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Rivers efficiently collect, process, and transport terrestrial‐derived carbon. River ecosystem metabolism is the primary mechanism for processing carbon. Diel cycles of dissolved oxygen (DO) have been used for decades to infer river ecosystem metabolic rates, which are routinely used to predict metabolism of carbon dioxide (CO2) with uncertainties of the assumed stoichiometry ranging by a factor of 4. Dissolved inorganic carbon (DIC) has been less used to directly infer metabolism because it is more difficult to quantify, involves the complexity of inorganic carbon speciation, and as shown in this study, likely requires a two‐station approach. Here, we developed DIC metabolism models using single‐ and two‐station approaches. We compared metabolism estimates based on simultaneous DO and DIC monitoring in the Upper Clark Fork River (USA), which also allowed us to estimate ecosystem‐level photosynthetic and respiratory quotients (PQE and RQE). We observed that metabolism estimates from DIC varied more between single‐ and two‐station approaches than estimates from DO. Due to carbonate buffering, CO2 is slower to equilibrate with the atmosphere compared to DO, likely incorporating a longer distance of upstream heterogeneity. Reach‐averaged PQE ranged from 1.5 to 2.0, while RQE ranged from 0.8 to 1.5. Gross primary production from DO was larger than that from DIC, as was net ecosystem production by 100mmolm−2d−1. The river was autotrophic based on DO but heterotrophic based on DIC, complicating our understanding of how metabolism regulated CO2 production. We suggest future studies simultaneously model metabolism from DO and DIC to understand carbon processing in rivers. [ABSTRACT FROM AUTHOR]

The Arctic Ocean is generally undersaturated in CO 2 and acts as a net sink of atmospheric CO 2. This oceanic uptake is strongly modulated by sea ice, which can prevent air–sea gas exchange and has major impacts on stratification and primary production. Moreover, carbon is stored in sea ice with a ratio of alkalinity to dissolved inorganic carbon that is larger than in seawater. It has been suggested that this storage amplifies the seasonal cycle of seawater p CO 2 and leads to an increase in oceanic carbon uptake in seasonally ice-covered regions compared to those that are ice-free. Given the rapidly changing ice scape in the Arctic Ocean, a better understanding of the link between the seasonal cycle of sea ice and oceanic uptake of CO 2 is needed. Here, we investigate how the storage of carbon in sea ice affects the air–sea CO 2 flux and quantify its dependence on the ratio of alkalinity to inorganic carbon in ice. To this end, we present two independent approaches: a theoretical framework that provides an analytical expression of the amplification of carbon uptake in seasonally ice-covered oceans and a simple parameterization of carbon storage in sea ice implemented in a 1D physical–biogeochemical ocean model. Sensitivity simulations show a linear relation between ice melt and the amplification of seasonal carbon uptake. A 30 % increase in carbon uptake in the Arctic Ocean is estimated compared to ice melt without amplification. Applying this relationship to different future scenarios from an earth system model that does not account for the effect of carbon storage in sea ice suggests that Arctic Ocean carbon uptake is underestimated by 5 % to 15 % in these simulations. [ABSTRACT FROM AUTHOR]

The Arctic Ocean is generally undersaturated in CO2 and acts as a net sink of atmospheric CO2. This oceanic uptake is strongly modulated by sea ice, which can prevent air-sea gas exchange and has major impacts on stratification and primary production. Moreover, carbon is stored in sea ice with a ratio of alkalinity to dissolved inorganic carbon that is larger than in seawater. It has been suggested that this storage amplifies the seasonal cycle of seawater p CO2 and leads to an increase in oceanic carbon uptake in seasonally ice-covered regions compared to those that are ice-free. Given the rapidly changing ice-scape in the Arctic Ocean, a better understanding of the link between the seasonal cycle of sea ice and oceanic uptake of CO2 is needed. Here, we investigate how the storage of carbon in sea ice affects the air-sea CO2 flux and quantify its dependence on the ratio of alkalinity to inorganic carbon in ice. To this end, we present two independent approaches: a theoretical framework that provides an analytical expression of the amplification of carbon uptake in seasonally ice-covered oceans, and a simple parameterization of carbon storage in sea ice implemented in a 1D physical-biogeochemical ocean model. Sensitivity simulations show a linear relation between ice melt and the amplification of seasonal carbon uptake. A 30 % increase in carbon uptake in the Arctic Ocean is estimated compared to ice melt without amplification. Applying this relationship to different future scenarios from an Earth System Model that does not account for the effect of carbon storage in sea ice suggests that Arctic Ocean carbon uptake is underestimated by 5 to 15 % in these simulations. [ABSTRACT FROM AUTHOR]

We introduce a new titration methodology, tracer monitored titration (TMT), in which analyses are free of volumetric and gravimetric measurements and insensitive to pump precision and reproducibility. Spectrophotometric monitoring of titrant dilution, rather than volume increment, lays the burden of analytical performance solely on the spectrophotometer. In the method described here, the titrant is a standardized mixture of acid-base indicator and strong acid. Dilution of a pulse of titrant in a titration vessel is tracked using the total indicator concentration measured spectrophotometrically. The concentrations of reacted and unreacted indicator species, derived from Beer's law, are used to calculate the relative proportions of titrant and sample in addition to the equilibrium position (pH) of the titration mixture. Because the method does not require volumetric or gravimetric additions of titrant, simple low-precision pumps can be used. Here, we demonstrate application of TMT for analysis of total alkalinity ([A.sub.T]). High-precision, high-accuracy seawater AT measurements are crucial for understanding, for example, the marine CaC[O.sub.3] budget and saturation state, anthropogenic C[O.sub.2] penetration into the oceans, calcareous phytoplankton blooms, and coral reef dynamics. We present data from 286 titrations on three types of total alkalinity standards: [Na.sub.2]C[O.sub.3] in 0.7 mol kg x [soln.sup.-1] NaCl, NaOH in 0.7 mol kg x [soln.sup.-1] NaCl, and a seawater Certified Reference Material (CRM). Based on [Na.sub.2]C[O.sub.3] standards, the accuracy and precision are [+ or -] 0.2 and [+ or -] 0.1% (4 and 2 [micro]mol kg x [soln.sup.-1] for [A.sub.T] ~2100-2500 [micro]mol kg x [soln.sup.-1], n = 242), using low-precision solenoid pumps to introduce sample and titrant. Similar accuracy and precision were found for analyses run 42 days after the initial experiments. Excellent performance is achieved by optimizing the spectrophotometric detection system and relying upon basic chemical thermodynamics for calculating the equivalence point. Although applied to acid-base titrations in this paper, the approach should be generally applicable to other types of titrations.

An autonomous sensor for long-term in situ measurements of the pH of natural waters is described. The system is based upon spectrophotometric measurements of a mixture of sample and sulfonephthalein indicator. A simple plumbing design, using a small, low-power solenoid pump and valve, avoids the need for quantitative addition of indicator. A ~50-[micro]L slug of indicator is pulled into the sample stream by the pump, and subsequent pumping and mixing results in a section of indicator and sample where absorbance measurements can be made. The design also permits direct determination of the indicator pH perturbation. Absorbances are recorded at three wavelengths (439, 579, and 735 nm) using a custom-built 1.7-cm path length fiber-optic flow cell. Solution blanks are obtained by periodically flushing the cell with sample. Field tests were performed in a local river over an 8-day period. The in situ accuracy, based on comparison with laboratory spectrophotometric pH measurements, was -0.003 pH unit (n = 16), similar to the measurement precision. No drift was observed during the 8-day period. The absorbance ratio used to calculate pH, in combination with a simple and robust optical design, imparts an inherent stability not achievable with conventional potentiometric methods, making the design feasible for long-term autonomous pH measurements.

Inland waters have an important role in the global carbon cycle, contributing significantly to terrestrial carbon fluxes through downstream export and exchange of CO2 with the atmosphere. However, large uncertainties in freshwater inorganic carbon fluxes remain. One contributing factor is uncertainty in carbonate system calculations for estimating the partial pressure of CO2 (pCO2) from pH and alkalinity in freshwater systems. The uncertainty stems largely from inaccurate pH values caused by glass pH electrode measurements in low ionic strength systems. This study compares indicator‐based spectrophotometric and electrochemical pH measurements and their application for calculating freshwater pCO2. Our study found that, compared to a pCO2 reference method, pH electrode‐based estimates of pCO2 were overestimated by 230 ± 200 μatm (n = 54) where indicator‐based spectrophotometric pH estimates of pCO2 were 58 ± 33 μatm (n = 34) over the range of 100–1600 μatm. Furthermore, we found that when ionic strength was assumed to be zero, calculated pCO2 error was ~ 20% of the reference pCO2. A 19‐d field study using autonomous spectrophotometric pH and pCO2 sensors found an average error in calculated pCO2 of −70 ± 57 μatm (n = 1685). Although, our focus is on riverine CO2, these findings and subsequent conclusions apply to all freshwater systems. Spectrophotometric pH measurements will improve future freshwater pCO2 calculations and better quantify inland waters' role in the global carbon budget. [ABSTRACT FROM AUTHOR]

Calibration-free optical chemical sensors are feasible, as gleaned from sensor intercomparisons and long-term studies of the sensor response. A reagent-based optical chemical sensor has been developed for autonomous mooring-based measurements of seawater pCO2. The sensor demonstrates excellent sensor-to-sensor reproducibility and long-term measurement stability. These show that such sensors could circumvent the need for periodic calibrations.

A new method, based on a renewable-reagent fiber optic sensor, for measuring the partial pressure of [CO.sub.2] (p[CO.sub.2]) in seawater is presented. The sensor operates by measuring the light intensity at the absorbing wavelengths of a colorimetric acid--base indicator which is continuously delivered to the fiber tips through capillary tubing. The light intensity is modulated by pH changes that occur when [CO.sub.2] diffuses across a gas-permeable membrane. The sensor operates both in a diffusion-dependent steady state and equilibrium regime depending upon the indicator flow rate. At low flow rates, an equilibrium model can be used to predict the response of the sensor. The results indicate that the sensor operates within the steady-state regime at flow rates higher than approximately 0.2 [micro]L/min. The optimal precision is [+ or -] 0.8 [micro]atm from 300--550 [micro]atm of [CO.sub.2], calculated from the response sensitivity and 3 X the root mean square noise. Response times (100%) range from 11 to 26 min and depend upon the indicator flow rate. Sensitivity to temperature and sample hydrodynamics is also discussed. The sensor performance was tested on a research cruise, and these results are compared to the underway [pCO.sub.2] measured simultaneously by an infrared [CO.sub.2] analyzer., The initial developmental stages of an instrument for the widespresd mapping of carbon dioxide disequilibrium partial pressure between ocean surface waters and the atmosphere is discussed. A renewable-reagent fiberoptic sensor is the basis of this new method which was utilized in both diffusion-dependent steady state and equilibrium regime. The expected superiority of the renewable-reagent approach was affirmed as the method's capacity was demonstrated.

Total alkalinity (AT) is an important parameter in the study of aquatic biogeochemical cycles, chemical speciation modeling, and many other important fundamental and anthropogenic (e.g., industrial) processes. We know little about its short‐term variability, however, because studies are based on traditional bottle sampling typically with coarse temporal resolution. In this work, an autonomous AT sensor, named the Submersible Autonomous Moored Instrument for Alkalinity (SAMI‐alk), was tested for freshwater applications. A comprehensive evaluation was conducted in the laboratory using freshwater standards. The results demonstrated excellent precision and accuracy (± 0.1%–0.4%) over the AT range from 800 to 3000 μmol L−1. The system had no drift over an 8 d test and also demonstrated limited sensitivity to variations in temperature and ionic strength. Three SAMI‐alks were deployed for 23 d in the Clark Fork River, Montana, with a suite of other sensors. Compared to discrete samples, in situ accuracy for the three instruments were within 10–20 μmol L−1 (0.3–0.6%), indicating good performance considering the challenges of in situ measurements in a high sediment, high biofouling riverine environment with large and rapid changes in temperature. These data reveal the complex AT dynamics that are typically missed by coarse sampling. We observed AT diel cycles as large as 60–80 μmol L−1, as well as a rapid change caused by a runoff event. Significant errors in inorganic carbon system modeling result if these short‐term variations are not considered. This study demonstrates both the feasibility of the technology and importance of high‐resolution AT measurements. [ABSTRACT FROM AUTHOR]

Chemists might debate the definition of a chemical sensor. but within the context of this article we consider a chemical sensor to be an instrument or probe that can be [...]

Changes may be occurring in the carbonate chemistry of fjords due to natural and anthropogenic disturbance of major freshwater sources. We present a high‐frequency time series study of seasonal pH and CO2 partial pressure (pCO2) in a north Patagonian fjord with a focus on changes in freshwater inflows and biological processes. To do this, we monitored pH and pCO2 in situ, along with river streamflow, salinity, temperature, and dissolved oxygen (DO) in the Reloncaví Fjord (41.5°S) for a full year (January to December 2015). Strong seasonal variability was observed in the pCO2, pH, and DO of the fjord's surface waters. During the summer, pCO2 reached its annual minimum (range: 187–571 μatm) and pH its maximum (range: 7.98–8.24), coinciding with lower freshwater inflows (204–307 m3/s) and high DO (280–378 μmol/kg), as well as aragonite saturation states (ΩArag) higher than 1. In contrast, in winter, pCO2 ranged from 461–1,008 μatm and pH from 7.57–8.03, coinciding with high freshwater inflows (1,049–1,402 m3/s), lower oxygen (216–348 μmol/kg), and constant undersaturation of ΩArag. Reloncaví Fjord had an annual air‐water CO2 flux of 0.716 ± 2.54 mol·m−2·year−1 during 2015 and thus acted as a low emission system. The annual cycle was mainly governed by seasonal changes in biological processes that enhanced the shift from a CO2 sink in late spring and summer, caused by high primary production rates, to a CO2 source during the rest of the year caused by high community respiration due to allochthonous organic carbon inputs. Key Points: North Patagonian fjords are aquaculture‐heavy used environments, and there are not available data for understanding major biogeochemical process like air‐sea CO2 fluxes and aragonite saturationA high‐frequency time series of the annual cycle of CO2, pH, and dissolved oxygen in a north Patagonian fjord is evaluatedThe fjord inorganic carbon cycle is very dynamic, primarily driven by varying contributions from primary production and respiration of allochthonous organic carbon [ABSTRACT FROM AUTHOR]

Solute exclusion during sea ice formation is a potentially important contributor to the Arctic Ocean inorganic carbon cycle that could increase as ice cover diminishes. When ice forms, solutes are excluded from the ice matrix, creating a brine that includes dissolved inorganic carbon (DIC) and total alkalinity (AT). The brine sinks, potentially exporting DIC and AT to deeper water. This phenomenon has rarely been observed, however. In this manuscript, we examine a ~1 year pCO2 mooring time series where a ~35‐μatm increase in pCO2 was observed in the mixed layer during the ice formation period, corresponding to a simultaneous increase in salinity from 27.2 to 28.5. Using salinity and ice based mass balances, we show that most of the observed increases can be attributed to solute exclusion during ice formation. The resulting pCO2 is sensitive to the ratio of AT and DIC retained in the ice and the mixed layer depth, which controls dilution of the ice‐derived AT and DIC. In the Canada Basin, of the ~92 μmol/kg increase in DIC, 17 μmol/kg was taken up by biological production and the remainder was trapped between the halocline and the summer stratified surface layer. Although not observed before the mooring was recovered, this inorganic carbon was likely later entrained with surface water, increasing the pCO2 at the surface. It is probable that inorganic carbon exclusion during ice formation will have an increasingly important influence on DIC and pCO2 in the surface of the Arctic Ocean as seasonal ice production and wind‐driven mixing increase with diminishing ice cover. Plain Language Summary: When sea ice forms, the dissolved solutes are squeezed out of the ice matrix, forming a dense brine that can sink into deeper water. This process has a potentially important role in the inorganic carbon cycle because dissolved inorganic carbon can sink with the brine, exporting it from the surface ocean and increasing the sea surface CO2 partial pressure (pCO2). We deployed a pCO2 sensor at 30‐m depth in the Canada Basin, the largest of the Arctic Ocean subbasins, that documented an increase in pCO2 during the fall to winter ice formation period. This increase correlated with an increase in salinity over the same time period. A simple model calculation supports that the increase in pCO2 and inorganic carbon is primarily due to ice formation. This finding has important implications for the future inorganic carbon cycle as seasonal ice formation increases due to loss of perennial ice cover in the Arctic. Key Points: We deployed a pCO2 sensor in the northwest Canada Basin of the Arctic Ocean; the time series shows a steady increase from fall to winterThe pCO2 increase is attributed to sea ice formation that excludes inorganic carbon; the inorganic carbon concentrated in the mixed layerIce exclusion of inorganic carbon could become an increasingly important part of the Arctic Ocean carbon cycle with diminishing ice cover [ABSTRACT FROM AUTHOR]