ΔO2/N2’ as a Tracer of Mixed Layer Net Community Production: Theoretical
Considerations and Proof-of-Concept
Abstract
The biological oxygen (O2) saturation anomaly ΔO2/Ar is a tracer for net
community production (NCP) in marine surface waters, with argon (Ar)
normalization used to correct for physical effects on O2
supersaturation. Ship-board mass spectrometry has been used for ΔO2/Ar
measurements, but this approach may not be accessible to many research
groups. Here, we present a proof-of-concept for NCP estimates based on
underway measurements of ΔO2/N2, which can be obtained from deployments
of O2-Optodes and gas tension devices (GTD). We used a one-dimensional
mixed layer model, validated against field observations, to evaluate
divergence in ΔO2/Ar and ΔO/N2 resulting from differences in the
sensitivity of Ar and nitrogen (N2) to various physical processes.
Changes in sea surface temperature and responses in air-sea exchange
most strongly decouple surface Ar and N2 with additional excess N2
associated with bubble-injection during high-wind conditions and
vertical mixing in regions of elevated subsurface N2. In contrast,
biological N2-fixation has a negligible contribution to the observed
divergence between Ar and N2. Based on readily available environmental
data, we present an approach to correct for Ar and N2 differences,
yielding a new tracer, N2’, that is a near analog of Ar. We show that
ΔO2/N2’ provides an excellent approximation to ΔO2/Ar, and that
uncertainty and biases in ΔO2/N2’ are small relative to other errors in
NCP calculations. Our results demonstrate the potential for ΔO2/N2’
measurements to expand NCP estimates from oceanographic research
surveys, vessels of opportunity or autonomous surface vehicles.