Posts Tagged 'methods'

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Controls on the spatio-temporal distribution of microbialite crusts on the Great Barrier Reef over the past 30,000 years

Published 3 September 2020 Science Closed
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Highlights

  • Comprehensive dataset of reefal microbial crusts over the past 30,000 years.
  • Modern 3D analysis to assess heterogeneity of microbialites in reef frameworks.
  • Radiocarbon ages show microbialite development coeval with and postdating framework.
  • Microbialite thickness correlates with changes in carbonate saturation level and pH.

Abstract

Calcification of microbial mats adds significant amounts of calcium carbonate to primary coral reef structures that stabilizes and binds reef frameworks. Previous studies have shown that the distribution and thicknesses of late Quaternary microbial crusts have responded to changes in environmental parameters such as seawater pH, carbonate saturation state, and sediment and nutrient fluxes. However, these studies are few and limited in their spatio-temporal coverage. In this study, we used 3D and 2D examination techniques to investigate the spatio-temporal distribution of microbial crusts and their responses to environmental changes in Integrated Ocean Drilling Program (IODP) Expedition 325 (Great Barrier Reef Environmental Changes) fossil reef cores that span 30 to 10 ka at two locations on the GBR reef margin. Our GBR microbialite record was then combined with a meta-analysis of 17 other reef records to assess global scale changes in microbialite development (i.e., presence/absence, thickness) over the same period. The 3D results were compared with 2D surface area measurements to assess the accuracy of 2D methodology. The 2D technique represents an efficient and accurate proxy for the 3D volume of reef framework components within the bounds of uncertainty (average: 9.45 ± 4.5%). We found that deep water reef frameworks were most suitable for abundant microbial crust development. Consistent with a previous Exp. 325 study (Braga et al., 2019), we also found that crust ages were broadly coeval with coralgal communities in both shallow water and fore-reef settings. However, in some shallow water settings they also occur as the last reef framework binding stage, hundreds of years after the demise of coralgal communities. Lastly, comparisons of crust thickness with changes in environmental conditions between 30 and 10 ka, show a temporal correlation with variations in partial pressure of CO2 (pCO2), calcite saturation state (Ωcalcite), and pH of seawater, particularly during the past ~15 kyr, indicating that these environmental factors likely played a major role in microbialite crust development in the GBR. This supports the view that microbialite crust development can be used as an indicator of ocean acidification.

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Preparation of 2‐amino‐2‐hydroxymethyl‐1,3‐propanediol (TRIS) pHT buffers in synthetic seawater

Published 17 August 2020 Science Closed
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Buffers of known quality for the calibration of seawater pHT measurements are not widely or commercially available. Although there exist published compositions for the 0.04 mol kg‐H2O−1 equimolar buffer 2‐amino‐2‐hydroxymethyl‐1,3‐propanediol (TRIS)‐TRIS · H+ in synthetic seawater, there are no explicit procedures that describe preparing this buffer to achieve a particular pHT with a known uncertainty. Such a procedure is described here which makes use of easily acquired laboratory equipment and techniques to produce a buffer with a pHT within 0.006 of the published pHT value originally assigned by DelValls and Dickson (1998), 8.094 at 25°C. Such a buffer will be suitable for the calibration of pH measurements expected to fulfil the “weather” uncertainty goal of the Global Ocean Acidification Observation Network of 0.02 in pHT, an uncertainty goal appropriate to “identify relative spatial patterns and short‐term variation.”

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Interpreting measurements of total alkalinity in marine and estuarine waters in the presence of proton-binding organic matter

Published 13 August 2020 Science Closed
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Highlights

• Model simulates alkalinity differences arising from different titration methodologies.

• Differences between measured and calculated alkalinity caused by organics are computed.

• Alkalinity differences vary with titration method and organic proton-binding affinity.

• Organics create errors in carbonate system calculations and interpretations.

• Marine alkalinity assessments should account for the ubiquitous presence of organics

Abstract

Total alkalinity (AT) is one of four measurable cornerstone parameters for characterizing the marine carbonate system, yet its measurement by standard titration methods is subject to systematic misinterpretations in the presence of uncharacterized dissolved organic molecules in ocean and estuarine waters. A consequence of these misinterpretations may be the lack of thermodynamic consistency that is routinely observed among measured and calculated parameters of the carbonate system. In this work, a numerical model is used to illustrate (a) how proton-binding dissolved organic molecules influence the reported results of total alkalinity titrations in marine and estuarine settings and (b) how errors in interpretations of reported AT values can then propagate through carbonate system calculations, thus distorting biogeochemical interpretations of calculated parameters. We examine five distinct approaches for alkalinity measurement by titration. Ideally, the difference between the measured (reported) AT and the conventional (thermodynamic) definition of inorganic alkalinity (Ainorg) would be zero. However, in the presence of titratable organic matter, our model results show consistent non-zero differences that vary with the chemical properties of the organic matter. For all five titration approaches, the differences between reported AT and Ainorg are greatest when the negative logarithm of the organic acid dissociation constant (pKorg) is between approximately 5 and 7. The differences between reported AT and Ainorg also display previously undescribed variation among measurement approaches, most significantly when pKorg is between approximately 3 and 6 (typical of carboxylic acid groups). The measurement approaches that are most effective at limiting the unfavorable influence of these relatively low-pK organic acids on AT are closed-cell titrations and single-step titrations that are terminated at a relatively high pH. For calculated carbonate system parameters relevant to in situ conditions (e.g., pH, pCO2, calcium carbonate mineral saturation states), errors resulting from the presence of proton-binding organics are largest when calculations are based on the input pair of directly measured dissolved inorganic carbon (CT) and directly measured AT, and can vary in magnitude depending on the titration approach that is used to obtain AT. The modeling results presented in this work emphasize the importance of (a) determining AT in a manner that accounts for the ubiquity of organic alkalinity in marine and estuarine waters and (b) working toward a clearer understanding of the phenomena underlying the routine lack of internal consistency between measured versus calculated carbonate system parameters. Total alkalinity measurements should begin to incorporate either implicit or explicit evaluations of the titration characteristics of the natural organic carbon present in each sample. To that end, we recommend use of secondary titrations to directly measure organic alkalinity (sample-by-sample), characterization of relationships between total dissolved organic carbon concentrations and organic alkalinity (on local to regional scales), and/or exploration of novel curve-fitting procedures to infer the behavior of organic functional groups from titration data.

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Current estimates of K1* and K2* appear inconsistent with measured CO2 system parameters in cold oceanic regions

Published 11 August 2020 Science Closed
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Seawater absorption of anthropogenic atmospheric carbon dioxide (CO2) has led to a range of changes in carbonate chemistry, collectively referred to as ocean acidification. Stoichiometric dissociation constants used to convert measured carbonate system variables (pH, pCO2, dissolved inorganic carbon, total alkalinity) into globally comparable parameters are crucial for accurately quantifying these changes. The temperature and salinity coefficients of these constants have generally been experimentally derived under controlled laboratory conditions. Here, we use field measurements of carbonate system variables taken from the Global Ocean Data Analysis Project version 2 and the Surface Ocean CO2 Atlas data products to evaluate the temperature dependence of the carbonic acid stoichiometric dissociation constants. By applying a novel iterative procedure to a large dataset of 948 surface-water, quality-controlled samples where four carbonate system variables were independently measured, we show that the set of equations published by Lueker et al. (2000), currently preferred by the ocean acidification community, overestimates the stoichiometric dissociation constants at temperatures below about 8 ∘C. We apply these newly derived temperature coefficients to high-latitude Argo float and cruise data to quantify the effects on surface-water pCO2 and calcite saturation states. These findings highlight the critical implications of uncertainty in stoichiometric dissociation constants for future projections of ocean acidification in polar regions and the need to improve knowledge of what causes the CO2 system inconsistencies in cold waters.

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Purified meta-cresol purple dye perturbation: how it influences spectrophotometric pH measurements

Published 15 July 2020 Science Closed
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Highlights

• The addition of an indicator dye perturbs the sample original pH and limits the spectrophotometric pH measurement accuracy.

• Dye perturbation on the sample pH varies depending on the sample properties and the indicator dye properties.

• To experimentally correct dye perturbation, sample properties like total alkalinity and salinity should be taken into consideration.

• A MATLAB function is proposed to calculate theoretical dye perturbation.

Abstract

Ocean acidification, a phenomenon of seawater pH decreasing due to increasing atmospheric CO2, has a global effect on seawater chemistry, marine biology, and ecosystems. Ocean acidification is a gradual and global long-term process, the study of which demands high-quality pH data. The spectrophotometric technique is capable of generating accurate and precise pH measurements but requires adding an indicator dye that perturbs the sample original pH. While the perturbation is modest in well-buffered seawater, applications of the method in environments with lower buffer capacity such as riverine, estuarine, sea-ice meltwater and lacustrine environments are increasingly common, and uncertainties related to larger potential dye perturbations need further evaluation. In this paper, we assess the effect of purified meta-Cresol Purple (mCP) dye addition on the sample pH and how to correct for this dye perturbation. We conducted numerical simulations by incorporating mCP speciation into the MATLAB CO2SYS program to examine the changes in water sample pH caused by the dye addition and to reveal the dye perturbation mechanisms. Then, laboratory experiments were carried out to verify the simulation results. The simulations suggest that the dye perturbation on sample pH is a result of total alkalinity (TA) contributions from the indicator dye and chemical equilibrium shifts that are related to both the water sample properties (pH, TA, and salinity) and the indicator dye solution properties (pH and solvent matrix). The laboratory experiments supported the simulation results; the same dye solution can lead to different dye perturbations in water samples with different pH, TA, and salinity values. The modeled adjustments agreed well with the empirically determined adjustments for salinities >5, but it showed greater errors for lower salinities with disagreements as large as 0.005 pH units. Adjustments are minimized when the pH and salinity of the dye are matched to the sample. When the dye is used over a wide range of salinity, we suggest that it should be prepared in deionized water to minimize the dye perturbation effect on pH in the fresher sample waters with less well-constrained perturbation adjustments. We also suggest that the dye perturbation correction should be based on double dye addition experiments performed over a wide range of pH, TA, and salinity. Otherwise, multiple volume dye addition experiments are recommended for each sample to determine the dye perturbation adjustment. We further create a MATLAB function dyeperturbation.m that calculates the expected dye perturbation. This function can be used to validate empirically-derived adjustments or in lieu of empirical adjustments if dye addition experiments are unfeasible (e.g., for historical data). This study of dye perturbation evaluation and correction will improve the accuracy of the pH data, necessary for monitoring the long-term anthropogenic-driven changes in the seawater carbonate system.

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Evaluation of a new carbon dioxide system for autonomous surface vehicles

Published 2 July 2020 Science Closed
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Current carbon measurement strategies leave spatiotemporal gaps that hinder the scientific understanding of the oceanic carbon biogeochemical cycle. Data products and models are subject to bias because they rely on data that inadequately capture mesoscale spatiotemporal (kilometers and days to weeks) changes. High-resolution measurement strategies need to be implemented to adequately evaluate the global ocean carbon cycle. To augment the spatial and temporal coverage of ocean-atmosphere carbon measurements, an Autonomous Surface Vehicle CO2 (⁠⁠) system was developed. From 2011 to 2018, ASVCO2 systems were deployed on seven Wave Glider and Saildrone missions along the U.S. Pacific and Australia’s Tasmanian coastlines and in the tropical Pacific to evaluate the viability of the sensors and their applicability to carbon cycle research. Here we illustrate that the ASVCO2 systems are capable of long-term oceanic deployment and robust collection of air and seawater pCO2 within ± 2 µatm based on comparisons with established ship-board underway systems, with previously described MAPCO2 systems, and with companion ASVCO2 systems deployed side-by-side.

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Reversible and high accuracy pH colorimetric sensor array based on a single acid-base indicator working in a wide pH interval

Published 1 July 2020 Science Closed
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Highlights

• Reversible pH CSA working within 7 pH units by using only one pH indicator.

• Use of a suitable surfactant to vary the pKa of the indicator over 7 pH units.

• Error minimization (0.01 pH units) with a model computing the optimal spots number.

• A pH CSA with analytical performance comparable to the glass electrode.

Abstract

A pH colorimetric sensor array (CSA) with fast response time (<1 min) using only one acid-base indicator, Bromothymol Blue (BB), was prepared and characterized by modulating the amount, C, of the surfactant Hexadecyltrimethylammonium p-toluenesulfonate between 0 and 0.3725 gCTApTs/gprecursor with a constant amount of the OrMoSil precursors. The effect of the C increase is a continuous acidic shift of the calibration position, i.e. a huge variation of the pKa value of BB in the pH range 5.80-13.50. The precision error decreased with increasing C from 0.096 pH units (lower C values) to 0.023 pH units (larger C values). This result led to the development of a model to determine the number of spots with suitable C values required for having a similar value of precision in the entire working interval of the CSA. By selecting only 4 spots the precision error is < 0.100 pH units in the pH range 5.80–13.50. With 256 spots (diameter of each spot ≈ 3 mm), the model predicted an error almost constant (≈0.010) in the entire pH range.

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Calcification of planktonic foraminifer Pulleniatina obliquiloculata controlled by seawater temperature rather than ocean acidification

Published 19 June 2020 Science Closed
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Highlights

• A method is provided to correct the dissolution effect on foraminiferal SNW

• Core-top ISNWP. obli is positively correlated with calcification temperature

• ISNWP. obli linked to seawater temperature, but not atmospheric pCO2, since 250 ka

• Temperature is the dominant factor controlling P. obliquiloculata calcification

Abstract

Planktonic foraminifera represent a major component of global marine carbonate production, and understanding environmental influences on their calcification is critical to predicting marine carbon cycle responses to modern climate change. The present study investigated the effects of different environmental influences on calcification of the planktonic foraminifer Pulleniatina obliquiloculata. By correcting the dissolution effect on the size-normalized weight (SNW) of P. obliquiloculata from deep-sea sediments, we provide a means of estimating initial size-normalized weight (ISNW) from which to assess secular changes in the degree of calcification of P. obliquiloculata. Core-top ISNW in P. obliquiloculata from the global tropical oceans is significantly positively correlated with calcification temperature, suggesting that temperature is the dominant control on calcification. Using Neogloboquadrina dutertrei SNW as an independent deep-water Δ[CO32−] proxy, we present an ISNW record for P. obliquiloculata from the western tropical Pacific since 250 ka. The response of ISNW to past seawater temperature variations further confirms the dominant influence of temperature on P. obliquiloculata calcification. A potential increase in calcification as a result of ocean warming may have reduced oceanic uptake of CO2 from the atmosphere and increased atmospheric pCO2, generating a positive feedback for global warming.

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Global ocean spectrophotometric pH assessment: consistent inconsistencies

Published 15 June 2020 Science Closed
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Ocean Acidification (OA)—or the decrease in seawater pH resulting from ocean uptake of CO2 released by human activities—stresses ocean ecosystems and is recognized as a Climate and Sustainable Development Goal Indicator that needs to be evaluated and monitored. Monitoring OA related pH changes requires a high level of precision and accuracy. The two most common ways to quantify seawater pH are to measure it spectrophotometrically or to calculate it from Total Alkalinity (TA) and Dissolved Inorganic Carbon (DIC). However, despite decades of research, small but important inconsistencies remain between measured and calculated pH. To date, this issue has been circumvented by examining changes only in consistently-measured properties. Currently, the oceanographic community is defining new observational strategies for OA and other key aspects of the ocean carbon cycle based on novel sensors and technologies, that rely on validation against data records and/or synthesis products. Comparison of measured spectrophotometric pH to calculated pH from TA and DIC measured during the 2000s and 2010s eras, reveals that: 1) there is an evolution towards a better agreement between measured and calculated pH over time from 0.02 pH units in the 2000s to 0.01 pH units in the 2010s at pH>7.6; 2) a disagreement greater than 0.01 pH units persists in waters with pH<7.6, and 3) inconsistencies likely stem from variations in the spectrophotometric pH standard operating procedure (SOP). A reassessment of pH measurement and calculation SOPs and metrology is urgently needed.

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The pH dependency of the boron isotopic composition of diatom opal (Thalassiosira weissflogii) (update)

Published 29 May 2020 Science Closed
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The high-latitude oceans are key areas of carbon and heat exchange between the atmosphere and the ocean. As such, they are a focus of both modern oceanographic and palaeoclimate research. However, most palaeoclimate proxies that could provide a long-term perspective are based on calcareous organisms, such as foraminifera, that are scarce or entirely absent in deep-sea sediments south of 50∘ S in the Southern Ocean and north of 40∘ N in the North Pacific. As a result, proxies need to be developed for the opal-based organisms (e.g. diatoms) found at these high latitudes, which dominate the biogenic sediments recovered from these regions. Here we present a method for the analysis of the boron (B) content and isotopic composition (δ11B) of diatom opal. We apply it for the first time to evaluate the relationship between seawater pH, δ11B and B concentration ([B]) in the frustules of the diatom Thalassiosira weissflogii, cultured across a range of carbon dioxide partial pressure (pCO2) and pH values. In agreement with existing data, we find that the [B] of the cultured diatom frustules increases with increasing pH (Mejía et al., 2013). δ11B shows a relatively well defined negative trend with increasing pH, completely distinct from any other biomineral previously measured. This relationship not only has implications for the magnitude of the isotopic fractionation that occurs during boron incorporation into opal, but also allows us to explore the potential of the boron-based proxies for palaeo-pH and palaeo-CO2 reconstruction in high-latitude marine sediments that have, up until now, eluded study due to the lack of suitable carbonate material.

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Mid-infrared sensor system based on tunable laser absorption spectroscopy for dissolved carbon dioxide analysis in the South China Sea: system-level integration and deployment

Published 20 May 2020 Science Closed
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System-level integration of a mid-infrared carbon dioxide (CO2) sensor system based on tunable laser absorption spectroscopy (TLAS) was realized for the analysis of dissolved CO2 in seawater employing an interband cascade laser (ICL) centered at 4319 nm and a multi-pass cell (MPC) with an optical path length of 29.8 m. At low measurement pressure of 30 Torr, three absorption lines of 12CO2 were selected to realize different measurement ranges, and a 13CO2 absorption line was targeted for simultaneous isotopic abundance analysis of δ13CO2. The sensor system was compactly integrated into a standalone system with automatic operation for underwater field deployment, and the working process was controlled by a specially-designed electrical system. A gas-liquid separator system was developed for CO2 extraction from water, and a pressure-control mechanism with two operation modes (i.e. static and dynamic mode) was proposed to make the sensor system applicable under deep-sea environment. Series of experiments were carried out in laboratory for performance assessment of the developed sensor system employed for the analysis of dissolved CO2 in water. The sensor was deployed for a field test for natural gas hydrates exploration at an underwater depth of 0−2000 m in the South China Sea, with the sensor operating normally during the deployment.

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Technical note: Seamless gas measurements across Land-Ocean Aquatic Continuum – corrections and evaluation of sensor data for CO2, CH4 and O2 from field deployments in contrasting environments

Published 12 May 2020 Science Closed
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Comparatively the ocean and inland waters are two separate worlds, with concentrations in greenhouse gases having orders of magnitude in difference between the two. Together they create the Land-Ocean Aquatic Continuum (LOAC), which comprises itself largely of areas with little to no data in regards to understanding the global carbon system. Reasons for this include remote and inaccessible sample locations, often tedious methods that require collection of water samples and subsequent analysis in the lab, as well as the complex interplay of biological, physical and chemical processes. This has led to large inconsistencies, increasing errors and inevitably leading to potentially false upscaling. Here we demonstrate successful deployment in oceanic to remote inland regions, over extreme concentration ranges with multiple pre-existing oceanographic sensors combined set-up, allowing for highly detailed and accurate measurements. The set-up consists of sensors measuring pCO2, pCH4 (both flow-through, membrane-based NDIR or TDLAS sensors), O2, and a thermosalinograph at high-resolution from the same water source simultaneously. The flexibility of the system allowed deployment from freshwater to open ocean conditions on varying vessel sizes, where we managed to capture day-night cycles, repeat transects and also delineate small scale variability. Our work demonstrates the need for increased spatiotemporal monitoring, and shows a way to homogenize methods and data streams in the ocean and limnic realms.

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Subsurface Automated Samplers (SAS) for ocean acidification research

Published 12 May 2020 Science Closed
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Ocean acidification (OA) is the process whereby anthropogenic carbon dioxide is absorbed into seawater, resulting in altered carbonate chemistry and a decline in pH. OA will negatively impact numerous marine organisms, altering the structure and function of entire ecosystems. The progression of OA, while faster than has occurred in recent geological history, has been subtle and detection may be complicated by high variability in shallow-water environments. Nevertheless, comprehensive monitoring and characterization is important given the scale and severity of the problem. Presently, technologies used to measure OA in the field are costly and limited by their detection of only one carbonate chemistry parameter, such as pH. Discrete water samples, by contrast, offer a means of measuring multiple components of the carbonate system, including parameters of particular explanatory value (e.g., total alkalinity, dissolved inorganic carbon), for which field-based sensors do not presently exist. Here we describe the design, use, and performance of a low-cost (<$220 USD) Subsurface Automated Sampler (SAS), suitable for the collection of water for carbonate chemistry analysis. Each sampler is field-programmable using a remote control, performs in depths up to 55 m seawater, collects two separately preserved samples, and logs temperature at the time of collection. SASs are designed from the ground up to be open source with respect to physical design and sampling components, electronic hardware, and software. Build instructions, parts lists, and printable 3D files are provided along with code to ultimately lower the cost of OA monitoring, facilitate further research, and encourage application-specific customization.

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A high precision method for calcium determination in seawater using ion chromatography

Published 4 May 2020 Science Closed
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Calcium (Ca2+) is an important major cation in seawater, which is closely related to the oceanic biogeochemistry cycle. Direct and accurate determination of Ca2+ concentration is required for a more comprehensive study of the carbonate system in seawater. Due to the high background concentration of Ca2+ in seawater and small variances of Ca2+ during CaCO3 precipitation and dissolution process, a precision of better than 0.1% (of approximately ± 10 μmol/kg) is very much desired for carbonate chemistry related studies. In this study, a simple, non-toxic and labor-saving technique using ion chromatography (IC) has been developed to determine Ca2+ in seawater with an overall precision of better than 0.1%. Due to lack of available commercial calcium standard in seawater matrix, IAPSO seawater was selected as the reference after calibration. This proposed method can get a result within 15 min and only requires a small sample volume (∼1 ml). The concentrations and flow rates of the eluent have been optimized to achieve the best chromatographic separation (20 mmol/L and 1.0 mL/min were selected in this study). Our work suggests that sample dilutions by weighing have no discernible effect on Ca2+ determinations. However, the measured Ca2+ concentration shows a linear decrease with the increasing Mg/Ca ratio in samples, which could be corrected by a derived formula to achieve high accuracy within 0.1%. This optimized method has been applied to the analysis of Ca2+ distribution in Southwest Indian Ocean and the laboratory study on the calcite precipitation in seawater.

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Pressure compensated pH sensor (United States Patent)

Published 11 March 2020 Science Closed
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Inventors: Daryl Allen Carlson, Jesse John Bauman, David Dahl Walter, Matthew Eric D’Asaro

An embodiment a pressure compensated pH sensor apparatus, including: a pH sensing component comprising a sensing portion that is exposed to a fluid source when in use; a pressure chamber located in a position under the sensing portion and that envelopes all of the sensing portion not exposed to the fluid source when in use; and a pressure compensation mechanism located within the pressure chamber, wherein the pressure compensation mechanism reacts to pressure from an environment outside the apparatus, thereby support the sensing portion.

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Ultrasensitive seawater pH measurement by capacitive readout of potentiometric sensors

Published 10 March 2020 Science Closed
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Potentiometric pH probes remain the gold standard for the detection of pH but are not sufficiently sensitive to reliably detect ocean acidification at adequate frequency. Here, potentiometric probes are made dramatically more sensitive by placing a capacitive electronic component in series to the pH probe while imposing a constant potential over the measurement circuit. Each sample change now triggers a capacitive current transient that is easily identified between the two equilibrium states, and is integrated to reveal the accumulated charge. This affords dramatically higher precision than with traditional potentiometric probes. pH changes down to 0.001 pH units are easily distinguished in buffer and seawater samples, at a precision (standard deviation) of 28 μpH and 67 μpH, respectively, orders of magnitude better than what is possible with potentiometric pH probes.

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Shallow coral reef free ocean carbon enrichment: novel in situ flumes to manipulate pCO2 on shallow tropical coral reef communities

Published 5 March 2020 Science Closed
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Given the severe implications of climate change and ocean acidification (OA) for marine ecosystems, there is an urgent need to quantify ecosystem function in present‐day conditions to determine the impacts of future changes in environmental conditions. For tropical coral reefs that are acutely threatened by these effects, the metabolism of benthic communities provides several metrics suitable for this purpose, but the application of infrastructure to manipulate conditions and measure community responses is not fully realized. To date, most studies of the effects of OA on coral reefs have been conducted ex situ, and while greater ecological relevance can be achieved through free ocean carbon enrichment (FOCE) experiments on undisturbed areas of reef, such approaches have been deterred by technical challenges (e.g., spatial scale and duration, stable maintenance of conditions). In this study, we describe novel experimental infrastructure called shallow coral reef (SCoRe) FOCE to overcome these challenges and present data from a proof of concept application in Mo’orea, French Polynesia. Our objectives were to (1) implement an autonomous system that could be deployed kilometers from shore, (2) regulate the chemical (pCO2) and physical properties of seawater over undisturbed, shallow (∼2–5‐m depth) coral reef over multiple weeks, and (3) measure the metabolic response of the coral community to the treatment conditions. We describe the design, function, and application of the SCoRe FOCE, and present data demonstrating its efficacy. This infrastructure has great potential for advancing ecologically relevant studies of the effects of changing environmental conditions on coral reefs.

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Intercomparison of four methods to estimate coral calcification under various environmental conditions (update)

Published 26 February 2020 Science Closed
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Coral reefs are constructed by calcifiers that precipitate calcium carbonate to build their shells or skeletons through the process of calcification. Accurately assessing coral calcification rates is crucial to determine the health of these ecosystems and their response to major environmental changes such as ocean warming and acidification. Several approaches have been used to assess rates of coral calcification, but there is a real need to compare these approaches in order to ascertain that high-quality and intercomparable results can be produced. Here, we assessed four methods (total alkalinity anomaly, calcium anomaly, 45Ca incorporation, and 13C incorporation) to determine coral calcification of the reef-building coral Stylophora pistillata. Given the importance of environmental conditions for this process, the study was performed under two starting pH levels (ambient: 8.05 and low: 7.2) and two light (light and dark) conditions. Under all conditions, calcification rates estimated using the alkalinity and calcium anomaly techniques as well as 45Ca incorporation were highly correlated. Such a strong correlation between the alkalinity anomaly and 45Ca incorporation techniques has not been observed in previous studies and most probably results from improvements described in the present paper. The only method which provided calcification rates significantly different from the other three techniques was 13C incorporation. Calcification rates based on this method were consistently higher than those measured using the other techniques. Although reasons for these discrepancies remain unclear, the use of this technique for assessing calcification rates in corals is not recommended without further investigations.

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Autonomous, ISFET-based total alkalinity and pH measurements on a barrier reef of Kāneʻohe Bay

Published 17 February 2020 Science Closed
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Here we present first of its kind high frequency Total Alkalinity (AT) and pH data from a single solid-state autonomous sensor collected during a 6-day deployment at a barrier reef in Kāneʻohe Bay on the CRIMP-2 buoy. This dual parameter sensor is capable of rapid (<60 s), near simultaneous measurement of the preferred seawater carbonate system parameters, pH and AT without requiring any external reagents or moving parts inherent to the sensor. Its solid state construction, low power consumption, and low titrated volume (nanoliters) requirement make this sensor ideal for in situ monitoring of the aqueous carbon dioxide system. Through signal averaging, we estimate the pH-AT sensor is capable of achieving 2-10 μmol kg-1 precision in AT and 0.005 for pH. The CRIMP-2 site in Hawaiʻi provided an excellent means of validation of the prototype pH-AT sensor due to the extensive observations routinely collected at this site and large daily fluctuations in AT (~116 μmol kg-1) driven primarily by high calcification during the day and occasional CaCO3 mineral dissolution at night. High frequency sampling by the pH-AT sensor reveals details in the diurnal cycle that are nearly impossible to observe by discrete sampling. Greater temporal resolution of the aqueous carbon dioxide system is essential for differentiating various drivers of coral reef health and the response to external influences such as ocean warming and acidification.

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The ebb and flow of protons: a novel approach for the assessment of estuarine and coastal acidification

Published 10 February 2020 Science Closed
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Highlights

• Proton production and transport are responsible for estuarine acidification.

• Proton fluxes (mmol/h) were quantified between an estuary and bay.

• Fluxes calculated using high frequency [H+] and tidal discharge measurements.

• Non-tidal proton fluxes are directed upstream with seasonal changes in magnitude.

• Delaware Bay contributes to the acidification of the Murderkill Estuary.

Abstract

The acidification of coastal waters is a consequence of both natural (e.g., aerobic respiration) and anthropogenic (e.g., combustion of fossil fuels, eutrophication) processes and can negatively impact the surrounding ecosystems. Until recently it was difficult to accurately measure estuarine pH, and thus total proton concentrations (), when salinities vary significantly and rapidly as a consequence of tidal mixing. Proton production and transport are ultimately responsible for acidification in coastal environments, and the uncertainty surrounding proton concentrations measured at high frequency has hindered our understanding of the net impact of global and local processes on estuarine acidification. Here, we quantify the rate of proton exchange between an estuary and bay to assess the extent of acidification by using the novel combination of high frequency pHT (total hydrogen ion concentration scale) data from an autonomous SeapHOx™ sensor and continuous tidal discharge measurements made between the eutrophic Murderkill Estuary and Delaware Bay. Proton fluxes reverse with each tide. However, the net non-tidal proton fluxes are directed upstream and display seasonal changes in magnitude. Our results indicate that Delaware Bay contributes to the acidification of the Murderkill Estuary, yet the degree of acidification is reduced in the summer. Using proton concentrations measured at high temporal resolution to calculate proton fluxes provides a new and relatively simple approach for quantifying the acidification of dynamic nearshore environments.

Continue reading ‘The ebb and flow of protons: a novel approach for the assessment of estuarine and coastal acidification’

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