Ocean Acidification

The partial pressure of CO₂ (p(CO₂)) dissolved in seawater is often indirectly determined through a mathematical calculation involving two variables of the carbonate system, which requires the use of expensive equipment, materials, and reagents, and is usually demanding in terms of human resources. In this study, we present a simple, low-cost method for directly measuring dissolved CO₂ (p(CO₂)) in coastal waters using a small gas equilibrator coupled with a non-dispersive infrared (NDIR) detector in a closed-loop system. The cost of this methodology can be further reduced by utilizing a low-cost NDIR detector and inexpensive materials. Its analytical performance was validated against the conventional indirect approach based on pH and dissolved inorganic carbon (DIC), showing high accuracy (91%), excellent linearity (r² = 0.998), and a quantification limit (∼21 μatm) approximately 20 times lower than the average surface seawater p(CO₂). The method proved robust to typical variations in salinity (15–45 PSU) and temperature (20–25 °C), though a new calibration is recommended for salinities below 15 PSU. Application to samples from a tropical coastal lagoon and adjacent oceanic area showed statistically comparable results with both methodologies. This approach could enable direct measurement in the field, thereby reducing labor, equipment, and material costs. This low-cost approach enhances in situ p(CO₂) monitoring, offering significant potential for long-term ocean acidification studies in resource-limited settings.

During the continuous monitoring of pH in environmental water, the temperature of the environmental water changes at each moment. Coral reefs are likely to be strongly affected by ocean acidification, so accurate and continuous pH monitoring is important. On the other hands, the water temperature in coral reef areas varies greatly not only due to solar radiation but also due to the mixing of open ocean water and land water. This temperature variation affects the accurate pH measurement. Preparing calibration values for the pH electrodes at all water temperatures is impossible, but the monitoring accuracy could be improved by predicting the constituent values at the on-site water temperature from the electrode calibration results at one or two temperatures. Therefore, we calibrate the pH at one or two temperatures and use the result for estimating the calibration value at another temperature. For this purpose, we estimated the pH change of the internal liquid of the electrode (assumed as a phosphate buffer) under saturated and unsaturated conditions of the coexisting KCl in the buffer.

As a key index to measure the buffering capacity of marine carbonate systems, the total alkalinity (TA) of seawater plays an important role in monitoring and predicting ocean acidification. Given the limitation of the existing TA measuring techniques to laboratory environments and the difficulty in continuous real-time monitoring of marine fields, the existing TA analyzer was optimized based on single-step titration spectrophotometry. Then, a hardware system covering sample collection, measurement and analysis, data processing and storage, power supply, and control system was established by integrating the overall architectural design and development of the raft microcirculation system, dual-path photoelectric detection system, and mini-instrument. Besides, the corresponding instrument control, data processing and analysis software and user-friendly interface were developed, because of which the detection error and precision of the analyzer were < 1 % and ± 3.6 μmol/kg, respectively. The study results provide a high-precision and high-efficiency seawater TA measuring tool for scientific marine research and environmental monitoring.

Human activities have significantly increased carbon dioxide emissions, leading to global warming and ocean acidification, which threaten marine ecosystems, including coral reefs with high biodiversity. Coral reef maintenance relies on a balance between calcium carbonate formation and dissolution. Among the processes, sandy sediments, covering vast areas and highly sensitive to ocean acidification, require urgent investigations to elucidate their dissolution mechanisms. However, conventional glass electrodes have limitations in continuous monitoring of the spatiotemporal distribution of pH within sediment. To address this, we developed a multichannel ion-sensitive field-effect transistor (ISFET)-pH sensor array with a tantalum oxide sensing membrane, which was embedded in the sediment to enable high-resolution and continuous pH monitoring. A 24-h pH monitoring experiment was conducted in coral reef sediments to validate the method. The performance of the sensor was evaluated through both laboratory and field observations, and a comparison with a conventional glass electrode confirmed that the ISFET-pH sensor provided stable pH measurements within the uncertainty range of the glass electrode. The developed sensor array is a low-cost and durable automatic measurement system, offering an alternative to conventional glass electrodes, which are expensive and fragile. However, optimizing sputtering conditions, annealing processes, and data processing techniques is necessary to reduce environmental influences and enhance measurement accuracy. The proposed array-based observation method enables the acquisition of high-resolution vertical pH profiles and is expected to contribute to the quantitative evaluation of the chemical role of sandy sediments and the elucidation of carbon cycling in coral reef ecosystems.

The ocean has absorbed 25% of anthropogenic carbon dioxide emissions over the past 40 years, effectively slowing atmospheric carbon dioxide growth but causing ocean acidification. As acidification intensifies, the seawater absorption capacity for carbon dioxide will decline. While the Revelle factor has been used to assess carbon dioxide absorption, it becomes inapplicable at pH < 7.5. Here, we propose a new factor, γ_CO2, to better measure the absorption capacity for carbon dioxide and acidification. γ_CO2 decreases with increased partial pressure of carbon dioxide and decreased pH, indicating reduced absorption capacity and intensified acidification. In 2020, global surface ocean γ_CO2 was 15.50 ± 0.21, a 13% decline since 1992. Projections under SSP5-8.5 anticipate an average γ_CO2 of 4.72 by 2100, with 61.5% of global ocean regions below the critical threshold of γ_CO2 = 3.0, potentially harming aragonite-based organisms.

pH, which reflects the thermodynamic balance of acid-base systems in seawater, serves as a key indicator of the interplay between acidic and basic components in marine environments. When combined with another parameter, such as TA, DIC, or pCO2 the entire inorganic carbon system can be derived. However, each parameter presents methodological challenges that may introduce random or systematic errors, which then propagate through subsequent calculations. In coastal and estuarine environments, errors can become more pronounced, as standard operating procedures (SOPs) developed for open-ocean conditions may not adequately address the complexities unique to these regions. Measuring more than two parameters enables further insight into systematic errors through the evaluation of internal consistency, where existing data products often reveal pH-dependent offsets between measured pH and pH calculated from measured TA and DIC. These offsets may arise from errors in pH measurements, TA and DIC measurements, or the equilibrium constants used in the calculations, and are therefore difficult to tease apart. Comparing measurements from different research groups can help identify the specific measurement biases responsible for these offsets; however, the lack of inter-comparison studies, particularly in field settings, hinders our understanding. This work advocates for integrating internal consistency and inter-comparison studies in field conditions, as conducting them at sea provides a realistic evaluation of reproducibility between research groups. Chapter 2 utilizes this method by comparing at-sea spectrophotometric pH (pHspec) measurements from two research groups aboard the R/V Coriolis in June 2022 in the Gulf of St. Lawrence and the Lower St. Lawrence Estuary during the Tracer Release Deep Experiment 2 (TReX2) cruise. This combined analysis of reproducibility and internal consistency highlights how even minor methodological differences can substantially affect data quality, and in turn, shape data interpretation. These impacts are particularly pronounced when estimating potential bias from unidentified, excess components of TA (TAx), expected to be non-negligible in estuarine environments, where the two groups had notably different estimates. Chapter 3 draws conclusions from the discussion of data quality in estuarine environments from Chapter 2, focusing on the potential role of pHspec in Monitoring, Reporting, and Verification (MRV) frameworks for ocean alkalinity enhancement (OAE), a proposed marine carbon dioxide removal (mCDR) strategy. It incorporates insights from OAE field trial work in the Bedford Basin, Halifax, a fjord-like estuarine system, to assess the quality of pHspec, TA, and DIC data, offering an assessment of the reliability of these measurements for interpreting potential carbon dioxide removal. This chapter also includes suggestions for a future protocol for observational components of MRV frameworks.