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

Abstract

AbstractWe 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.