Socratis Loucaides

The highly productive Amundsen Sea Polynya in West Antarctica is bounded in the south by rapidly‐melting ice shelves, including the Dotson Ice Shelf (DIS). We present a high‐resolution, full‐depth pH data set within 10 km of DIS using a Lab‐On‐Chip (LOC) spectrophotometric pH sensor on an ocean glider. The LOC performs well with a 0.01 difference in pH between sensor measurements and discrete samples. Over 19 days in January–February 2022, the pH sensor captured the variability resulting from summertime biological and physical processes. At depth, modified Circumpolar Deep Water entering the DIS cavity exhibited a relatively low pH (7.91). The outflow had a higher average pH (7.94) as a result of dilution by glacial meltwater, but its pH was 0.01 lower than that of surrounding water at the same density. The Antarctic Coastal Current transported unventilated surface water with a low pH emerging from below fast ice east of Bear Island, westward to the eastern side of DIS. Net primary production increased pH in the upper 50 m in front of DIS, whilst temperature change was responsible for 5 of the change in pH. This study provides the first direct measurements of pH in the Amundsen Sea capable of deducing the impact of glacial meltwater on pH. It demonstrates the value, particularly in remote regions, of LOC pH sensors that have the longevity and stability to measure ocean pH without sensor drift.

Marine carbon dioxide removal (mCDR) techniques have the potential to remove legacy emitted carbon dioxide from the atmosphere and store it durably in ocean reservoirs. While the ocean’s vast surface area and buffering capacity offer benefits for scaling removal, monitoring remains challenging. This comprehensive review of marine carbonate system observing and monitoring technologies finds that mCDR processes span multiple scales, often at the edges of existing platform and sensing capability. While some mCDR observing and monitoring is possible today, challenges will arise as experiments and operations expand. Leaps forward in assessing the novel scales of mCDR processes will emerge from low-energy, low-cost, highly durable sensors; innovative deployment platforms; cooperative sensing networks; dedicated model validation programs; and decision support tools. This review is intended to serve as a guide to the history, ongoing innovation, and transformative vision of future marine carbonate system observing and monitoring informed by mCDR.

Coral reefs are increasingly threatened by local and global stressors. Traditional visual reef monitoring methods are time‐intensive and provide only a snapshot of reef condition. Recent efforts aim to measure reef metabolism, or the balance of net community production (NCP) and net community calcification (NCC), as coral reef carbonate budgets shift globally toward declining NCC and greater NCP influence. Resolving these metabolic changes requires comprehensive, high‐resolution biogeochemical measurements unattainable by traditional sampling alone. This work reports the novel multi‐month deployment of a high‐resolution, autonomous total alkalinity (TA) sensor alongside pH, partial pressure of , and dissolved oxygen (DO) sensors at Cement Wreck, a central Red Sea offshore reef. Observations from the back reef and fore reef revealed strong diel variability and distinct reef‐zone biogeochemical dynamics linked to contrasting benthic compositions. On the back reef, pH and DO were tightly coupled and closely matched a Redfield‐based model , while the fore reef relationship was weaker . TA observations allowed characterization of the back reef carbonate system, with TA–DIC stoichiometry yielding a slope of 0.26, indicating NCP accounted for approximately 87% of observed carbonate system variability. Compared to global reefs, Cement Wreck exhibits high % NCP contribution. However, the observed 2023 TA–DIC slope closely matches those from Red Sea observations from 1997 to 2013, suggesting that while reefs around the world shift, Red Sea reefs like Cement Wreck represent a persistent metabolic state. Scaled‐up autonomous deployments, like the one presented here, will better enable predictions of global reef health in future conditions.

For the first time, the Autosub Long Range (ALR) completed a fully autonomous, long‐distance (2,000 km) scientific mission, delivering new insight into coastal carbonate dynamics and air‐sea fluxes. Equipped with a suite of oceanographic sensors, including a Lab‐on‐Chip (LOC) pH sensor and a Sea‐Bird SeaFET pH sensor, the mission generated nearly 50,000 high‐resolution pH measurements, providing one of the most detailed continuous coastal carbonate data sets collected to date in the region. We evaluated the adjustment of the SeaFET reference potential ( k ₀ ), testing both the co‐deployed LOC sensor and neural network estimates as reference pH. Before correction, the LOC and SeaFET sensors showed close agreement ( = 0.013 0.009), which improved to = 0.00004 0.007 after LOC‐based k ₀ adjustment. Both sensors diverged from model estimates, indicating reduced ability of models to resolve fine‐scale coastal variability and reinforcing the need for direct in situ observations. Total alkalinity (TA) was derived from salinity‐based relationships and model predictions, and paired with pH (SeaFET, LOC, and modeled) to estimate the partial pressure of ( p ), which ranged 263–598 27 μatm. Resulting air‐sea fluxes ranged −17.0 to 7.1 1.09 mmol , with the Celtic Margin acting as a net sink in May–June of 2022. p and flux proved sensitive to subtle pH differences, but less so to TA estimates. Our findings demonstrate the critical role of high‐resolution autonomous observations in quantifying coastal carbonate dynamics and fluxes, capturing processes and variability that are largely unresolved by ship‐based surveys or global models.

We present a method to quantify benthic ecosystem calcification from simultaneous pH (proton) and O₂ eddy covariance flux measurements. In benthic ecosystems, photosynthesis is a proton sink, while calcification is a proton source. Where calcification is the dominant nonmetabolic proton source, it can be isolated as the residual between the measured proton flux and the flux predicted from O₂-derived metabolism. We demonstrate this technique in Posidonia oceanica seagrass meadows near Ischia, Italy, where coralline algae epiphytes are the primary calcifiers. The method resolved a diurnal calcification signal consistent in magnitude with previous estimates for seagrass epiphytes. However, our pH measurements and proton fluxes also revealed widespread, diffusive CO₂ vent influence at both the vent-adjacent site and the control site (670 m away), demonstrating that control sites near natural CO₂ vents may not provide the stable baseline often assumed. Excluding the vent-affected data removed substantial portions of the data set, resulting in high uncertainty, while also illustrating the insights that high-speed multiparameter sensing provides. Our error analysis identifies accuracy in pH, alkalinity, and the ecosystem photosynthetic quotient as critical constraints on this and other pH-O₂ based calcification measurements, particularly in environments where calcification rates are small relative to metabolic fluxes.

The development of marine autonomous platforms has improved our capability to gather ocean observations at fine spatial scales and high temporal frequency, which can be used to better measure, characterize, and model ocean carbon. As part of the OCEANIDS program, novel carbonate sensors were integrated into the Autosub Long-Range (ALR) autonomous underwater vehicle (AUV) and deployed in the Celtic Sea. Autonomous Lab-On-Chip (LOC) sensors measured pH and total alkalinity (TA) while onboard the ALR. Using interpolation, the ALR-sensor data set is compared against CTD co-samples. The average differences between the LOC sensor and co-sample pH range from -0.011 to -0.015. The TA sensor data agrees with co-samples within 1-2 μmol kg^-1 on average. Biogeochemical water properties differing between CTD and ALR observations reveal correlations to carbonate parameter variations. The LOC sensors enabled the characterization of the marine carbonate system from autonomous subsurface measurements for the first time. Sensor pH and TA data were used to calculate dissolved inorganic carbon (DIC), partial pressure of CO₂ (pCO₂), and aragonite saturation state (Ω_Ar) and are compared with CTD co-samples with mean residuals of 4-7 μmol kg^-1, 10-17 μatm, and -0.03 to -0.06, respectively. Future perspectives on sensor deployment and analysis are discussed.

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.