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

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 (O-2/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 (O-2/N-2) 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 O-2 and N-2 makes this tracer pair less accurate than O-2/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 O-2/ Ar and O-2/N-2 approaches were coherent throughout this cruise, with median mixed layer integrated NCP of 9.3 +/- 2.8 and 7.9 +/- 3.2 mmol O-2 m(-2) day(-1), respectively. The range of NCP was large, from less than zero to >100 mmol O-2 m 2 day 1, with some of the largest NCP estimates measured at well-established hotspots in the Pacific Arctic. While O-2/Ar and O-2/N-2 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.

Automated summary

Spatial and temporal patterns of primary productivity in the Arctic are expected to change with warming. Historically, productivity measurements have been limited in spatial and temporal resolution. Recent advancements in measuring dissolved oxygen/argon ratios (O-2/Ar) and oxygen/nitrogen ratios (O-2/N-2) have provided higher resolution assessments of net community production (NCP). However, the accuracy of the O-2/N-2 approach is lower due to differences in solubility between oxygen and nitrogen.