High-resolution biological net community production in the Pacific-influenced Arctic as constrained by O2/Ar and O2/N2 observations

Abstract

Spatial and temporal patterns of primary productivity in the Arctic are expected to change with warming-associated changes in ice cover and stratification, yet productivity measurements are historically spatially and temporally limited. Over the last two decades, an approach that uses measurement of dissolved oxygen/argon ratios (O2/Ar) from a vessel's underway seawater system has emerged as an established method to assess net community production (NCP) rates with high spatial and/or temporal resolution. More recently, the measurement of oxygen/nitrogen ratios (O2/N2) with a gas tension device (GTD) and optode have been piloted in underway settings to provide comparable NCP estimates. The GTD/optode approach has several advantages: instrumentation is small, inexpensive, and suitable for autonomous deployments; however, dissimilarity in solubility between O2 and N2 makes this tracer pair less accurate than O2/Ar. We conducted a side-by-side ship-based comparison of a GTD/optode and Equilibrator Inlet Mass Spectrometer (EIMS) in the Pacific Arctic during one of the North Pacific Research Board Integrated Ecosystem Research Program cruises in 2019. NCP from O2/Ar and O2/N2 approaches were coherent throughout this cruise, with median mixed layer integrated NCP of 9.3 ± 2.8 and 7.9 ± 3.2 mmol O2 m−2 day−1, respectively. The range of NCP was large, from less than zero to >100 mmol O2 m−2 day−1, with some of the largest NCP estimates measured at well-established hotspots in the Pacific Arctic. While O2/Ar and O2/N2 largely tracked each other, deviations were observed, principally in the Bering Sea where wind-induced bubbles were a primary driver, while a combination of temperature and wind drove differences over the majority of the cruise. The GTD/optode can be used to enhance spatial and temporal coverage of NCP measurements, yet the uncertainty makes this approach better-suited to regions with higher overall rates of NCP, while regions near-equilibrium may result in unacceptably high uncertainty. Additionally, the GTD/optode is reliant on well-calibrated oxygen observations, a potential challenge if autonomously deployed.