FerryBox-assisted monitoring of mixed layer pH in the Norwegian Coastal Current

• Automated, flow-through, spectrophotometric pH detector was developed for FerryBox platforms.
• In situ uncertainty < 0.003 and compliant with best practices.
• Norwegian Coastal Current wintertime pH was relatively low and stable.
• Spring and summertime pH was higher and more variable, coincident with higher chlorophyll.

The evaluation of marine carbonate system variability and the impacts of ocean acidification (OA) on coastal marine ecosystems increasingly rely on monitoring platforms capable of delivering near real-time in situ carbonate system observations. These observations are also used for developing models and scenarios of OA, including potential impacts on marine ecosystem structure and function. An embedded flow-through spectrophotometric pH detection system has been developed alongside an underway seawater sampling system – termed a FerryBox – operating on ships of opportunity (SOOP), and can deliver a continuous data stream of mixed layer seawater pH with an in situ uncertainty of < 0.003. We report metrological approaches behind the pH detection procedure and the evaluation of dye addition perturbation with analytical precision as low as 0.0005. In addition, we present field-based observations from a deployment of the pH detection system along the Norwegian Coastal Current in winter, spring, and summer periods of 2015. Spring and summertime pH was generally ~ 0.1 higher, and up to ~ 0.255 higher, in comparison to winter pH observations. Here we show the necessity for a regular, high density monitoring approach, and the suitability of this pH detection technique for unmanned observational platforms.