PhRePhOx — an in-situ approach to quantifying carbon cycling processes

The quantification of the marine biogeochemical cycling is crucial for global carbon cycle modeling efforts. However, the magnitudes of marine fluxes are poorly constrained due to limited data collection and methods in marine environments. Of vital importance is the marine water column productivity, which remains unknown due to the large uncertainty in available estimates. The lack of marine metabolic state validations hinders the predictive understanding of how global biogeochemical cycles respond to stressors (e.g. ocean acidification, rising temperatures, hydrologic cycle changes). To bridge the gap, we built an in-situ prototype sensor package to measure Photosynthesis, Respiration, and Photo-Oxidation (PhRePhOx). The package is designed to collect data autonomously, using continuous measurements of dissolved oxygen, dissolved inorganic carbon (DIC), and temperature within closed chambers. The package consists of a three-chamber incubator that flushes and pumps water into the 3 separate chambers producing dark, visible light, and ultraviolet (UV) + visible light incubations. During the in situ incubation period, the oxygen content, DIC, and temperature are monitored using oxygen optodes, a Channelized Optical System II (CHANOS II) DIC sensor and temperature sensors. The package is equipped with UV LEDs to minimize biofouling, which can bias measurements by obstructing the light path for photosynthesis/photooxidation and by enhancing rates through biological growth on the chamber walls. This prototype is the first of its kind that is capable of continuous sampling at high frequency (~ 1hr) for constructing high-resolution time series of in-situ ecosystem processes. The future integration of this package on various vessels and platforms will greatly improve understanding of marine biogeochemical cycles.