Dissolved Inorganic Carbon Concentrations Research Articles

Riverine carbon fluxes to the South China Sea

AbstractThe high precipitation around Southeast Asia results in abundant freshwater outflow, which transports terrestrial dissolved and particulate material to the world's largest marginal sea, namely, the South China Sea (SCS). To estimate the riverine carbon flux to the SCS, carbonate data from 42 rivers were collected. These results, combined with literature data for 13 rivers, indicate that the concentrations of dissolved inorganic carbon (DIC) are positively correlated with the cation exchange capacity and the bulk density, except for the Low Discharge category. The highest DIC concentration and flux are in the Low and Medium Discharge categories. Negative correlations exist between dissolved organic carbon (DOC) concentrations and the base saturation in the Low and High Discharge categories and between DOC and bulk density in the Low and Medium Discharge categories. However, the correlation between the DOC concentration and the organic carbon content in the soil is only significant in the Medium Discharge category. Once the negative exponential relationships between the particulate inorganic carbon (PIC) or the particulate organic carbon (POC) and the total suspended matter are determined, these can be used to estimate the PIC and POC carbon fluxes. Of the annual riverine carbon flux, 83.0 ± 8.1 Tg C, 40.14 ± 6.11 Tg is DIC, 25.03 ± 4.54 Tg is DOC, 1.83 ± 0.35 Tg is PIC, and 16.03 ± 2.87 Tg is POC. The total discharge amounts to 6.2–10.3% of the global riverine discharge for a sea covering only 1% of the world's ocean surface area and for watersheds covering only 2.2% of the global land mass.

A new version of the Hydrochemical Dystrophy Index to evaluate dystrophy in lakes

Dystrophic freshwaters referred to as humic, brown, or black waters are typical for boreal or some mountainous regions where fens and coniferous forests form a significant part of basins. I evaluated the usefulness of the Hydrochemical Dystrophy Index (HDI) which has been developed for dystrophic lakes in Poland, on the basis of a rich database for lakes in Sweden and data provided by other researchers for lakes from Finland and Russia. I propose a new version of HDI for synthetic and quantitative description of habitat conditions in lakes and for use in limnological monitoring of protected areas. The state of dystrophy was evaluated using data for surface water pH, electric conductivity (EC), DIC (dissolved inorganic carbon) and DOC (dissolved organic carbon) concentrations. HDI values between 50 and 65 indicate semidystrophic conditions and with advanced a real dystrophy, they reach HDI values from 65 up to 100. Long term data shows that lake dystrophy is fairly steady with seasonal fluctuations. I show that the level of lake dystrophy is not correlated to latitude, but rather to small lake areas, regional geochemical spots, and favorable local hydrological conditions. A summary description of the main parameters of various types of humic lakes is presented.

An exploiting of cost-effective direct current conductivity detector in gas diffusion flow injection system

In this work, a homemade direct current (DC) conductivity detector as an alternative cost-effective detection device has been fabricated and investigated to use in flow analysis system. Under the selected appropriate conditions of flow system, the electrolysis of a carrier stream at the conductivity detector was negligible and provides well-defined signal. The cost-effective DC conductivity detector was demonstrated to couple with gas diffusion flow injection system for determination of dissolved inorganic carbon (DIC) in water. The method is based on the conversion of DIC (dissolved CO2, HCO3- and CO32-) presented in the injected sample to carbon dioxide in an acidic donor stream and then CO2 gas diffuses through a hydrophobic porous membrane to an acceptor stream. As a result, the change of conductivity signal was observed corresponding to DIC concentration. A linear calibration range of DIC in 1.0–10mmolL−1, with limit of detection of 70µmolL−1, repeatability of <3% RSD and 15 injections h−1 sample throughput can be obtained. This method was applied for DIC determination in natural water.

MoDIE: Moderate dissolved inorganic carbon (DI13C) isotope enrichment for improved evaluation of DIC photochemical production in natural waters

Photochemical reactions in natural waters are a sink for dissolved organic carbon (DOC) and a source of dissolved inorganic carbon (DIC). Although considered significant to the carbon budget of the oceans, DIC photoproduction rates remain poorly constrained due to the analytical challenge involved in accurately measuring very low production rates (likely sub-μM h− 1 for blue ocean waters) relative to high background DIC concentrations (~ 2 mM). In an attempt to overcome this analytical limitation, almost all previous DIC photoproduction studies in marine systems have relied on the stripping of background DIC prior to irradiation, with unknown consequences for the integrity of the DOC pool and its photoreactivity, and none have resulted in a satisfactory determination of photoproduced DIC in open ocean waters. Here, we use small additions of NaH13CO3 (< 10% of background DIC concentrations) to achieve moderate DI13C isotope enrichment (MoDIE) of river, estuarine, and seawater samples. We then quantify the shift in δ13C of DIC in irradiated samples via liquid chromatography - isotope ratio mass spectrometry (LC-IRMS) and use these shifts to calculate photoproduced DIC. MoDIE provides a new method to more precisely (± 56 to ± 340 nM DIC) and more rapidly quantify DIC photoproduction compared to previous methods. MoDIE was evaluated by determining initial DIC photoproduction rates, along with associated broadband photochemical efficiency data (ranging from 144 to 280 μmol DIC per mol photons absorbed), in riverine to offshore waters, and produced the earliest time point and most precise measurement of DIC photoproduction in unmodified, low-CDOM, blue water (ag(325) = 0.30 m− 1) reported to date. In addition to measuring small DIC changes from photochemistry, the use of MoDIE could provide a major advance for other ocean carbon cycling studies, including examination of oceanic respiration, carbonate mineral precipitation/dissolution, and assessment of ocean acidification.

Transport, anoxia and end-product accumulation control carbon dioxide and methane production and release in peat soils

Anaerobic respiration and methanogenesis have been found to slow-down in water saturated peat soils with accumulation of metabolic end-products, i.e. dissolved inorganic carbon (DIC) and methane (CH4), due to a lack of solute and gas transport. So far it is not well understood how solute and gas transport may control this effect. We conducted a column experiment with homogenized ombrotrophic peat over a period of 300 days at 20 °C. We specifically evaluated the effects of diffusive flux as control, downward advective water flux, intensified ebullition by conduit gas transport and diffusive oxygen supply on controlling anaerobic decomposition rates and carbon (C) turnover. To simulate advective flux, water and solutes were recirculated downward through the column after stripping of dissolved gases. We analyzed DIC and CH4 concentrations, production rates and fluxes, gas filled porosity, oxygen profiles (O2) and microbial C biomass over time. DIC residence time thereby served as proxy to characterize transport. A slowdown of anaerobic respiration and methanogenesis evolved with the accumulation of the end-products DIC and CH4 and set in after 150 days. This slow-down was accompanied by a decrease in the distribution of microbial biomass C with depths. Anaerobic DIC and CH4 production rates were fastest close to the water table and sharply slowed with depth. Accumulation of DIC and CH4 in the homogeneous peat material throughout the column decreased decomposition constants from about 10−5 near the surface to 10−9 year−1 deeper in the profile. Advective water transport extended the zone of active methanogenesis compared to a diffusive system; experimental enhancement of ebullition had little or no effect as well as strictly anoxic conditions. DIC residence time was negatively correlated to anaerobic respiration suggesting this parameter to be a predictor of anaerobic peat decomposition in peatlands. Overall, this study suggests that burial of peat and accumulation of metabolic end-products effectively slows decomposition and that this effect needs to be considered to explain peat accumulation and the response of peat mineralization rates to changes in environmental conditions.

Technical note: Coupling infrared gas analysis and cavity ring down spectroscopy for autonomous, high-temporal-resolution measurements of DIC and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C–DIC

Abstract. A new approach to autonomously determine concentrations of dissolved inorganic carbon (DIC) and its carbon stable isotope ratio (δ13C–DIC) at high temporal resolution is presented. The simple method requires no customised design. Instead it uses two commercially available instruments currently used in aquatic carbon research. An inorganic carbon analyser utilising non-dispersive infrared detection (NDIR) is coupled to a Cavity Ring-down Spectrometer (CRDS) to determine DIC and δ13C–DIC based on the liberated CO2 from acidified aliquots of water. Using a small sample volume of 2 mL, the precision and accuracy of the new method was comparable to standard isotope ratio mass spectrometry (IRMS) methods. The system achieved a sampling resolution of 16 min, with a DIC precision of ±1.5 to 2 µmol kg−1 and δ13C–DIC precision of ±0.14 ‰ for concentrations spanning 1000 to 3600 µmol kg−1. Accuracy of 0.1 ± 0.06 ‰ for δ13C–DIC based on DIC concentrations ranging from 2000 to 2230 µmol kg−1 was achieved during a laboratory-based algal bloom experiment. The high precision data that can be autonomously obtained by the system should enable complex carbonate system questions to be explored in aquatic sciences using high-temporal-resolution observations.

Continuous CO2 escape from the hypersaline Dead Sea caused by aragonite precipitation

Chemical precipitation of CaCO3 occurs in diverse marine and lacustrine environments. In the hypersaline Ca-chloride lakes that have been occupying the Dead Sea basin since the late Pleistocene, CaCO3 precipitated, mostly as aragonite. The aragonite sediments precipitated mainly during periods of high lake level stands as a result of mixing of bicarbonate-rich freshwater runoff with Dead Sea brine, that is Ca-rich and have high Mg/Ca ratio. During periods of arid conditions with limited freshwater inflow, water level declined, salinity increased and gypsum and halite became the dominant evaporitic minerals to precipitate.The present study investigates the carbon cycle of the Dead Sea under the current limited water and bicarbonate supply to the brine, representing periods of extremely arid conditions. The decrease of inflows to the Dead Sea in recent years stems mainly from diversion of freshwater from the drainage basin and results in dramatic water level decline and massive halite precipitation. During 2013–2014, bi-monthly depth profiles of total alkalinity, dissolved inorganic carbon (DIC) and its isotopic composition (δ13C) were conducted in the Dead Sea, from surface down to the bottom of the lake (290m). Mass balance calculations conducted for the period 1993–2013 show that while inventories of conservative ions such as Mg2+ remained constant, the net DIC inventory of the lake decreased by ∼10%. DIC supply to the lake during this period, however, amounted to ∼10% of lake’s inventory indicating that during 20years, the lake lost ∼20% of its 1993s inventory. Compilation of historical data with our data shows that during the past two decades the lake's low DIC (∼1mmolkg−1) and very high PCO2 (1800ppmV) remained relatively constant, suggesting that a quasi-steady-state situation prevails. In spite of the surprisingly stable DIC and CO2 concentrations, during this 20year period δ13CDIC increased significantly, from 1.4‰ to 2.7‰. An isotopic mass balance calculation together with the high PCO2 of the brine show that during that period the lake lost about 3.7·1010 moles of DIC, of which ∼60% by CO2 degassing and ∼40% by aragonite precipitation. The deviation from the common 1:1 CO2:CaCO3 ratio in most aqueous systems is facilitated by the dominance of the borate alkalinity of the Dead Sea. Nucleation and crystal growth experiments suggest that throughout this time the Dead Sea remained supersaturated with respect to aragonite, a situation facilitated by combination of slow nucleation rates and absence of growth surfaces. The very high PCO2 of the Dead Sea is maintained by the low CO2 piston velocity of the brine, calculated to be only ∼4.5myr−1, more than an order of magnitude slower than seawater value.

Stable carbon isotopic composition of submerged plants living in karst water and its eco-environmental importance

The stable carbon isotopic composition of submerged plants (δ13CP) can be controlled by physiological and environmental factors. Herein, we took advantage of a short, natural karst river with an annual mean bicarbonate (HCO3−) value of 3.8mmolL−1 to study the stable carbon isotopic composition of submerged plants along the river and the influence of environmental conditions on the δ13CP values. The δ13CP values of Ottelia acuminata, Potamogeton wrightii, Vallisneria natans, and Hydrilla verticillata from upstream to downstream show a gradient and ranged from −34.8‰ to −27.8‰, −36.6‰ to −23.7‰, −35.1‰ to −25.3‰, and −38.6‰ to −26.3‰, respectively and even more depleted values for the first two species at the uppermost site. Diurnal variation of water chemistry and concentration of the dissolved inorganic carbon (DIC) and the stable carbon isotopic composition of DIC (δ13CD) indicate that the macrophytes and other primary producers in the river have a very high net photosynthetic rate. The gradient of δ13CP values was consistent with CO2 being a declining source of inorganic carbon for photosynthesis in the downstream transect. The results demonstrate that the high DIC concentration with lower negative δ13C value, particularly in karst water environment has a significant role in controlling the stable carbon isotopic composition of submerged plants living in it.

Stable isotope mass balances versus concentration differences of dissolved inorganic carbon – implications for tracing carbon turnover in reservoirs

ABSTRACTThe aim of this study was to identify sources of carbon turnover using stable isotope mass balances. For this purpose, two pre-reservoirs in the Harz Mountains (Germany) were investigated for their dissolved and particulate carbon contents (dissolved inorganic carbon (DIC), dissolved organic carbon, particulate organic carbon) together with their stable carbon isotope ratios. DIC concentration depth profiles from March 2012 had an average of 0.33 mmol L–1. Increases in DIC concentrations later on in the year often corresponded with decreases in its carbon isotope composition (δ13CDIC) with the most negative value of –18.4 ‰ in September. This led to a carbon isotope mass balance with carbon isotope inputs of −28.5 ‰ from DOC and −23.4, −31.8 and −30.7 ‰ from algae, terrestrial and sedimentary matter, respectively. Best matches between calculated and measured DIC gains were achieved when using the isotope composition of algae. This shows that this type of organic material is most likely responsible for carbon additions to the DIC pool when its concentrations and δ13CDIC values correlate negatively. The presented isotope mass balance is transferable to other surface water and groundwater systems for quantification of organic matter turnover.

Temporal and Spatial Variations of Dissolved Inorganic Carbon and Its Stable Isotopic Composition in the Surface Stream of Karst Groundwater Recharge

Stable carbon isotope of dissolved inorganic carbon (δ13CDIC), which is mainly constituted by HCO3- in karst water, is widely used to trace the different sources and influential factors of dissolved inorganic carbon (DIC). In order to understand the distribution of DIC and δ13CDIC in subtropical karst area, this paper researched the water chemistry and δ13CDIC in a karst surface stream in detail, which is fed by Guancun subterranean stream in Liuzhou City, Guangxi Province, in the southwest of China. The results showed that the contents of DIC in subterranean stream outlet (G1 site) ranged from 4.60 to 4.90 mmol·L-1 with an average of 4.73 mmol·L-1 in dry season, and from 2.80 to 4.70 mmol·L-1 with an average of 4.23 mmol·L-1 in rainy season. The contents of DIC in 1.35 km downstream site (G2 site) ranged from 4.30 to 4.90 mmol·L-1 with an average of 4.56 mmol·L-1 in dry season, and from 3.00 to 4.70 mmol·L-1 with an average of 4.20 mmol·L-1 in rainy season. The δ13CDIC of subterranean stream outlet (G1 site) varied from -12.8‰ to -11.53‰ with an average of -12.22‰ in dry season, and from -13.12‰ to -11.01‰ with an average of -12.28‰ in rainy season. The δ13CDIC of stream downstream site (G2 site) ranged from -11.71‰ to -9.55‰ with an average of -10.73‰ in dry season, and ranged from -12.18‰ to -9.85‰ with an average of -11.10‰ in rainy season. The contents of DIC of G1 site were higher than those of G2 site. The DIC contents in dry season in both G1 and G2 site were higher than those in rainy season. The values of δ13CDICof G1 and G2 site in dry season were more positive than those in rainy season. The δ13CDICvalue of G1 site was more negative than that of G2 site. The main sources of DIC in underground river and surface stream were soil CO2and carbonate dissolution. However, the differences of DIC and δ13CDICbetween G1 and G2 site showed that CO2degassing and photosynthesis of aquatic plants had significant influence on water DIC and δ13CDIC value. This study is helpful to understand the dynamic change and distribution of DIC and δ13CDIC in karst surface stream.

Carbonate system biogeochemistry in a subterranean estuary – Waquoit Bay, USA

Quantifying carbon fluxes associated with submarine groundwater discharge (SGD) remains challenging due to the complex biogeochemistry of the carbonate system in the subterranean estuary (STE). Here we conducted time series measurements of total alkalinity (TAlk) and dissolved inorganic carbon (DIC) in a well-studied coastal aquifer (Waquoit Bay, Massachusetts, USA). Groundwater samples were collected monthly from May 2009 to June 2010 across the freshwater-saltwater mixing zone of the Waquoit Bay (WB) STE. The concentrations of both TAlk and DIC in zero-salinity groundwater were variable, but were lower than those in the bay water (S∼28). DIC underwent slightly non-conservative mixing between low and intermediate salinities while there was an apparent additional DIC source at high salinity (>20) in all seasons. TAlk concentrations exhibited even stronger variations, with evidence of both production and consumption in high salinity zones, and consistent TAlk consumption at intermediate salinity in summer and fall (June–December, 2009). The increases in DIC and TAlk at high salinity were attributed to aerobic respiration and denitrification in WB sediments during bay water recharge of the STE. We infer that the loss of TAlk at intermediate salinity reflects H+ production as reduced compounds (e.g. Fe2+) are oxidized within the STE. In terms of impacts on surface water inorganic carbon budgets, the SGD-derived DIC flux was mainly controlled by seasonal changes in SGD while a combination of TAlk concentration variability and SGD drove the TAlk flux. SGD-derived DIC, aqueous CO2, and H+ fluxes to the bay were ∼40–50% higher in summer vs. in winter, a result of enhanced marine groundwater flux and significant TAlk removal (proton addition) during periods of high seawater intrusion. Furthermore, the SGD-derived DIC flux was consistently greater than TAlk flux regardless of season, indicating that SGD serves to reduce the CO2 buffering capacity of surface water. Our results highlight the importance of seasonality and subsurface biogeochemical processes on the subterranean estuary carbonate system and the resulting impact on SGD-derived TAlk, DIC, aqueous CO2, and H+ fluxes to the coastal ocean.

Dissolved Inorganic Carbon (DIC) Contents in Middle and Lower Reaches of Lancang River: Related to Water Environments and Dams

Carbon cycle is one of the focuses of climate change, river carbon is an important part, while dissolved inorganic carbon (DIC) has a high proportion of river carbon flux. In this study, we did the research on the Lancang River, an important international river in the southwest of China. Water samples were obtained from 16 sections of the middle and lower reaches of the Lancang River in 2016 (11 months), then we monitored some water quality indicators and DIC content, finally analyzed the temporal-spatial distribution characteristics of DIC and the relationship between DIC content and water environment factors. The results showed that: (1) DIC contents in the middle and lower reaches of the Lancang River varied from 1.1840 mmol/L to 3.1440 mmol/L, with a mean value of about 2.2155 mmol/L. (2) At a time scale, DIC contents of dry season (spring, autumn and winter) were higher than rainy season (summer). At a space scale, DIC contents of the middle and lower reaches of the Lancang River gradually decreased from north to south, and each reservoir had the same characteristics, that is, DIC contents at upstream of the dam was lower than those at downstream of the dam. Compared to other rivers with cascade dams around the world, DIC contents within studied river were at similar level. And the reservoir’s effect of the Lancang River were not obvious, however, DIC contents in the water sampled upstream the dams had a slight stratification. (3) Water temperature, conductivity, turbidity were important factors affecting DIC content of water, and the effect of oxidation and reduction potential (ORP) and pH on DIC was relatively small.

“Old” carbon entering the South China Sea from the carbonate-rich Pearl River Basin: Coupled action of carbonate weathering and aquatic photosynthesis

Generally, negative Δ14C values of riverine particulate organic carbon (POC) are interpreted as old carbon derived from the erosion of deep soils and sedimentary rocks. Here we present natural 14C and 13C data from the carbonate-rich Pearl River Basin that discharges into the South China Sea. We found that the Δ14C values of POC and DIC (dissolved inorganic carbon) transported by the carbonate-rich river are all negative. This, however, does not mean the POC is necessarily old but indicates control of carbonate weathering (producing “old” DIC with negative Δ14C values) coupled with contemporary aquatic photosynthesis (producing new autochthonous POC but with negative Δ14C values) through the “dead carbon” effect of carbonate rocks, which was further evidenced by particular seasonal change in Δ14C values of DIC and POC (both higher in the rainy season and lower in dry season), spatial variation (both getting higher downstream), and negative correlation between δ13C and “age” of POC. This finding indicates that previous studies suggesting that riverine POC depleted in 14C is old may be problematic in carbonate-dominated river basins. The finding that river basins rich in carbonates can release “old” POC may have important implications for the interpretation of organic carbon age in rivers and coastal oceans affected by the runoff from this basin type. It also indicates that it is necessary to examine the concentrations of both DIC and autochthonous organic carbon in rivers to correctly assess the carbon sink produced by rock weathering.

The seasonal cycle of carbonate system processes in Ryder Bay, West Antarctic Peninsula

The carbon cycle in seasonally sea-ice covered waters remains poorly understood due to both a lack of observational data and the complexity of the system. Here we present three consecutive seasonal cycles of upper ocean dissolved inorganic carbon (DIC) and total alkalinity measurements from Ryder Bay on the West Antarctic Peninsula. We attribute the observed changes in DIC to four processes: mixing of water masses, air–sea CO2 flux, calcium carbonate precipitation/dissolution and photosynthesis/respiration. This approach enables us to resolve the main drivers of the seasonal DIC cycle and also investigate the mechanisms behind interannual variability in the carbonate system. We observe a strong, asymmetric seasonal cycle in the carbonate system, driven by physical processes and primary production. In summer, melting glacial ice and sea ice and a reduction in mixing with deeper water reduce the concentration of DIC in surface waters. The dominant process affecting the carbonate system is net photosynthesis which reduces DIC and the fugacity of CO2, making the ocean a net sink of atmospheric CO2. In winter, mixing with deeper, carbon-rich water and net heterotrophy increase surface DIC concentrations, resulting in pH as low as 7.95 and aragonite saturation states close to 1. We observe no clear seasonal cycle of calcium carbonate precipitation/dissolution but some short-lived features of the carbonate time series strongly suggest that significant precipitation of calcium carbonate does occur in the Bay. The variability observed in this study demonstrates that changes in mixing and sea-ice cover significantly affect carbon cycling in this dynamic environment. Maintaining this unique time series will allow the carbonate system in seasonally sea-ice covered waters to be better understood.

Identification of groundwater contamination zone around a reclaimed landfill using carbon isotopes.

Chemical and isotopic analyses of groundwater from piezometers localized around a reclaimed landfill were performed in order to identify the boundaries of groundwater contamination zone. Spatial distribution of dissolved inorganic carbon (DIC) concentration and stable carbon isotopes in the groundwater was used to distinguish the piezometers localized within the contaminated aquifer. Background groundwater was characterized by low DIC concentration (from 1.8 to 5.0 mmol/L) and negative values of δ13CDIC (from -20.6‰ to -12.4‰). Higher DIC concentrations (from 6.0 to 12.5 mmol/L) and higher values of δ13CDIC (from -10.9 to +3.6‰) were determined in groundwater contaminated by landfill leachate. The study confirmed that δ13CDIC value in the groundwater was a useful tracer in determining the extent of the contamination zone around the landfill. In general, upgradient from the landfill, carbon isotopic composition of groundwater depended on natural sources of carbon and δ13CDIC values were negative. Downgradient from the landfill, where groundwater was contaminated by the landfill leachate, δ13CDIC values were higher, sometimes even positive.

Relationship of dissolved inorganic carbon (DIC) concentrations with some environmental variables in the Red River water in the period 2008 - 2015

Dissolved Inorganic Carbon (DIC) is one of the main chemical components in water and very sensitive with environmental changes. DIC content in river water closely relates with natural weathering process and human activities in the whole basin. Therefore, DIC concentration reflects the impact of natural conditions and human activities in the basin to river water quality. This paper presents the survey results of the DIC concentrations at 4 sites in the Red River system during the period from January 2008 to December 2015 and simultaneously considers the relationship between some environmental variables and the DIC concentrations in the river water. The survey results showed that the DIC concentrations in the Red River water varied from 9.1 to 29.9 mgC.L-1, averaging 19.6 mgC.L-1 during the study period. The DIC concentrations are positively correlated with pH values and some major ions concentrations in river water, such as K+, Na+, Ca2+, SO42- but negatively correlated with water temperature. These relationships also indicate that the DIC concentrations in the Red River water are influenced by natural conditions, rather than by human activities in the river basin.&#x0D; Cacbon vô cơ hòa tan (DIC) là thành phần hóa học cơ bản trong nước và rất nhạy cảm với những thay đổi của các yếu tố môi trường. DIC có mối liên hệ chặt chẽ với sự phong hóa tự nhiên và các hoạt động của con người trên quy mô toàn lưu vực. Do đó, DIC phản ánh mức độ tác động của các điều kiện tự nhiên và con người trong lưu vực tới chất lượng nước sông. Bài báo trình bày kết quả quan trắc hàm lượng DIC trong nước sông Hồng, đồng thời xem xét mối quan hệ giữa một số yếu tố môi trường với hàm lượng DIC trong nước sông trong giai đoạn 1/2008 – 4/2015. Kết quả cho thấy hàm lượng DIC trong nước sông Hồng dao động trong khoảng 9,1 to 29,9 mgC.L-1, trung bình đạt 19,6 mgC.L-1 trong giai đoạn tháng 1 năm 2008 – tháng 12 năm 2015. Hàm lượng DIC có mối tương quan theo tỷ lệ nghịch với nhiệt độ nước sông; đồng thời có mối tương quan tỷ lệ thuận với giá trị pH và hàm lượng một số ion như K+, Na+, Ca2+, SO42-... tại 4 vị trí quan trắc trên sông Hồng. Các mối quan hệ này thể hiện rằng hàm lượng DIC trong nước sông Hồng chịu ảnh hưởng chính của các đặc điểm tự nhiên hơn là các tác động của con người trong lưu vực.

Inorganic carbon dominates total dissolved carbon concentrations and fluxes in British rivers: Application of the THINCARB model – Thermodynamic modelling of inorganic carbon in freshwaters

River water-quality studies rarely measure dissolved inorganic carbon (DIC) routinely, and there is a gap in our knowledge of the contributions of DIC to aquatic carbon fluxes and cycling processes. Here, we present the THINCARB model (THermodynamic modelling of INorganic CARBon), which uses widely-measured determinands (pH, alkalinity and temperature) to calculate DIC concentrations, speciation (bicarbonate, HCO3−; carbonate, CO32−; and dissolved carbon dioxide, H2CO3⁎) and excess partial pressures of carbon dioxide (EpCO2) in freshwaters. If calcium concentration measurements are available, THINCARB also calculates calcite saturation. THINCARB was applied to the 39-year Harmonised Monitoring Scheme (HMS) dataset, encompassing all the major British rivers discharging to the coastal zone. Model outputs were combined with the HMS dissolved organic carbon (DOC) datasets, and with spatial land use, geology, digital elevation and hydrological datasets. We provide a first national-scale evaluation of: the spatial and temporal variability in DIC concentrations and fluxes in British rivers; the contributions of DIC and DOC to total dissolved carbon (TDC); and the contributions to DIC from HCO3− and CO32− from weathering sources and H2CO3⁎ from microbial respiration. DIC accounted for >50% of TDC concentrations in 87% of the HMS samples. In the seven largest British rivers, DIC accounted for an average of 80% of the TDC flux (ranging from 57% in the upland River Tay, to 91% in the lowland River Thames). DIC fluxes exceeded DOC fluxes, even under high-flow conditions, including in the Rivers Tay and Tweed, draining upland peaty catchments. Given that particulate organic carbon fluxes from UK rivers are consistently lower than DOC fluxes, DIC fluxes are therefore also the major source of total carbon fluxes to the coastal zone. These results demonstrate the importance of accounting for DIC concentrations and fluxes for quantifying carbon transfers from land, via rivers, to the coastal zone.

Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)

Abstract. The Rhône River is the largest source of terrestrial organic and inorganic carbon for the Mediterranean Sea. A large fraction of this terrestrial carbon is either buried or mineralized in the sediments close to the river mouth. This mineralization follows aerobic and anaerobic pathways, with a range of impacts on calcium carbonate precipitation and dissolution in the sediment near the sediment–water interface. This study focuses on the production of dissolved inorganic carbon (DIC) and total alkalinity (TA) by early diagenesis, consequential pH variations and the effect on calcium carbonate precipitation or dissolution. The sediment porewater chemistry was investigated along a transect from the Rhône River outlet to the continental shelf. TA and concentrations of DIC, SO42− and Ca2+ were analyzed on bottom waters and extracted sediment porewaters, whereas pH and oxygen concentrations were measured in situ using microelectrodes. The average oxygen penetration depth into the sediment was 1.7 ± 0.4 mm close to the river mouth and 8.2 ± 2.6 mm in the continental shelf sediments, indicating intense respiration rates. Diffusive oxygen fluxes through the sediment–water interface ranged between 3 and 13 mmol O2 m−2 d−1. In the first 30 cm of the sediment, TA and DIC porewater concentrations increased with depth up to 48 mmol L−1 near the river outlet and up to 7 mmol L−1 on the shelf as a result of aerobic and anaerobic mineralization processes. Due to aerobic processes, at all stations pH decreased by 0.6 pH units in the oxic layer of the sediment accompanied by a decrease of the saturation state regarding calcium carbonate. In the anoxic layer of the sediments, sulfate reduction was the dominant mineralization process and was associated with an increase of porewater saturation state regarding calcium carbonate. Ultimately anoxic mineralization of organic matter caused calcium carbonate precipitation demonstrated by a large decrease in Ca2+ concentration with depth in the sediment. Carbonate precipitation decreased in the offshore direction, together with the carbon turnover and sulfate consumption in the sediments. The large production of porewater alkalinity characterizes these sediments as an alkalinity source to the water column, which may increase the CO2 buffering capacity of these coastal waters. Estuarine sediments should therefore receive more attention in future estimations of global carbon fluxes.

Organic carbon source tracing and DIC fertilization effect in the Pearl River: Insights from lipid biomarker and geochemical analysis

The photosynthetic conversion of dissolved inorganic carbon (DIC) into organic carbon (OC) by using aquatic phototrophs in rivers may serve as a potential carbon sink, especially in the carbonate rock areas, thereby offering a clue for finding the missing carbon sink. However, primary-produced autochthonous OC is erroneously considered as terrestrial-derived allochthonous OC. Thus, carbonate weathering-related carbon sink is underestimated if only DIC concentrations sampled at river mouths are considered, and the transformation of DIC to autochthonous OC is neglected. Therefore, distinguishing sources of autochthonous and allochthonous OC is vital in the assessment of carbon sink. In this study, source-specific biomarkers, in association with chemical compositions and phytoplankton proxies in water samples collected from the Pearl River, were analyzed to determine OC sources. Results showed that biomarkers in the Pearl River were quite abundant, and the calculated average autochthonous OC was approximately 65% of the total OC, indicating intense in-river primary productivity. Moreover, phytoplankton biomass and DIC concentration were positively related, indicating the DIC fertilization effect on aquatic photosynthesis. High total suspended solid (TSS) on the water surface blocked the sunlight and then reduced phytoplankton production. However, in situ photosynthesis of phytoplankton could also produce autochthonous OC, even larger than the allochthonous source at sites with high DIC, and even with higher TSS concentrations. These findings comprehensively elucidated the formation of autochthonous OC based on the coupling action of rock weathering and photosynthetic activity in the riverine system, suggesting a potential direction for finding the missing carbon sink.

Carbon and nitrogen tracers of land use effects on net ecosystem metabolism in mangrove estuaries, southwest Florida

Four estuaries in southwest Florida with different land-use characteristics in their watersheds were chosen to investigate the effects of anthropogenic land use on estuarine biogeochemical cycling. We compared C:N ratios, concentrations of dissolved inorganic carbon (DIC), chlorophyll-a (chl-a) and particulate organic carbon (POC), stable isotope ratios of DIC (δ13CDIC) and POC (δ13CPOC), and nitrogen isotope ratios of particulate organic nitrogen (δ15NPON) among these estuaries. Values of δ13CDIC ranged from −14.1 to +0.9‰. The more negative values occurred upstream, resulting from DIC inputs derived from both the degradation of organic carbon and dissolution of carbonates. Upstream DIC concentrations were as high as 8066 μmol L−1, suggesting high respiration rates. Further, a comparison of DIC values to a conservative mixing model indicates net heterotrophic metabolic state in all four estuaries. Supporting this interpretation, low δ13CPOC values suggest that terrestrial plants were the main source of POC in the upstream sampling points. However, C:N ratios ranged from 7.2 to 13.4, and were consistent with the decomposition of both terrestrial and aquatic sources. Chl-a concentrations were variable and typically below 20 μg L−1, indicating moderate to low levels of autotrophy in all estuaries. Elevated chl-a concentrations indicative of increased primary productivity occurred at intermediate salinities, and were possibly caused by the mixing front at mid-estuary locations. There were no apparent differences in δ15NPON among estuaries, suggesting that the N sources to these estuaries are comparable. The combined results show no differences between near-natural and anthropogenically influenced estuaries, indicating a minimal effect of anthropogenic activities on the parameters measured, possibly as a result of the filtering capacity of the extensive surrounding mangrove vegetation.