Abstract
We compared field measurements of the biological O2 saturation anomalies, ΔO2/Ar and ΔO2/N2, 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 ΔO2/N2 as an alternative to ΔO2/Ar for estimates of mixed layer net community production (NCP). We observed strong spatial coherence between ΔO2/Ar and ΔO2/N2, with small offsets resulting from differences in the solubility properties of Ar and N2 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 N2 and Ar, deriving the tracer ΔO2/N2′. Over most of our survey regions, ΔO2/N2′ provided a better analog for ΔO2/Ar, and thus more accurate NCP estimates than ΔO2/N2. However, in coastal Arctic waters, ΔO2/N2 and ΔO2/N2′ performed equally well as NCP tracers. On average, mixed layer NCP estimated from ΔO2/Ar and ΔO2/N2′ agreed to within ∼2 mmol O2 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 O2/N2 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.
Highlights
Marine net community production (NCP) represents the difference between gross photosynthesis and community-wide respiration, exerting a first-order control on the ocean’s capacity to support upper trophic level biomass and sequester atmospheric carbon dioxide via the biological pump (Volk and Hoffert, 1985; Ware and Thomson, 2005)
Our combined dataset allowed us to examine a range of conditions under which measurements of O2/N2 or O2/N2 can be used for accurate NCP derivation
In the Arctic, we identify nearshore waters as those occurring over the Baffin Bay continental shelf and in the Canadian Arctic Archipelago (CAA), and offshore waters of Baffin Bay (BB; excluding shelf regions < 6,000 km along-track)
Summary
Marine net community production (NCP) represents the difference between gross photosynthesis and community-wide respiration, exerting a first-order control on the ocean’s capacity to support upper trophic level biomass and sequester atmospheric carbon dioxide via the biological pump (Volk and Hoffert, 1985; Ware and Thomson, 2005). Oceanic responses to climate change are likely to alter marine biological production (e.g., Moore et al, 2018), but our capacity to predict these changes is limited, in part, by poor data coverage. Since the O2 saturation state is sensitive to both biological and physical processes, including temperature and salinity-dependent solubility effects and bubble injection, O2 measurements alone are insufficient to accurately resolve NCP. To address this limitation, O2 concentrations can be normalized to argon (Ar), a biologically inert gas with solubility properties that are virtually identical to O2 (Craig and Hayward, 1987). The so-called “biological O2 saturation anomaly,” O2/Ar (Eq 1), defined by normalizing the seawater O2/Ar ratio ([O2/Ar]sw) to the equilibrium ratio ([O2/Ar]eq), isolates the biological processes affecting O2
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