Ocean Acidification

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.

Total alkalinity (TA) is one of the measurable parameters that characterize the oceanic carbonate system. A high temporal and spatial frequency in TA data can lead to better measurements, modeling, and understanding of the carbon cycle in aquatic systems, providing insights into problems from global climate change to ecosystem functioning. However, there are very few autonomous technologies for in situ TA measurements, and none with field demonstrations below 2 m depth. To meet this need in marine observing capabilities, we present a submersible sensor for autonomous in situ TA measurements to full ocean depths. This sensor uses lab-on-a-chip technology to sample seawater and perform single-point open-cell titration with an optical measurement. It can carry multiple calibration materials on board, allowing for routine recalibration and quality checks in the field. The sensor was characterized in the laboratory and in a pressure testing facility to 600 bar (equivalent to 6 km depth) and deployed in a shallow estuary, on a lander at 120 m depth, and on an autonomous underwater vehicle. With a demonstrated precision and accuracy regularly better than 5 μmol kg^–1 in field deployments, this sensor has the potential to dramatically expand our ability to perform long-term autonomous measurements of the marine carbonate system.

Providing reliable instrumentation that enables collection of high-​quality, comparable data is one of the most challenging aspects of establishing ocean acidification monitoring programs. This is especially true for under-resourced countries, where such data are mostly unavailable. In 2016, The Ocean Foundation (TOF) worked with international bodies, including the International Atomic Energy Agency’s Ocean Acidification International Coordination Centre and the Global Ocean Acidification Observing Network (GOA-ON), and subject matter experts to develop a set of equipment known as the “GOA-ON in a Box” kit (The Ocean Foundation, 2017). This comprehensive kit provides researchers with everything needed—down to specialized rubber bands—to obtain weather-quality carbonate system measurements as defined by GOA-ON (Newton et al., 2015). Data are generated from spectrophotometric measurements of pH_T and manual titrations for total alkalinity from discrete samples as well as in situ sensors, the iSAMI-pH and a CTD. The kit’s modular design, composed of nearly 100 unique items, makes it much less expensive than comparable integrated systems and allows for easier troubleshooting and replacement of supplied spare components.