, precipitation, and riverine discharge all contribute to the distinct molecular composition of their dissolved organic matter (DOM). We compared DOM composition of both rivers over several years and found that hydrology is an important driver for determining the DOM composition of both rivers; however, it is more important for the Amazon than Congo, which had DOM that was more similar across the wet and dry seasons. Overall, both rivers had more aromatic and oxygen‐rich DOM than temperate or arctic rivers, but DOM from the Congo had a more terrestrial signature than the Amazon River DOM. Finally, we found that DOM molecular formulae thought to be stable over long periods of time in marine systems were exported in similar amounts by both rivers, even though the Amazon exports about twice as much DOC annually compared to the Congo, suggesting that the Congo is more than twice as efficient per unit volume at exporting molecular formulae that is thought to be stable in the ocean for long periods of time. Key Points: Congo River dissolved organic matter (DOM) composition remains mostly stable across the year, while Amazon River DOM varies seasonally with dischargeAmazon River DOM is less aromatic, less oxygenated, and more heteroatom‐rich than Congo River DOMBoth rivers export similar amounts of island of stability molecular formulae but the Congo is over twice as efficient [ABSTRACT FROM AUTHOR]

Tropical rivers emit large amounts of carbon dioxide (CO 2) to the atmosphere, in particular due to large wetland-to-river carbon (C) inputs. Yet, tropical African rivers remain largely understudied, and little is known about the partitioning of C sources between wetland and well-drained ecosystems to rivers. In a first-order sub-catchment (0.6 km 2) of the Nyong watershed (Cameroon 27 800 km 2), we fortnightly measured C in all forms and ancillary parameters in groundwater in a well-drained forest (hereafter referred to as non-flooded forest groundwater) and in the stream. In the first-order catchment, the simple land use shared between wetland and well-drained forest, together with drainage data, allowed the partitioning of C sources between wetland and well-drained ecosystems to the stream. Also, we fortnightly measured dissolved and particulate C downstream of the first-order stream to the main stem of order 6, and we supplemented C measurements with measures of heterotrophic respiration in stream orders 1 and 5. In the first-order stream, dissolved organic and inorganic C and particulate organic C (POC) concentrations increased during rainy seasons when the hydrological connectivity with the riparian wetland increased, whereas the concentrations of the same parameters decreased during dry seasons when the wetland was shrinking. In larger streams (order > 1), the same seasonality was observed, showing that wetlands in headwaters were significant sources of organic and inorganic C for downstream rivers, even though higher POC concentration evidenced an additional source of POC in larger streams during rainy seasons that was most likely POC originating from floating macrophytes. During rainy seasons, the seasonal flush of organic matter from the wetland in the first-order catchment and from the macrophytes in higher-order rivers significantly affected downstream metabolism, as evidenced by higher respiration rates in stream order 5 (756 ± 333 gC-CO 2 m -2 yr -1) compared to stream 1 (286 ± 228 gC-CO 2 m -2 yr -1). In the first-order catchment, the sum of the C hydrologically exported from non-flooded forest groundwater (6.2 ± 3.0 MgC yr -1) and wetland (4.0 ± 1.5 MgC yr -1) to the stream represented 3 %–5 % of the local catchment net C sink. In the first-order catchment, non-flooded forest groundwater exported 1.6 times more C than wetland; however, when weighed by surface area, C inputs from non-flooded forest groundwater and wetland to the stream contributed to 27 % (13.0 ± 6.2 MgC yr -1) and 73 % (33.0 ± 12.4 MgC yr -1) of the total hydrological C inputs, respectively. At the Nyong watershed scale, the yearly integrated CO 2 degassing from the entire river network was 652 ± 161 GgC-CO 2 yr -1 (23.4 ± 5.8 MgC CO 2 km -2 yr -1 when weighed by the Nyong watershed surface area), whereas average heterotrophic respiration in the river and CO 2 degassing rates was 521 ± 403 and 5085 ± 2544 gC-CO 2 m -2 yr -1 , which implied that only ∼ 10 % of the CO 2 degassing at the water–air interface was supported by heterotrophic respiration in the river. In addition, the total fluvial C export to the ocean of 191 ± 108 GgC yr -1 (10.3 ± 5.8 MgC km -2 yr -1 when weighed by the Nyong watershed surface area) plus the yearly integrated CO 2 degassing from the entire river network represented ∼ 11 % of the net C sink estimated for the whole Nyong watershed. In tropical watersheds, we show that wetlands largely influence riverine C variations and budget. Thus, ignoring the river–wetland connectivity might lead to the misrepresentation of C dynamics in tropical watersheds. [ABSTRACT FROM AUTHOR]

Measurements of turbulence, as rate of dissipation of turbulent kinetic energy (ε), adjacent to the air‐water interface are rare but essential for understanding of gas transfer velocities (k) used to compute fluxes of greenhouse gases. Variability in ε is expected over diel cycles of stratification and mixing. Monin‐Obukhov similarity theory (MOST) predicts an enhancement in ε during heating (buoyancy flux, β+) relative to that for shear (u*w3/κz where u*w is water friction velocity, κ is von Karman constant, z is depth). To verify and expand predictions, we quantified ε in the upper 0.25 m and below from profiles of temperature‐gradient microstructure in combination with time series meteorology and temperature in a tropical reservoir for winds <4 m s−1. Maximum likelihood estimates of near‐surface ε during heating were independent of wind speed and high, ∼5 × 10−6 m2 s−3, up to three orders of magnitude higher than predictions from u*w3/κz, increased with heating, and were ∼10 times higher than during cooling. k, estimated using near‐surface ε, was ∼10 cm hr−1, validated with k obtained from chamber measurements, and 2–5 times higher than computed from wind‐based models. The flux Richardson number (Rf) varied from ∼0.4 to ∼0.001 with a median value of 0.04 in the upper 0.25 m, less than the critical value of 0.2. We extend MOST by incorporating the variability in Rf when scaling the influence of β+ relative to u*w3/κz in estimates of ε, and by extension, k, obtained from time series meteorological and temperature data. Plain Language Summary: Fluxes of climate forcing trace gases in aquatic systems depend on physical processes, specifically turbulence for light to moderate winds, and the concentration gradient at the air‐water interface. The physical processes can be represented as a gas transfer velocity (k). We analyze physical and biogeochemical data from a tropical reservoir and provide an algorithm for computing near‐surface turbulence, as rate of dissipation of turbulent kinetic energy (ε), and k during heating and light winds, as commonly occur during morning in both lakes and oceans and during heat waves at all latitudes. During heating when wind speeds were less than 4 m s−1, near‐surface ε increased with heating and was higher than would be expected based on wind speed alone. Enhancement was moderated by reduced efficiency of mixing. k averaged 10 cm hr−1, ∼5 times higher than computed from wind‐based models. The equations, which include wind, heating, and mixing efficiency, will enable improved estimates of fluxes of CO2 and CH4 used in biogeochemical models of oceans and inland waters during heating and will be especially important in tropical water bodies with high concentrations of climate forcing trace gases. Key Points: Dissipation rates (ε) in the upper 0.25 m of a tropical water body reached 10−5 m2 s−3 under light winds and heatingFor winds up to 3.5 m s−1, ε depended on heating rather than wind speed, and ε was 10 times higher under heating than coolingGas transfer velocities were four times higher under light winds and heating than predicted from wind‐based models [ABSTRACT FROM AUTHOR]

CO 2 efflux at the water–air interface is an essential component of the riverine carbon cycle. However, the lack of spatially resolved CO 2 emission measurements prohibits reliable estimation of the global riverine CO 2 emissions. By deploying floating chambers, seasonal changes in river water CO 2 partial pressure (p CO 2) and CO 2 emissions from the Dong River in south China were investigated. Spatial and temporal patterns of p CO 2 were mainly affected by terrestrial carbon inputs (i.e., organic and inorganic carbon) and in-stream metabolism, both of which varied due to different land cover, catchment topography, and seasonality of precipitation and temperature. Temperature-normalized gas transfer velocity (k600) in small rivers was 8.29 ± 11.29 and 4.90 ± 3.82 m d -1 for the wet season and dry season, respectively, which was nearly 70 % higher than that of large rivers (3.90 ± 5.55 m d -1 during the wet season and 2.25 ± 1.61 m d -1 during the dry season). A significant correlation was observed between k600 and flow velocity but not wind speed regardless of river size. Most of the surveyed rivers were a net CO 2 source while exhibiting substantial seasonal variations. The mean CO 2 flux was 300.1 and 264.2 mmol m -2 d -1 during the wet season for large and small rivers, respectively, 2-fold larger than that during the dry season. However, no significant difference in CO 2 flux was observed between small and large rivers. The absence of commonly observed higher CO 2 fluxes in small rivers could be associated with the depletion effect caused by abundant and consistent precipitation in this subtropical monsoon catchment. [ABSTRACT FROM AUTHOR]

High‐altitude lakes are sentinels of change. Anticipated increases in inputs of dissolved organic carbon (DOC) from catchments in high‐altitude lakes could affect planktonic interactions, especially if inputs occur during the winter ablation phase. However, the responses of plankton communities to added DOC interacting with nutrients during this period are still not well understood.We sampled under‐ice water and used a multifactorial experimental design to investigate the DOC effect on planktonic organisms (phytoplankton and heterotrophic prokaryotic plankton [HPP]) and their interactions during the winter ablation phase. The plankton community was subjected to DOC additions (0, 2, 4, and 6 mg C/L) under two nutrient‐availability treatments (natural concentrations or enriched in N and P), two light conditions (dark or light), and two temperature conditions (10 or 18℃).We found HPP to be co‐limited by N, P, and C in our microcosms. Added glucose as available C‐source for bacterioplankton was highly consumed at the end of the experiments. This consumption was not always related to an increase of the HPP biomass, due to a rising predatory control by ciliates and mixotrophic phytoplankton over bacteria. In dark conditions, the biomasses of both autotrophic and mixotrophic phytoplankton were substantially reduced, and the HPP biomass under DOC and nutrient additions principally depended on the predatory control exerted by ciliates. In light conditions, a diversification of top‐down control over bacteria was observed, with the HPP response to DOC and nutrient additions depending on both mixotrophs and ciliates. Moreover, when heterotrophic ciliates and mixotrophs were present together in the experiments, the ciliates replaced the mixotrophs in phagotrophy over bacteria.Our experimental results indicate that DOC inputs can rapidly alter the trophic interactions of the planktonic food web, depending on nutrient limitation in high‐altitude lakes. We found decreased commensalistic interactions between bacteria and phytoplankton, but increased competition and predation, after DOC additions.The expected future scenario of global change during the ablation phase is likely to change the balance between heterotrophic and autotrophic processes in planktonic food webs by increasing heterotrophic processes. These changes could have major repercussions on energy transfer from the base to the top of the food chain in high‐altitude lakes.Exploring the planktonic response to rapid environmental changes during the ablation phase appears to be crucial in order to determine how vulnerable high‐altitude lakes are to future change. [ABSTRACT FROM AUTHOR]

Lakes receive and transform significant amounts of terrestrial carbon and are often considered a source of atmospheric carbon dioxide (CO2). Yet, continuous direct measurements of lake‐atmosphere CO2 exchange with high temporal resolution are sparse. In this study, we measured the CO2 exchange of a mountain lake in the eastern Austrian Alps continuously for one year using the eddy covariance (EC) and the boundary layer model (BLM) approaches. Results from both the EC and the BLM methods indicated the lake to be a small source of atmospheric CO2 with highest emissions in fall. EC flux measurements were affected by low‐frequency contributions especially during low wind conditions. The CO2 concentration gradient at the air‐water interface decreased during night‐time due to an increase in atmospheric CO2 above the lake, likely caused by cold and CO2‐rich air draining from the surrounding land. Consequently, BLM fluxes were lower during night‐time than during daytime. This diel pattern was lacking in the EC flux measurements because the EC instruments deployed at the shore of the lake did not capture low nocturnal lake CO2 fluxes due to the local wind regime. Overall, this study illustrates the effect of the surrounding landscape on lake‐atmosphere flux measurements. We conclude that estimating CO2 evasion from lakes situated in complex topography needs to explicitly account for biases in EC flux measurements caused by low‐frequency contributions and local wind regimes. Plain Language Summary: Lakes and rivers are an important link in the global carbon cycle transporting carbon from the land to the oceans. However, part of the carbon entering the water is stored in lake sediments or released from the water into the atmosphere as carbon dioxide (CO2). Therefore, lakes are considered important sources of atmospheric CO2, yet continuous direct measurements of this water‐air CO2 exchange are sparse. In this study, we used two different methods to measure the CO2 exchange of a small lake in the Austrian Alps for an entire year. We found that during the ice covered period, in spring, and in summer the CO2 exchange between lake and air was small. In fall, the lake released CO2 at higher rates. Overall, the lake was only a small source of CO2. We also found significant differences in the results of the two measuring methods. Those results demonstrated that measurements of lake‐atmosphere CO2 exchange are complex and—especially if the lake is small and situated in a mountainous landscape—the surrounding land can influence the measurements. Key Points: We estimated the carbon dioxide (CO2) exchange of a small mountain lake using the eddy covariance and the boundary layer model approachesCO2 fluxes were small and variable, and the variation in local atmospheric CO2 concentration was an important driver of lake CO2 exchangeThe study demonstrates that the influence of the surrounding land has to be considered in lake‐atmosphere flux measurements [ABSTRACT FROM AUTHOR]

Increasing evidence indicates that aquaporins (AQPs) exert an influence in cell signaling by the interplay with the transient receptor potential vanilloid 4 (TRPV4) channel. We previously found that TRPV4 physically and functionally interacts with AQP2 in cortical collecting ducts (CCD) cells, favoring cell volume regulation and cell migration. Because TRPV4 was implicated in ATP release in several tissues, we investigated the possibility that TRPV4/AQP2 interaction influences ATP release in CCD cells. Using two CCD cell lines expressing or not AQP2, we measured extracellular ATP (ATPe) under TRPV4 activation and intracellular Ca2+ under ATP addition. We found that AQP2 is critical for the release of ATP induced by TRPV4 activation. This ATP release occurs by an exocytic and a conductive route. ATPe, in turn, stimulates purinergic receptors leading to ATPe‐induced ATP release by a Ca2+‐dependent mechanism. We propose that AQP2 by modulating Ca2+ and ATP differently could explain AQP2‐increased cell migration. [ABSTRACT FROM AUTHOR]

The patterns of spatial and temporal variability in CO2 and CH4 emission from reservoirs are still poorly studied, especially in tropical regions where hydropower is growing. We performed spatially resolved measurements of dissolved CO2 and CH4 surface water concentrations and their gas‐exchange coefficients (k) to compute diffusive carbon flux from four contrasting tropical reservoirs across Brazil during different hydrological seasons. We used an online equilibration system to measure dissolved CO2 and CH4 concentrations; we estimated k from floating chamber deployments in conjunction with discrete CO2 and CH4 water concentration measurements. Diffusive CO2 emissions were higher during dry season than during rainy season, whereas there were no consistent seasonal patterns for diffusive CH4 emissions. Our results reveal that the magnitude and the spatial within‐reservoir patterns of diffusive CO2 and CH4 flux varied strongly among hydrological seasons. River inflow areas were often characterized by high seasonality in diffusive flux. Areas close to the dam generally showed low seasonal variability in diffusive CH4 flux but high variability in CO2 flux. Overall, we found that reservoir areas exhibiting highest emission rates ("hotspots") shifted substantially across hydrological seasons. Estimates of total diffusive carbon emission from the reservoir surfaces differed between hydrological seasons by a factor up to 7 in Chapéu D'Úvas, up to 13 in Curuá‐Una, up to 4 in Furnas, and up to 1.8 in Funil, indicating that spatially resolved measurements of CO2 and CH4 concentrations and k need to be performed at different hydrological seasons in order to constrain annual diffusive carbon emission. Plain Language Summary: Reservoirs are key for flood control, water supply, and hydropower generation. However, reservoirs are usually not carbon neutral. Studies worldwide point to reservoirs as important net sources of anthropogenic carbon emission to the atmosphere. Carbon emission from reservoirs derives from the decomposition of organic matter. Although carbon emission from reservoirs has been increasingly studied over the past two decades, most studies do not sufficiently describe emissions across space and time. Our study applies highly resolved spatial coverage of dissolved surface water concentrations and gas‐exchange coefficients of CO2 and CH4 to compute rates of CO2 and CH4 diffusion to the atmosphere across distinct hydrological seasons in four contrasting tropical reservoirs. We found that emissions varied substantially over both space and time. More specifically, we found that reservoir areas exhibiting highest emission rates ("hotspots") shifted substantially between dry and rainy seasons. Overlooking the spatial within‐reservoir variability across seasons may result in serious underestimation or overestimation of total diffusive carbon emission from reservoirs, depending on the time and space that studies focus their sampling on. Our work may support scientists in adopting more comprehensive sampling strategies relevant for better constrained upscaling, and, consequently, support informed policy decisions and management actions. Key Points: Reservoir regions displaying highest emission rates shifted substantially between sampling occasions at hydrologically different seasonsRiver inflow areas were often characterized by high seasonality in diffusive carbon fluxSpatially resolved measurements need to be conducted at different hydrological seasons in order to constrain diffusive emission [ABSTRACT FROM AUTHOR]

Purpose: Dams require high-volume of construction materials and operations over the life cycle. This paper aims to select a proper type of dam structure that can significantly contribute to the sustainability of dam projects. Design/methodology/approach: This research proposes a complementary fuel consumption and carbon dioxide (CO2) emission assessment method for the alternate dam structure types to assist decision-makers in selecting sustainable choices. Related equations are developed for two common earthen and rock-fill dam structures types in Iran. These equations are then successfully applied to two real dam project cases where the significance of the achieved results are assessed and discussed. Findings: The achieved results of the case studies demonstrate a high deviation of up to 41.3% in CO2 emissions comparing alternate dam structure scenarios of earthen and rock-fill dam structures. This high deviation represents an important potential for CO2 emission reduction considering the high volume of the emission in large dam projects. Originality/value: The life cycle emission assessment of the alternate dam structures, proposed in this research as a novel complementary factor, can be used in the decision-making process of dam projects. The results in this research identify high potential sustainability improvement of dam projects as a result of the proposed method. [ABSTRACT FROM AUTHOR]

River deltas, with their mosaic of ponds, channels and seasonally inundated areas, act as the last continental hot spots of carbon turnover along the land–ocean aquatic continuum. There is increasing evidence for the important role of riparian wetlands in the transformation and emission of terrestrial carbon to the atmosphere. The considerable spatial heterogeneity of river deltas, however, forms a major obstacle for quantifying carbon emissions and their seasonality. The water chemistry in the river reaches is defined by the upstream catchment, whereas delta lakes and channels are dominated by local processes such as aquatic primary production, respiration or lateral exchange with the wetlands. In order to quantify carbon turnover and emissions in the complex mosaic of the Danube Delta, we conducted monthly field campaigns over 2 years at 19 sites spanning river reaches, channels and lakes. Here we report on the greenhouse gas fluxes (CO 2 and CH 4) from the freshwater systems of the Danube Delta and present the first seasonally resolved estimates of its freshwater carbon emissions to the atmosphere. Furthermore, we quantify the lateral carbon transport of the Danube River to the Black Sea. We estimate the delta's CO 2 and CH 4 emissions to be 65 GgC yr -1 (30–120 GgC yr -1 , a range calculated using 25 to 75 percentiles of observed fluxes), of which about 8 % are released as CH 4. The median CO 2 fluxes from river branches, channels and lakes are 25, 93 and 5.8 mmol m -2 d -1 , respectively. Median total CH 4 fluxes amount to 0.42, 2.0 and 1.5 mmol m -2 d -1. While lakes do have the potential to act as CO 2 sinks in summer, they are generally the largest emitters of CH 4. Small channels showed the largest range in emissions, including a CO 2 and CH 4 hot spot sustained by adjacent wetlands. Thereby, the channels contribute disproportionately to the delta's emissions, considering their limited surface area. In terms of lateral export, we estimate the net total export (the sum of dissolved inorganic carbon, DIC, dissolved organic carbon, DOC, and particulate organic carbon, POC) from the Danube Delta to the Black Sea to be about 160 ± 280 GgC yr -1 , which only marginally increases the carbon load from the upstream river catchment (8490 ± 240 GgC yr -1) by about 2 %. While this contribution from the delta seems small, deltaic carbon yield (45.6 gC m -2 yr -1 ; net export load/surface area) is about 4 times higher than the riverine carbon yield from the catchment (10.6 gC m -2 yr -1). [ABSTRACT FROM AUTHOR]

Freshwaters are important emitters of carbon dioxide (CO2) and methane (CH4), two potent greenhouse gases (GHGs). While aquatic surface GHG fluxes have been extensively measured, there is much less information about their underlying sources. In lakes and reservoirs, surface GHG can originate from horizontal riverine flow, the hypolimnion, littoral sediments, and water column metabolism. These sources are generally studied separately, leading to a fragmented assessment of their relative role in sustaining CO2 and CH4 surface fluxes. In this study, we quantified sources and sinks of CO2 and CH4 in the epilimnion along a hydrological continuum in a permanently stratified tropical reservoir (Borneo). Results showed that horizontal inputs are an important source of both CO2 and CH4 (>90 % of surface emissions) in the upstream reservoir branches. However, this contribution fades along the hydrological continuum, becoming negligible in the main basin of the reservoir, where CO2 and CH4 are uncoupled and driven by different processes. In the main basin, vertical CO2 inputs and sediment CH4 inputs contributed to on average 60 % and 23 % respectively to the surface fluxes of the corresponding gas. Water column metabolism exhibited wide amplitude and range for both gases, making it a highly variable component, but with a large potential to influence surface GHG budgets in either direction. Overall our results show that sources sustaining surface CO2 and CH4 fluxes vary spatially and between the two gases, with internal metabolism acting as a fluctuating but key modulator. However, this study also highlights challenges and knowledge gaps related to estimating ecosystem-scale CO2 and CH4 metabolism, which hinder aquatic GHG flux predictions. [ABSTRACT FROM AUTHOR]

As the second largest area of contiguous tropical rainforest and second largest river basin in the world, the Congo Basin has a significant role to play in the global carbon (C) cycle. For the present day, it has been shown that a significant proportion of global terrestrial net primary productivity (NPP) is transferred laterally to the land–ocean aquatic continuum (LOAC) as dissolved CO2 , dissolved organic carbon (DOC), and particulate organic carbon (POC). Whilst the importance of LOAC fluxes in the Congo Basin has been demonstrated for the present day, it is not known to what extent these fluxes have been perturbed historically, how they are likely to change under future climate change and land use scenarios, and in turn what impact these changes might have on the overall C cycle of the basin. Here we apply the ORCHILEAK model to the Congo Basin and estimate that 4 % of terrestrial NPP (NPP = 5800±166 Tg C yr -1) is currently exported from soils and vegetation to inland waters. Further, our results suggest that aquatic C fluxes may have undergone considerable perturbation since 1861 to the present day, with aquatic CO2 evasion and C export to the coast increasing by 26 % (186±41 to 235±54 Tg C yr -1) and 25 % (12±3 to 15±4 Tg C yr -1), respectively, largely because of rising atmospheric CO2 concentrations. Moreover, under climate scenario RCP6.0 we predict that this perturbation could continue; over the full simulation period (1861–2099), we estimate that aquatic CO2 evasion and C export to the coast could increase by 79 % and 67 %, respectively. Finally, we show that the proportion of terrestrial NPP lost to the LOAC could increase from approximately 3 % to 5 % from 1861–2099 as a result of increasing atmospheric CO2 concentrations and climate change. However, our future projections of the Congo Basin C fluxes in particular need to be interpreted with some caution due to model limitations. We discuss these limitations, including the wider challenges associated with applying the current generation of land surface models which ignore nutrient dynamics to make future projections of the tropical C cycle, along with potential next steps. [ABSTRACT FROM AUTHOR]

We report here an increase in the amount of carbon dioxide in Taal lake during the year preceding the January 2020 eruption. Starting in February 2019, the CO2 emissions from the lake increased from background value (700 t day−1) to a flux close to 2,400 t day−1 at the time of the eruption. We show that the lake acts as a highly sensitive gasometer where CO2 (aq) reflects the balance between CO2 supplied to the lake (by hot springs) and CO2 lost by diffusion at air‐water interface. The lake waters are extremely enriched in dissolved carbon dioxide with pCO2 values as high as 0.35 atm (350,000 ppmV) equivalent to a CO2(aq) of 9.32 mmol l−1. The residence time of CO2 in the lake is around 1 week which allows for fast detection of change in magma degassing and makes carbon dioxide a very promising tool for volcano monitoring. Plain Language Summary: Volcanic gas fuels volcanic eruptions. Among the dominant species, sulfur dioxide is easily monitored using remote sensing and ground‐based instruments. Carbon dioxide remains however challenging to detect and quantify due to its high background in the atmosphere. We have developed a new methodology for monitoring CO2 by using a volcanic lake in an innovative way. It allows to quantify CO2 emissions by the lake, thereby providing critical insights into the change in degassing magma at depth. Using this new methodology, we were able to detect the earlier precursory signs of the January 2020 eruption of Taal volcano in the Philippines which affected over 380,000 people from local communities. The Taal volcanic lake is a unique location to study CO2 emission with spectacular degassing from more than 500 subaqueous vents that were active for many years. The lake waters are extremely enriched in carbon dioxide, to our knowledge, the highest value ever recorded in a lake. Our method also provides the longest time series to date of continuous CO2 emission from a volcano. Key Points: pCO2 in a volcanic lake reflects the balance between magmatic CO2 supplied by hot springs and CO2 lost by diffusion to the atmosphereThe continuous monitoring of CO2 flux by the volcanic lake is achieved using in situ measurement of lake pCO2 with a unique sensorThe increase in pCO2 in Taal volcanic lake is a precursory sign of the January 2020 eruption [ABSTRACT FROM AUTHOR]

Variability in greenhouse gas emissions from reservoirs creates uncertainty in global estimates of C emissions from reservoirs. This study examines the temporal and spatial variability in CO2 and CH4 surface water concentrations and diffusive fluxes from an Amazonian reservoir using an original data set combining both a high temporal (1 central site × 22 years) and spatial (44 sites × 1 season) resolution monitoring. The gas concentrations at the central site decreased over time and suggested reduced bioavailability of C in the initial flooded soil but exhibited strong seasonal variation. Not accounting for this variability may result in uncertainties in estimates of annual concentrations (ranging from −68.9% to +260% for CH4 and from −71.5% to +156% for CO2) and thus in estimates of diffusive gas emissions. Gas concentrations and diffusive fluxes exhibited high spatial variability in the reservoir, 24 years after impoundment. In particular, diffusive fluxes were higher in littoral and transitional areas than in open areas, suggesting a large contribution of allochthonous C to current gaseous emissions. Not accounting for this spatial variability in diffusive fluxes may underestimate the total emissions expressed in CO2 equivalents from the whole reservoir by 50.7%. Our study stresses the importance of well‐resolved temporal and spatial monitoring to provide reliable estimated of C emissions and a comprehensive understanding of the processes involved; both of these inputs are needed to support decision‐making for developing energy strategies. Plain Language Summary: While touted as a low carbon source of electricity, some tropical hydropower reservoirs were reported to emit substantial quantities of dioxide carbon and methane. Tropical reservoirs, especially those located in the lowland reaches of Amazon basin, are the greatest reservoir emitters on the planet. Despite this, hundreds of dams are set to be constructed in these areas. In the context of global warming and the expanding growth of hydroelectricity, the quantification of greenhouse gas emissions from tropical reservoirs is recognized as a priority. Yet emissions vary greatly in time and space. Such variability has to be addressed to provide reliable estimates of carbon emissions from reservoirs and to guide energy decision‐making. With long‐term monitoring of a 24 year old Amazonian reservoir, we reported high gaseous concentrations in the first years following impoundment due to the mineralization of the large stock of carbon coming from the flooded rainforest. The past 10 years, concentrations have significantly decreased with the reduced bioavailability of flooded carbon. Yet the variability of gaseous concentrations within the reservoir reveals the importance of carbon inputs coming from numerous tributaries and the surrounding rainforest for explaining long‐term emissions from the reservoir. Not accounting for such spatial variability may lead to significant underestimates of gross greenhouse emissions from Amazonian reservoirs. Key Points: Petit‐Saut reservoir exhibited high temporal and spatial variability in dissolved CO2 and CH4 concentrationsVariability in dissolved gas concentrations was likely related to heterogeneity of carbon sources that also vary in time and spaceTransitional and littoral areas exhibited higher C emissions than open areas suggesting the importance of allochthonous inputs for the current gross emissions [ABSTRACT FROM AUTHOR]

Evaporation of water bodies is significantly controlled by the available energy (net radiation Rn minus heat storage G). Compared to deep lakes or lakes in the high‐latitude or high‐altitude regions, the heat storage effects of shallow lakes in subtropical regions are less well studied. In this paper, we tested the heat storage effect on the latent heat fluxes estimation using the Priestley‐Taylor (PT) equation and numerical water temperature simulations in an ephemeral and shallow lake, Poyang Lake, in China. Results showed that the estimation bias was significantly correlated with heat storage changes (r = 0.97). Overlooking the impacts of the heat storage changes will induce an increase in the RMSE (root‐mean‐square error) of the evaporation estimation from 10.1 to 54.4 W m−2. Evaporation was overestimated in June and underestimated in August because the daytime Rn tended to be stored in water in June, whereas the heat storage during daytime tended to be released as nighttime evaporation in August, when the vapor gradient between the saturated surface and the air was generally larger than in other months. Water temperature simulations showed that the diurnal changes in the heat storage in the layer that were less than 2 m deep govern G of the total water column. PT estimation of a shallow lake can be improved if the temperature of the top 2 m of water is known. Also, from the modeling perspective, because G was linearly correlated with Rn (r = 0.77), a simple equation (a*Rn + b) that approximates the available energy can improve the evaporation estimate. Key Points: We study the heat storage effect on the PT estimation in an ephemeral and shallow lake, Poyang LakeThe overlook of heat storage induced the RMSE of evaporation estimation increased over three timesThe conversion of heat storage in the water into nighttime LE was the key for evaporation estimation [ABSTRACT FROM AUTHOR]

High stocking density aquaculture can result in major environmental impacts such as waste disposal with high organic matter. In order to overcome this issue, biofloc technology (BFT) has arisen as an alternative to lower effluent nutrient loading by promoting dense microbial communities that control water quality through the assimilation of inorganic nutrients. Given the central role of microorganisms in nutrients storage, the aim of this study was to evaluate the elemental composition of bioflocs and free‐living microorganisms and to determine which environmental factors are associated with the variation in their stoichiometric ratios (C:N:P) in shrimp farming systems. Samples were taken in marine shrimp farms, and the stoichiometric ratios from bioflocs and free‐living microorganisms were analysed, as well as physical and chemical water quality variables. The C:N ratios were lower, and N:P ratios were higher in the free‐living microorganisms than bioflocs. The C:P and N:P ratios of the biofloc were lower at higher temperature and total suspended solids (TSS). We concluded that the elemental composition of the bioflocs is richer in C and P and that systems with higher temperature and higher TSS have a greater ability to absorb and immobilize these elements in the bioflocs. [ABSTRACT FROM AUTHOR]

In this paper, we quantify the CO2 and N2O emissions from denitrification over the Amazonian wetlands. The study concerns the entire Amazonian wetland ecosystem with a specific focus on three floodplain (FP) locations: the Branco FP, the Madeira FP and the FP alongside the Amazon River. We adapted a simple denitrification model to the case of tropical wetlands and forced it by open water surface extent products from the Soil Moisture and Ocean Salinity (SMOS) satellite. A priori model parameters were provided by in situ observations and gauging stations from the HYBAM Observatory. Our results show that the denitrification and the trace gas emissions present a strong cyclic pattern linked to the inundation processes that can be divided into three distinct phases: activation, stabilization and deactivation. We quantify the average yearly denitrification and associated emissions of CO2 and N2O over the entire watershed at 17.8 kgN ha -1 yr -1 , 0.37 gC- CO2 m -2 yr -1 and 0.18 gN- N2O m -2 yr -1 respectively for the period 2011–2015. When compared to local observations, it was found that the CO2 emissions accounted for 0.01 % of the integrated ecosystem, which emphasizes the fact that minor changes to the land cover may induce strong impacts on the Amazonian carbon budget. Our results are consistent with the state of the art of global nitrogen models with a positive bias of 28 %. When compared to other wetlands in different pedoclimatic environments we found that the Amazonian wetlands have similar emissions of N2O with the Congo tropical wetlands and lower emissions than the temperate and tropical anthropogenic wetlands of the Garonne (France), the Rhine (Europe) and south-eastern Asia rice paddies. In summary our paper shows that a data-model-based approach can be successfully applied to quantify N2O and CO2 fluxes associated with denitrification over the Amazon basin. In the future, the use of higher-resolution remote sensing products from sensor fusion or new sensors like the Surface Water and Ocean Topography (SWOT) mission will permit the transposition of the approach to other large-scale watersheds in tropical environments. [ABSTRACT FROM AUTHOR]

The breakdown of allochthonous plants is of great importance in providing energy and nutrients in streams. In this regard, shredder macroinvertebrates play an important role in decomposing organic matter. Changes in land use strongly influence the type of material entering a stream, which ultimately affects the food chain dependent on this material. This study compared the decay of C3 (Mollinedia schottiana) and C4 (Brachiaria brizantha) plants in the montane Atlantic Forest of Brazil using litterbag experiments in two streams draining watersheds of different land uses. Concomitantly, we investigated the colonisation and assimilation of these plant detritus by aquatic macroinvertebrates. The breakdown of C4 plants in the forest stream was faster than that of C3 plants; however, aquatic macroinvertebrates did not assimilate the C4 carbon. These results support other studies that have shown a greater abundance of shredders in montane tropical streams (lower temperature) than in lowland streams. Moreover, the findings of this study support the view that changes in land use alter the structure of the benthic community, and that these changes can alter the leaf breakdown process. We compared aquatic leaf breakdown between an arboreal and a grass species in two streams of the Brazilian Atlantic Forest, one draining mostly forests and the other draining mostly pasture lands. Grass breakdown was faster than that of arboreal plants in the forest stream, but not in the pasture stream. In both streams, aquatic invertebrates assimilated more forest leaves than grass leaves. [ABSTRACT FROM AUTHOR]

To get a deep understand of the carbon cycle of Tibetan lakes, the CO2 flux over Ngoring Lake (NL), a large freshwater lake in the source region of the Yellow River, was monitored from 2011 to 2013 using an eddy covariance system. Based on the monthly mean CO2 flux, NL was a significant sink of atmospheric CO2 in 2012, with the greatest negative (downward) CO2 flux of −0.83 (±0.27) g C m−2 d−1 occurring in October. While in 2011 and 2013, NL was in quasi‐equilibrium with atmospheric CO2. The process controlling the daily variation in the CO2 flux was investigated using statistical analysis. The daily mean of the surface wind speed was significantly correlated with the observed daily CO2 flux. Under the assumption that the vertical gradient in the CO2 concentration near the water surface varied slowly, this correlation confirms the rationality of using the surface wind to express the gas transfer velocity. Following this consideration, because the thermal contrast between NL and the surrounding grassland was also significantly correlated with the daily CO2 flux, a new parameterization scheme of k600 is suggested in this study. Potential mechanisms for the coupling process between the surface wind and the thermal contrast of the lake are discussed. Based on the surface energy balance analysis, the heat storage rate of the lake played different roles in the CO2 fluxes during the three years, depending on the CO2 saturation of the lake water. The process controlling the monthly mean CO2 flux was also investigated. Plain Language Summary: There are more than 1,500 lakes on the Tibetan Plateau. Most of these lakes have not been thoroughly investigated, so little is known about the features of their carbon dynamics. To gain a better understand of the CO2 exchange over these lakes, an observation experiment was conducted over Ngoring Lake (a fresh water lake, with an area of 610.7 km2) from 2011 to 2013, and valuable data were obtained. By investigating these observations, we determined that the CO2 flux was negative (downward) during most of 2012, but it was in quasi‐equilibrium in 2011 and 2013. Second, the surface temperature difference between the lake and the surrounding grasslands may have largely controlled the daily variation in the gas transfer velocity near the water's surface. Because the surface temperature around a lake can be conveniently and accurately monitored by satellite, this finding may be useful in the estimation of the surface CO2 flux over other remote lakes worldwide. Key Points: Ngoring Lake significantly sequestrated atmospheric CO2 in 2012, with the smallest CO2 flux of −0.83 g C m−2 d−1 in OctoberIf any atmospheric variable has an effect with k600, its daily mean may be highly correlated with the CO2 fluxThe thermal contrast of a lake can satisfactorily express the daily variation in gas transfer velocity [ABSTRACT FROM AUTHOR]

The article presents a study which examined the diel variations in carbon dioxide (CO2) concentrations and exchanges with the atmosphere in the flooded forests in the Amazon basin. Also cited are the estimated air-water gas exchange, gas transfer velocities, and mean daily CO2 fluxes that were recorded in the research.

Carbon dioxide (CO2 ) and methane (CH4 ) were monitored at five sites spanning the upstream–downstream extent of the Mackenzie Delta channel network during May 2010, capturing the historically under-sampled ice-out period that includes the rising freshet, peak water levels and the early falling freshet (flood recession). Unexpectedly, partial pressures of CO2 in the Mackenzie River were undersaturated during the rising freshet before water levels peaked, indicating net CO2 invasion at instantaneous CO2 flux rates (F-CO2 ) ranging from –112 to –258 mg-C m-2 d-1. Net CO2 invasion was also observed around the time of peak water levels at sites in the middle and outer delta. Following peak water levels, the Mackenzie River switched to saturation and net CO2 evasion (F-CO2 from 74 to 177 mg-C m-2 d-1). Although the Peel River (which flows into the west side of the Mackenzie Delta) was a strong emitter of CO2 (F-CO2 from 373 to 871 mg-C m-2 d-1), overall, the Mackenzie River and Delta were weak emitters of CO2 during the 2010 ice-out period. All sites were strong emitters of CH4 during ice-out, however, with the highest evasive fluxes observed in the outer delta when the extent of flooded delta landscape was greatest. Estimated aerial fluxes from Mackenzie Delta channel surfaces during May 2010 ranged from 2.1 to 4.8 Gg-C as CO2, and 186 to 433 Mg-C as CH4 . These results provide critical information that can be used to refine gas flux estimates in high-latitude circumpolar river deltas during the relatively under-studied ice-out period. [ABSTRACT FROM AUTHOR]

Aromatic amines found in tobacco smoke are carcinogenic to humans. There are no doubts that they cause of cancers in the lungs, bladder, kidneys, pancreas, esophagus, larynx, pharynx, and oral cavities associated with public health. Therefore, monitoring, control, and awareness toward the smoking effects are important subjects to be conveyed. Thus, rapid, sensitive, simple and accurate analytical methods for the identification of aromatic amines and their metabolites are required to provide a clear and complete visualization for the occurrence of aromatic amines by investigating the urine of smokers and nonsmokers. This comprehensive review serves to give an overview of the previous and recent studies about the analytical trends for the determination of urinary aromatic amines. This review covers the sampling methodologies and sample preparation techniques such as solvent extraction, solid phase extraction, magnetic solid phase extraction, and solid phase microextraction couple with different separation methods including gas chromatography and liquid chromatography. To add more value to this review paper, the advantages, disadvantages, challenges, and the future prospects of these methods are discussed as well. This review is hopefully beneficial for researchers to access and monitor the level of potentially carcinogenic aromatic amines in human urine. [ABSTRACT FROM AUTHOR]

We carried out 10 field expeditions between 2010 and 2015 in the lowland part of the Congo River network in the eastern part of the basin (Democratic Republic of the Congo), to describe the spatial variations in fluvial dissolved carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) concentrations. We investigate the possible drivers of the spatial variations in dissolved CO2 , CH4 and N2O concentrations by analyzing covariations with several other biogeochemical variables, aquatic metabolic processes (primary production and respiration), catchment characteristics (land cover) and wetland spatial distributions. We test the hypothesis that spatial patterns of CO2 , CH4 and N2O are partly due to the connectivity with wetlands, in particular with a giant wetland of flooded forest in the core of the Congo basin, the "Cuvette Centrale Congolaise" (CCC). Two transects of 1650 km were carried out from the city of Kisangani to the city of Kinshasa, along the longest possible navigable section of the river and corresponding to 41 % of the total length of the main stem. Additionally, three time series of CH4 and N2O were obtained at fixed points in the main stem of the middle Congo (2013–2018, biweekly sampling), in the main stem of the lower Kasaï (2015–2017, monthly sampling) and in the main stem of the middle Oubangui (2010–2012, biweekly sampling). The variations in dissolved N2O concentrations were modest, with values oscillating around the concentration corresponding to saturation with the atmosphere, with N2O saturation level (% N2O , where atmospheric equilibrium corresponds to 100 %) ranging between 0 % and 561 % (average 142 %). The relatively narrow range of % N2O variations was consistent with low NH4+ (2.3±1.3 µ mol L -1) and NO3- (5.6±5.1 µ mol L -1) levels in these near pristine rivers and streams, with low agriculture pressure on the catchment (croplands correspond to 0.1 % of catchment land cover of sampled rivers), dominated by forests (∼70 % of land cover). The covariations in % N2O , NH4+ , NO3- and dissolved oxygen saturation level (% O2) indicate N2O removal by soil or sedimentary denitrification in low O2 , high NH4+ and low NO3- environments (typically small and organic matter rich streams) and N2O production by nitrification in high O2 , low NH4+ and high NO3- (typical of larger rivers that are poor in organic matter). Surface waters were very strongly oversaturated in CO2 and CH4 with respect to atmospheric equilibrium, with values of the partial pressure of CO2 (pCO2) ranging between 1087 and 22 899 ppm (equilibrium ∼400 ppm) and dissolved CH4 concentrations ranging between 22 and 71 428 nmol L -1 (equilibrium ∼2 nmol L -1). Spatial variations were overwhelmingly more important than seasonal variations for pCO2 , CH4 and % N2O as well as day–night variations for pCO2. The wide range of pCO2 and CH4 variations was consistent with the equally wide range of % O2 (0.3 %–122.8 %) and of dissolved organic carbon (DOC) (1.8–67.8 mg L -1), indicative of generation of these two greenhouse gases from intense processing of organic matter either in "terra firme" soils, wetlands or in-stream. However, the emission rate of CO2 to the atmosphere from riverine surface waters was on average about 10 times higher than the flux of CO2 produced by aquatic net heterotrophy (as evaluated from measurements of pelagic respiration and primary production). This indicates that the CO2 emissions from the river network were sustained by lateral inputs of CO2 (either from terra firme or from wetlands). The pCO2 and CH4 values decreased and % O2 increased with increasing Strahler order, showing that stream size explains part of the spatial variability of these quantities. In addition, several lines of evidence indicate that lateral inputs of carbon from wetlands (flooded forest and aquatic macrophytes) were of paramount importance in sustaining high CO2 and CH4 concentrations in the Congo river network, as well as driving spatial variations: the rivers draining the CCC were characterized by significantly higher pCO2 and CH4 and significantly lower % O2 and % N2O values than those not draining the CCC; pCO2 and % O2 values were correlated to the coverage of flooded forest on the catchment. The flux of greenhouse gases (GHGs) between rivers and the atmosphere averaged 2469 mmol m -2 d -1 for CO2 (range 86 and 7110 mmol m -2 d -1), 12 553 µ mol m -2 d -1 for CH4 (range 65 and 597 260 µ mol m -2 d -1) and 22 µ mol m -2 d -1 for N2O (range - 52 and 319 µ mol m -2 d -1). The estimate of integrated CO2 emission from the Congo River network (251±46 TgC (10 12 gC) yr -1), corresponding to nearly half the CO2 emissions from tropical oceans globally (565 TgC yr -1) and was nearly 2 times the CO2 emissions from the tropical Atlantic Ocean (137 TgC yr -1). Moreover, the integrated CO2 emission from the Congo River network is more than 3 times higher than the estimate of terrestrial net ecosystem exchange (NEE) on the whole catchment (77 TgC yr -1). This shows that it is unlikely that the CO2 emissions from the river network were sustained by the hydrological carbon export from terra firme soils (typically very small compared to terrestrial NEE) but most likely, to a large extent, they were sustained by wetlands (with a much higher hydrological connectivity with rivers and streams). [ABSTRACT FROM AUTHOR]

The Belo Monte hydropower complex located in the Xingu River is the largest run-of-the-river (ROR) hydroelectric system in the world and has one of the highest energy production capacities among dams. Its construction received significant media attention due to its potential social and environmental impacts. It is composed of two ROR reservoirs: the Xingu Reservoir (XR) in the Xingu's main branch and the Intermediate Reservoir (IR), an artificial reservoir fed by waters diverted from the Xingu River with longer water residence time compared to XR. We aimed to evaluate spatiotemporal variations in CO2 partial pressure (pCO2) and CO2 fluxes (FCO2) during the first 2 years after the Xingu River impoundment under the hypothesis that each reservoir has contrasting FCO2 and pCO2 as vegetation clearing reduces flooded area emissions. Time of the year had a significant influence on pCO2 with the highest average values observed during the high-water season. Spatial heterogeneity throughout the entire study area was observed for pCO2 during both low- and high-water seasons. FCO2 , on the other hand, only showed significant spatial heterogeneity during the high-water period. FCO2 (0.90±0.47 and 1.08±0.62 µ mol m 2 d -1 for XR and IR, respectively) and pCO2 (1647±698 and 1676±323 µ atm for XR and IR, respectively) measured during the high-water season were on the same order of magnitude as previous observations in other Amazonian clearwater rivers unaffected by impoundment during the same season. In contrast, during the low-water season FCO2 (0.69±0.28 and 7.32±4.07 µ mol m 2 d -1 for XR and IR, respectively) and pCO2 (839±646 and 1797±354 µ atm for XR and IR, respectively) in IR were an order of magnitude higher than literature FCO2 observations in clearwater rivers with naturally flowing waters. When CO2 emissions are compared between reservoirs, IR emissions were 90 % higher than values from the XR during low-water season, reinforcing the clear influence of reservoir characteristics on CO2 emissions. Based on our observations in the Belo Monte hydropower complex, CO2 emissions from ROR reservoirs to the atmosphere are in the range of natural Amazonian rivers. However, the associated reservoir (IR) may exceed natural river emission rates due to the preimpounding vegetation influence. Since many reservoirs are still planned to be constructed in the Amazon and throughout the world, it is critical to evaluate the implications of reservoir traits on FCO2 over their entire life cycle in order to improve estimates of CO2 emissions per kilowatt for hydropower projects planned for tropical rivers. [ABSTRACT FROM AUTHOR]

The explosive multiplication of phytoplankton caused by water eutrophication often occur in the intensive shrimp aquaculture. To comprehensively assess the diversity and community structure of phytoplankton in the waters of typical indoor industrial aquaculture system for Litopenaeus vannamei, a combination of high‐throughput sequencing and morphological identification methods were used in the present study. A total of 41 genera belong to nine phyla were detected by both methods. Chlorophyta, Cyanophyta and Bacillariophyta were found to be three dominant phyla. The high‐throughput sequencing revealed that green algae and cyanobacteria were the most dominant phytoplankton; however, diatoms were the first dominant phytoplankton by using the morphological identification. At the genus level, Picochlorum and Synechococcus were dominant, accounting for 20.94%–97.19% and 0.01%–52.81% of total phytoplankton, revealed by the high‐throughput sequencing. Therefore, more attention should be paid to their ecological impacts on the surrounding sea areas or potential toxicity to shrimp. Cyclotella was the most dominant genus revealed by the morphological identification. High‐throughput sequencing revealed a high diversity and small‐sized phytoplankton which were undetected by microscopy. Both methods provide similar information on the environmental drivers of phytoplankton community. NO3−, NH4+, DIP, DSi, DON and DOP concentrations were the main factors influencing the phytoplankton community structure and diversity. [ABSTRACT FROM AUTHOR]

Impounding alters the carbon transport in rivers. To quantify this effect, we measured CO2 effluxes from a mountainous valley-type reservoir in the upper Mekong River (known as Lancang River in China). CO2 evasion rates from the reservoir surface were 408±337 mg CO2 m -2 d -1 in the dry season and 305±262 mg CO2 m -2 d -1 in the rainy season much lower than those from the riverine channels (1567±2312 mg CO2 m -2 d -1 at the main stem and 905±1536 mg CO2 m -2 d -1 at the tributary). Low effluxes in the pelagic area were caused by low allochthonous organic carbon (OC) inputs and photosynthetic uptake of CO2. A negative relationship between CO2 efflux and water temperature suggested CO2 emissions at the pelagic area were partly offset by photosynthesis in the warmer rainy season. CO2 emissions from the reservoir outlet and littoral area, which were usually considered hotspots of CO2 emissions, had a low contribution to the total emission because of epilimnion water spilling and a small area of the littoral zones. Yet at the river inlets effluxes were much higher in the dry season than in the rainy season because different mixing modes occurred in the two seasons. When the river joined the receiving waterbody in the dry season, warmer and lighter inflow became an overflow and large amounts of CO2 were released to the atmosphere as the overflow contacted the atmosphere directly. Extended water retention time due to water storage might also help mineralization of OC. In the wet season, however, colder, turbid and heavier inflow plunged into the reservoir and was discharged downstream for hydroelectricity, leaving insufficient time for decomposition of OC. Besides, diurnal efflux variability indicated that the effluxes were significantly higher in the nighttime than in the daytime, which increased the estimated annual emission rate by half. [ABSTRACT FROM AUTHOR]

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Greenhouse gas evasion from inland waters is a globally significant yet highly uncertain flux, especially in regard to effects of wetlands and hydrologic variability. We sampled five first‐order and two second‐order streams with variable wetland influence during storm events for dissolved CO2, CH4, and N2O. We also calculated gas evasion rates. In first‐order streams, pCO2 and pN2O were significantly higher in the stream with the most wetland influence (mean ± 1 std: 3,965 ± 1,504 and 1.18 ± 0.37 μatm, respectively) than the forested stream (2,927 ± 439 and 0.47 ± 0.08 μatm, respectively). In second‐order streams, pCO2, pCH4, and pN2O were higher in the 14% wetland stream (3,274 ± 825, 501 ± 207, and 1.37 ± 0.43 μatm, respectively) than in the 2% wetland stream (1,858 ± 423, 137 ± 53, and 0.37 ± 0.08 μatm, respectively). In first‐order streams, pCO2 in streams with wetland influence increased during rain events, while pCO2 in streams with little to no wetland influence decreased or remained constant. Generally, pCH4 and pN2O followed the same trend, except in one stream with intermediate wetland influence. Gas transfer velocity increased in all streams during storm events. However, the forested streams had higher gas transfer velocities than the wetland streams due to steeper topography. CO2, CH4, and N2O evasion peaked in one of the intermediate wetland streams at high flow (maximum: 66 g C·m−2·day−1, 177 mg C·m−2·day−1, and 9.7 mg N·m−2·day−1, respectively). These findings suggest that gases are shunted downstream in flatter, wetland streams, while gases are evaded closer to their source in steeper, forested streams. Plain Language Summary: Greenhouse gas evasion from inland waters is an important component of the global carbon cycle. However, this flux has only just been included in the most recent IPCC carbon cycle (AR5). Headwater streams are areas where gases, such as carbon dioxide, methane, and nitrous oxide, tend to collect and are released to the atmosphere. Gas dynamics in headwater streams must be understood to improve global evasion estimates. We sampled streams covering a gradient of wetland to forest land covers during rainstorms to examine the effects on gas evasion. We found that gas concentrations generally increased during storms in wetland streams, while concentrations tended to decrease or remain constant in forested streams. However, we found that gases were more easily evaded from forested streams than from wetland streams. Forested streams tend to be found in steeper areas and have more turbulent flows than wetland streams, which promote evasion. Conversely, gas inputs to wetland streams are carried downstream and released farther from the source. The significance of this work is that greenhouse gas evasion from headwater streams is a spatially and temporally variable flux from the landscape. Inclusion of this variability in global models may support higher global evasion estimates. Key Points: Carbon dioxide, methane, and nitrous oxide fluxes are spatially and temporally variable in part due to land cover and storm dynamicsAqueous pCO2 increases with rain in wetland streams and decreases or remains constant in forest streamsWetland streams have lower gas transfer velocity than forest streams due to flatter topography, which favors downstream transport [ABSTRACT FROM AUTHOR]

At the global scale, inland waters are a significant source of atmospheric carbon (C), particularly in the tropics. The active pipe concept predicts that C emissions from streams, lakes and rivers are largely fuelled by terrestrial ecosystems. The traditionally recognized C transfer mechanisms from terrestrial to aquatic systems are surface runoff and groundwater drainage. We present here a series of arguments that support the idea that land flooding is an additional significant process that fuels inland waters with C at the global scale. Whether the majority of CO2 emitted by rivers comes from floodable land (approximately 10 % of the continents) or from well-drained land is a fundamental question that impacts our capacity to predict how these C fluxes might change in the future. Using classical concepts in ecology, we propose, as a necessary step forward, an update of the active pipe concept that differentiates floodable land from drained land. Contrarily to well-drained land, many wetlands (in particular riparian and littoral wetlands) combine strong hydrological connectivity with inland waters, high productivity assimilating CO2 from the atmosphere, direct transfer of litter and exudation products to water and waterlogged soils, a generally dominant allocation of ecosystem respiration (ER) below the water surface and a slow gas-exchange rate at the water–air interface. These properties force plants to pump atmospheric C to wetland waters and, when hydrology is favourable, to inland waters as organic C and dissolved CO2. This wetland CO2 pump may contribute disproportionately to CO2 emissions from inland waters, particularly in the tropics where 80 % of the global CO2 emissions to the atmosphere occur. In future studies, more care must be taken in the way that vertical and horizontal C fluxes are conceptualized along watersheds, and 2-D models that adequately account for the hydrological export of all C species are necessary. In flooded ecosystems, significant effort should be dedicated to quantifying the components of primary production and respiration by the submerged and emerged part of the ecosystem community and to using these metabolic rates in coupled hydrological–biogeochemical models. The construction of a global typology of wetlands that includes productivity, gas fluxes and hydrological connectivity with inland waters also appears necessary to adequately integrate continental C fluxes at the global scale. [ABSTRACT FROM AUTHOR]