Delta O-2/N-2 ' As A New Tracer Of Marine Net Community Production: Application And Evaluation In The Subarctic Northeast Pacific And Canadian Arctic Ocean

We compared field measurements of the biological O-2 saturation anomalies, Delta O-2/Ar and Delta O-2/N-2, from simultaneous oceanographic deployments of a membrane inlet mass spectrometer and optode/gas tension device (GTD). Data from the Subarctic Northeast Pacific and Canadian Arctic Ocean were used to evaluate Delta O-2/N-2 as an alternative to Delta O-2/Ar for estimates of mixed layer net community production (NCP). We observed strong spatial coherence between Delta O-2/Ar and Delta O-2/N-2, with small offsets resulting from differences in the solubility properties of Ar and N-2 and their sensitivity to vertical mixing fluxes. Larger offsets between the two tracers were observed across hydrographic fronts and under elevated sea states, resulting from the differential time-response of the optode and GTD, and from bubble dissolution in the ship's seawater lines. We used a simple numerical framework to correct for physical sources of divergence between N-2 and Ar, deriving the tracer Delta O-2/N-2'. Over most of our survey regions, Delta O-2/N-2' provided a better analog for Delta O-2/Ar, and thus more accurate NCP estimates than Delta O-2/N-2. However, in coastal Arctic waters, Delta O-2/N-2 and Delta O-2/N-2' performed equally well as NCP tracers. On average, mixed layer NCP estimated from Delta O-2/Ar and Delta O-2/N-2' agreed to within similar to 2 mmol O-2 m(-2) d(-1), with offsets typically smaller than other errors in NCP calculations. Our results demonstrate a significant potential to derive NCP from underway O-2/N-2 measurements across various oceanic regions. Optode/GTD systems could replace mass spectrometers for autonomous NCP derivation under many oceanographic conditions, thereby presenting opportunities to significantly expand global NCP coverage from various underway platforms.