Coastal Marine Carbon and Air‐Sea Fluxes Quantified From pH Sensors on an Extended AUV Deployment

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 CO2 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 (k0), testing both the co-deployed LOC sensor and neural network estimates as reference pH. Before correction, the LOC and SeaFET sensors showed close agreement (Δ pHT = 0.013 ± 0.009), which improved to Δ pHT = 0.00004 ±
0.007 after LOC-based k0 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 CO2 (pCO2), which ranged 263–598 ± 27 μatm. Resulting air-sea CO2 fluxes ranged −17.0 to 7.1 ± 1.09 mmol m−2 d−1, with the Celtic Margin acting as a net CO2 sink in May–June of 2022. pCO2 and CO2 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 CO2 fluxes, capturing processes and variability that are largely unresolved by ship-based surveys or global models.