For the first time, the Autosub Long Range (ALR) has completed a fully autonomous, long-distance scientific mission. The ALR was equipped with a suite of oceanographic sensors including a Lab-on-Chip (LOC) pH sensor, a SeaBird SeaFET pH sensor, a conductivity-temperature-depth sensor (CTD), and a dissolved oxygen sensor, and traveled across the continental shelf, and back. Deployed and recovered from the shoreline in Plymouth, UK, the ALR operated without shipboard assistance, covering 2,000 km over five weeks (May-June 2022) and reaching depths up to 1000 m. We examine the performance of the pH sensors and apply an adapted k0 adjustment using different reference pH inputs, including neural network estimates. We find the onboard pH sensors agree better with each other than either sensor with model estimates. Before correction, the SeaFET and LOC pH measurements agree well, with a mean ΔpHT of 0.013 ± 0.009. After correction (LOC as reference) agreement improved to 0.00004 ± 0.007. Total alkalinity (TA) was estimated from both salinity-based relationships and model predictions. The partial pressure of CO2 (pCO2) was calculated from combinations of pH (raw, corrected, and model estimates) and TA (algorithms and model estimates), and range from 263-598 µatm. Air-sea CO2 fluxes range -17.0-7.1 mmol m-2d-1. Both pCO2 and CO2 fluxes are strongly influenced by pH input, with much less sensitivity to TA. This study highlights that while established, robust carbon models and methods exist for the open ocean, improved coastal observation strategies are critical, given the region’s disproportionate role in carbon cycling and production.
Hammermeister E. M., Wimart-Rousseau C., Papadimitriou S., Trucco-Pignata P., Chaney E., Templeton R., Phillips A. B. & Loucaides S., 2025. A first of its kind: the extended autonomous deployment of the Autosub Long Range with dual pH sensors. ESS Open Archive. Article.
