Search

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]

Ocean Acidification (OA) is negatively affecting the physiological processes of marine organisms, altering biogeochemical cycles, and changing chemical equilibria throughout the world's oceans. It is difficult to measure pH broadly, in large part because accurate pH measurement technology is expensive, bulky, and requires technical training. Here, we present the development and evaluation of a hand-held, affordable, field-durable, and easy-to-use pH instrument, named the pHyter, which is controlled through a smartphone app. We determine the accuracy of pH measurements using the pHyter by comparison with benchtop spectrophotometric seawater pH measurements, measurement of a certified pH standard, and comparison with a proven in situ instrument, the iSAMI-pH. These results show a pHyter pH measurement accuracy of ±0.046 pH or better, which is on par with interlaboratory seawater pH measurement comparison experiments. We also demonstrate the pHyter's ability to conduct both temporal and spatial studies of coastal ecosystems by presenting data from a coral reef and a bay, in which the pHyter was used from a kayak. These studies showcase the instrument's portability, applicability, and potential to be used for community science, STEM education, and outreach, with the goal of empowering people around the world to measure pH in their own backyards. [ABSTRACT FROM AUTHOR]

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]

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]

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]

Multiple processes support the significant efflux of carbon dioxide (CO2) from rivers and streams. Attribution of CO2 oversaturation will lead to better quantification of the freshwater carbon cycle and provide insights into the net cycling of nutrients and pollutants. CO2 production is closely related to O2 consumption because of the metabolic linkage of these gases. However, this relationship can be weakened due to dissolved inorganic carbon inputs from groundwater, carbonate buffering, calcification, and anaerobic metabolism. CO2 and O2 concentrations and other water quality parameters were analyzed in two data sets: a synoptic field study and nationwide water quality monitoring data. CO2 and O2 concentrations were strongly negatively correlated in both data sets ( ρ = −0.67 and ρ = −0.63, respectively), although the correlations were weaker in high-alkalinity environments. In nearly all samples, the molar oversaturation of CO2 was a larger magnitude than molar O2 undersaturation. We used a dynamically coupled O2CO2 model to show that lags in CO2 air-water equilibration are a likely cause of this phenomenon. Lags in CO2 equilibration also impart landscape-scale differences in the behavior of CO2 between high- and low-alkalinity watersheds. Although the concept of carbonate buffering and how it creates lags in CO2 equilibration with the atmosphere is well understood, it has not been sufficiently integrated into our understanding of CO2 dynamics in freshwaters. We argue that the consideration of carbonate equilibria and its effects on CO2 dynamics are primary steps in understanding the sources and magnitude of CO2 oversaturation in rivers and streams. [ABSTRACT FROM AUTHOR]

Impurities in indicator salts can significantly bias spectrophotometric pH determinations. In this work, two purified sulfonephthalein indicators, meta-cresol purple (mCP) and phenol red (PR), were tested for analysis of freshwater pH on the free hydrogen ion concentration scale. These two purified indicators were characterized for the first time under low ionic strength conditions, providing their molar absorption coefficients and dissociation constants along with their temperature dependence from 8 °C to 30 °C. At 25 °C, the infinite dilution constants ( [ABSTRACT FROM AUTHOR]

Total alkalinity (AT ) is an important parameter for describing the marine inorganic carbon system and understanding the effects of atmospheric CO2 on the oceans. Measurements of AT are limited, however, because of the laborious process of collecting and analyzing samples. In this work we evaluate the performance of an autonomous instrument for high temporal resolution measurements of seawater AT. The Submersible Autonomous Moored Instrument for alkalinity (SAMI-alk) uses a novel tracer monitored titration method where a colorimetric pH indicator quantifies both pH and relative volumes of sample and titrant, circumventing the need for gravimetric or volumetric measurements. The SAMI-alk performance was validated in the laboratory and in situ during two field studies. Overall in situ accuracy was -2.2 ± 13.1 μmol kg-1 (n = 8 6 ), on the basis of comparison to discrete samples. Precision on duplicate analyses of a carbonate standard was ±4.7 μmol kg-1 (n = 22). This prototype instrument can measure in situ Ax hourly for one month, limited by consumption of reagent and standard solutions. [ABSTRACT FROM AUTHOR]

Coral reefs arc predicted to be one of the ecosystems most sensitive to ocean acidification. To improve predictions of coral reef response to acidification, wc need Ln better characterize the natural range of variability of pll, partial pressure of carbon dioxide (pCO2) and calcium carbonate saturation states (Ω). In this study, autonomous sensors for pH and pCO2 were deployed on Media Lima reef, Puerto Rico over three seasons from 2007 to 2008. High temporal resolution CaCO, saturation states were calculated from the in situ data, giving a much more detailed characterization of reef saturation states than previously possible. Reef pH, pCO2 and aragonite saturation (ΩAr) ranged from 7.89 to 8.17 pH units, 176-613 μatm and 2.7-4.7, respectively, in the range characteristic of most other previously studied reef ecosystems. The diel pH. pCO2 and Ω cycles were also large, encompassing about half of the seasonal range of variability. Wanning explained about 50% of the seasonal supersaturation in mean pCO2, with the remaining supersaturation primarily due to net heterotrophy and net CaCO3 production. Net hcterotrophy was likely driven by remineralization of mangrove derived organic carbon which continued into the fall, sustaining high pCO2 levels until early winter when the pCO2 returned to offshore values. As a consequence, the reef was a source of CO2 to the atmosphere during the summer and fall and a sink during winter, resulting in a net annual source of 0.73 ± 1.7 mol m-2 year-1. These results show that reefs are exposed to a wide range of saturation stales in their naLural environment. Mean ΩAr levels will drop to 3.0 when atmospheric CO2 increases to 500 μatm and ΩAr will be less than 3.0 for greater than 70% of die time in the summer. Long duration exposure to these low ΩAr levels are expected to significantly decrease calcification rates on the reef [ABSTRACT FROM AUTHOR]

Titrations, while primarily known as the chemical rite of passage for fledgling science students, are still widely used for chemical analysis. With its many years of existence and improvement, the method would seem an unlikely candidate for innovation, yet it is desirable, in this age of autonomous sensing where analyzers may be sent into space or to the bottom of the ocean, to have a simplified titrimetric method that does not rely upon volumetric or gravimetric measurement of sample and titrant. In previous work on the measurement of seawater alkalinity, we found that use of a tracer in the titrant eliminates the need to measure mass or volume. Here, we show the versatility of the method for diverse types of titrations and tracers. The results suggest that tracers may be employed in all types of titrations, opening the door for greatly simplified laboratory and field-based chemical analysis. [ABSTRACT FROM AUTHOR]

Two different sulfonephthalein indicators, cresol red (CR) with a pKa of ∼8.3 and bromothymol blue (BTB) with pKa of ∼7.4, were tested for an analysis of freshwater over a broad range of pH and total alkalinity values. Measurements from an autonomous sensor system using a 1 cm optical path length were compared to those using a 10 cm path length on a benchtop spectrophotometer. The indicator pH perturbation was quantified with a thermodynamic model and nonlinear least-squares analysis. The laboratory study found that the perturbation-corrected pH differed between the 1 cm (large indicator perturbation) and 10 cm (small indicator perturbation) optical path length measurements from -0.017 to +0.15 with a median of +0.0041 pH units for CR and from -0.015 to +0.026 with a median of -0.0008 pH units for BTB. Precision was ±0.0005-0.013 and ±0.0001-0.0027 pH units for the 1 and 10-cm-path-length measurements, respectively. The autonomous sensor was deployed for 14 days in a local creek. Simultaneous glass pH electrode measurements had a large negative and drifting offset (-0.15 to -0.40 pH units) compared to the indicator-based measurements. This study is the first in situ comparison between potentiometric and spectrophotometric pH methods in a freshwater system. [ABSTRACT FROM AUTHOR]

Sensors for the partial pressure of CO2 (pCO2) and dissolved O2 (DO) were deployed near the surface and bottom of a freshwater lake (Placid Lake, Montana) during the period from ice cover to seasonal stratification. Sources of variability were examined using one-dimensional physical and biogeochemical models. Model predictions for pCO2 and DO were compared to further constrain model parameters. A number of transient processes were documented that have not been well characterized in previous studies. The models made it possible to link these short-term events to specific forcings. We found that (1) 11 d of the 13-d turnover period occurred under ice through light-driven convective mixing, (2) phytoplankton biomass increased to its highest seasonal level under ice, (3) weak stratification set up immediately after ice-out, causing bottom water pCO2and DO to diverge from surface levels, (4) subsequent diel convective mixing brought bottom pCO2 and DO back toward surface levels, and (5) before stable stratification, vertical entrainment of CO2-rich water, net production, and air-water exchange drove 100-200 µatm daily changes in pCO2, but, because of their counterbalancing effects, surface pCO2 remained > 1,000 µatm for nearly 1 month after ice-out. Upon stable stratification, net production and air-water exchange overcame pCO2 gains from mixing and heating and reduced pCO2 to near atmospheric levels within 20 d. Net production and gas exchange accounted for ∼ 75% and 25%, respectively, of the decrease in surface pCO2 observed after ice-out. Diel convection was the dominant mixing process both under ice and after ice-out and may be an important underrepresented mechanism for CO2 and DO exchange between surface and bottom water. [ABSTRACT FROM AUTHOR]

Evaluates the important temporal scales of biogeochemical variability in Placid Lake, Montana. Relationship between biogeochemical cycles and physical processes during ice-covered periods using in situ chemical sensor technologies; Consequence of biological oxidation of organic matter; Sources of variability during ice-covered periods.

Probes the feasibility of calibration-free optical chemical sensors. Chemical detectors designed to have identical behavior from sensor to sensor without a drift; Design of an indicator-based partial pressure of carbon dioxide (pCO2) sensor; Single-ended renewable-reagent pCO2 sensor.

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 (AT). High-precision, high-accuracy seawater AT measurements are crucial for understanding, for example, the marine CaCO3 budget and saturation state, anthropogenic CO2 penetration into the oceans, calcareous phytoplankton blooms, and coral reef dynamics. We present data from 286 titrations on three types of total alkalinity standards: Na2CO3 in 0.7 mol kg·soln-1 NaCl, NaOH in 0.7 mol kg·soln-1 NaCl, and a seawater Certified Reference Material (CRM). Based on Na2CO3 standards, the accuracy and precision are ±0.2 and ±0.1% (4 and 2 μmol kg·soln-1 for AT∼2100–2500 μmol kg·soln-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. [ABSTRACT FROM AUTHOR]

Describes an autonomous sensor for long-term in situ measurements of the pH of natural waters. Direct determination of the indicator pH perturbation; Absorbance ratio; Use of fiber-optic flow cell.

Analysis of a global hydrographic data product showed a clear pH-dependent discrepancy between pH on the total scale measured spectrophotometrically (pH spec) using purified meta-Cresol Purple (mCP) and pH calculated from total alkalinity and dissolved inorganic carbon. However, this was based mainly on US cruises, and three recent Japanese cruises do not show this pH-dependent discrepancy. One potential explanation is that purified mCP batches obtained from different institutions lead to significantly different pH spec. Here, we tested this hypothesis by comparing the performance of purified mCP obtained from four different institutions. We demonstrate that consistent pH of ±0.0012 (95% C.I.) can be achieved regardless of the institution when impurities are properly removed. However, there was at least one batch from three of the four institutions that had significant pH-dependent errors that were as large as −0.008. The presence of impurities that led to pH-dependent errors was identified using HPLC and, for 8 out of the 9 cases, by spectrophotometry (although issues still remain for the latter). We conclude that pH-dependent errors due to impurities that remain after the purification process are, by themselves, too small to account for the differences observed between the recent set of cruises. Identifying the source of this difference should be a top priority. This study also highlights the importance of establishing robust quality assurance and quality control protocols to ensure consistent behavior with previously published equations to compute pH. We recommend a centralized system where one or a handful of institutions distribute purified mCP for the community, as this distribution approach will lead to lower prices and simplify quality assurance. • Consistent pH (±0.0012) can be obtained when impurities are properly removed. • Robust QA/QC protocols are required to prevent pH-dependent errors. • HPLC is an effective approach to identify the presence of impurities. • Impurities alone cannot explain different pH-dependent behavior in recent cruises. • Dye solutions are stable to ±0.001 for 2 years when stored properly. [ABSTRACT FROM AUTHOR]