Autonomous Ion-Sensitive Field Effect Transistor-Based Total Alkalinity and pH Measurements on a Barrier Reef of Kane’ohe Bay
Ellen M. Briggs, Eric H. De Carlo, Christopher L. Sabine, Noah M. Howins, Todd R. Martz
Abstract
Here, we present first of its kind high-frequency total alkalinity (AT) and pH data from a single solid-state autonomous sensor collected during a six-day deployment at a barrier reef in Ka̅ne’ohe 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 AT without requiring any external reagents or moving parts inherent to the sensor. Its solid-state construction, low power consumption, and low titrated volume (nanoliters) requirement make this sensor ideal for in situ monitoring of the aqueous carbon dioxide system. Through signal-averaging, we estimate the pH–AT sensor is capable of achieving 2–10 μmol kg–1 precision in AT and 0.005 for pH. The CRIMP-2 site in Hawai’i provided an excellent means of validation of the prototype pH–AT sensor because of extensive observations routinely collected at this site and large daily fluctuations in AT (∼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–AT 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.