UC San Diego

Anthropogenic activities are driving changes in ocean biogeochemistry, which can be monitored through instruments and sensors deployed across diverse platforms in even the harshest marine environments. Continued monitoring of these changes demands innovations in instrumentation, calibration and quality control to effectively capture dynamic signals and ensure comprehensive ocean coverage. This dissertation focuses on advancements in oceanographic pH sensors, starting with the longest near-continuous ocean pH dataset collected using ion-sensitive field effect transistor (ISFET) technology at Scripps Pier. A new in situ calibration approach, based on direct tris buffer injection, was compared to the traditional bottle collection method, yielding a fourfold improvement in repeatability with an uncertainty of 0.006 pH. Additionally, an automated calibration system integrated into the sensor package was evaluated, offering near real-time, self-calibrating capability for ocean acidification and biogeochemical monitoring programs. To continue the discourse of pH sensor technology in the second section of this dissertation, a novel optical pH sensor was evaluated in laboratory settings to establish its accuracy and precision, response time, temperature and pressure sensitivity, and calibration techniques which improved accuracy over factory methods. Field tests of the optical pH sensor across diverse marine environments—deep ocean, dynamic nearshore, and open ocean profiling—provided guidelines for field calibration, correction and optimal field use. In a scaled-up sense, the final section of this dissertation leveraged pH and other biogeochemical sensors on BGC-Argo profiling floats to explore biogeochemical variability in the equatorial Pacific from 2019 to 2024. While the region has extensive physical data, subsurface biogeochemical observations and their links to El Niño and La Niña cycles are sparse. These floats revealed distinct biogeochemical patterns driven by vertical movement of the mixed layer depth, meridional subtropical water transport and primary production shifts associated with ENSO phases. Overall, this work combines new sensor technologies and analytical methods to provide essential data, instrument guidelines and reveal insights into ocean biogeochemical phenomena. Ongoing instrumentation development and monitoring will be critical to expand and deepen our understanding of how human-driven impacts are transforming our oceans.