Autonomous Total Alkalinity and pH Measurements on a Barrier Reef of Kāneʻohe Bay

Here we present first of its kind high frequency Total Alkalinity (A_T) and pH data from a single solid-state autonomous sensor collected during a 6-day deployment at a barrier reef in Kāneʻohe Bay on the CRIMP-2 buoy. This dual parameter sensor is capable of rapid (<60 s), near simultaneous measurement of the preferred seawater carbonate system parameters, pH and A_T without requiring any external reagents or moving parts inherent to the sensor. Its solid state construction, low sample volume requirements (nanoliters), and low power consumption make this sensor ideal for in situ monitoring of the aqueous carbon dioxide system. Through signal averaging, we estimate the pH-A_T sensor is capable of achieving 2-10 μmol kg^˗1 precision in A_T and 0.005 for pH. The CRIMP-2 site in Hawaiʻi provided an excellent means of validation of the prototype pH-A_T sensor due to the extensive observations routinely collected at this site and large daily fluctuations in A_T (~116 μmol kg^˗1) driven primarily by high calcification during the day and occasional CaCO₃ mineral dissolution at night. High frequency sampling by the pH-A_T sensor reveals details in the diurnal cycle that are nearly impossible to observe by discrete sampling. Greater temporal resolution of the aqueous carbon dioxide system is essential for differentiating various drivers of coral reef health and the response to external influences such as ocean warming and acidification.