ΔO₂/N₂′ as a tracer of mixed layer net community production: Theoretical considerations and proof‐of‐concept

Abstract

We present a method to estimate net community production (NCP) from underway ΔO2/N2 observations. This approach, based on high‐resolution measurements using relatively low‐cost O2‐optode and gas tension device instrumentation, provides an alternative to NCP derivation from mass spectrometric analysis of ΔO2/Ar. To fully exploit ΔΟ2/Ν2 as an NCP tracer, it is necessary to account for differences in surface water argon (Ar) and nitrogen (Ν2) saturation states. Using a one‐dimensional mixed layer model, evaluated against field observations, we examined the divergence between ΔO2/Ar and ΔΟ2/Ν2 resulting from various physical processes. Results show that changes in sea surface temperature, bubble injection and vertical mixing can decouple mixed layer ΔO2/Ar and ΔΟ2/Ν2, while biological N2‐fixation has a negligible effect. Based on readily available environmental data, we developed a framework to correct for Ar and N2 solubility differences, yielding a new tracer, ΔΟ2/Ν2′, that is a near‐analog of ΔO2/Ar. Through model simulations and field measurements, we show that ΔΟ2/Ν2′ provides an excellent approximation to ΔO2/Ar, and that uncertainty and biases in ΔO2/N2′ are small relative to other errors in NCP calculations. This work demonstrates the potential for ΔO2/N2′ measurements to significantly expand NCP estimates from a variety of sampling platforms.

Design Update to “The Pressure of In-Situ Gases Instrument (PIGI) for Autonomous Shipboard Measurement of Dissolved O2 and N2 in Surface Ocean Waters”

“…The calculation includes temperature and salinity correction on both the O 2 and N 2 data based on measurements from the ship's thermosalinograph. Calculating NCP estimates following Izett and Tortell (2021) was easily performed as the required auxiliary data was collected by R/V Savannah, including the barometric pressure, wind speed at 10 m, and mixed layer depth (determined by CTD casts). These standard oceanographic vessel measurements facilitate calculations of NCP from PIGI data.…”

“…Similar to O 2 /Ar sampling, the PIGI system's NCP estimates assume that N 2 , like Ar, is a biologically inert analog for O 2 , meaning that the O 2 /N 2 ratio is unaffected by temperature and salinitydependent solubility effects, and that there is no biological impact on N 2 . Izett and Tortell (2021) and Izett et al (2022) tested the validity of these assumptions in model and field tests, demonstrating a negligible impact of biological N 2 cycling on mixed layer O 2 /N 2 signals. They also found near-equivalence between surface O 2 /Ar-and O 2 /N 2 -derived NCP estimates.…”