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Improved estimates of net community production in the Subarctic Pacific and Canadian Arctic Ocean using ship-based autonomous measurements and computational approaches Izett, Robert William

Abstract

This PhD thesis focuses on the development of new tools for measuring marine net community production (NCP), an important ecological variable quantifying the metabolic balance between photosynthesis and community-wide respiration. A common approach to estimating NCP exploits the seawater oxygen-to-argon ratio (O₂/Ar) and derived biological O₂ saturation anomaly, ΔO₂/Ar, as a tracer of net biological production. Using ship-based mass spectrometry, ΔO₂/Ar can be measured at high-resolution, enabling surface water NCP quantification from evaluations of the mixed layer O₂ budget. However, resulting NCP estimates may be biased by the vertical mixing flux of low- or high-O₂ water into the ocean’s surface, while the requirement of mass spectrometry to measure ΔO₂/Ar has largely constrained ship-based NCP quantification to ocean regions sampled by research vessels. These challenges have limited our ability to accurately observe NCP variability, particularly in coastal or under-sampled waters. This thesis addresses these limitations by combining high-resolution, underway sampling in the Subarctic Northeast Pacific and Canadian Arctic Ocean with instrumentation development and numerical analyses to yield new tools for NCP quantification from ship-based observations. Using measurements of nitrous oxide within and below the mixed layer and the output from a numerical ocean circulation model, I evaluate two approaches for correcting surface NCP estimates for biases caused by vertical mixing fluxes of O₂ in coastal and offshore waters. Applying these approaches produces new NCP estimates that reveal elevated productivity along the continental margin of the Subarctic Pacific and near regions of strong nutrient input from glaciers and wind-driven mixing in the Arctic. I also develop a new approach for NCP quantification based on surface seawater O₂ and nitrogen (N₂) observations obtained using a custom-built autonomous ship-based measurement system. This approach combines high-resolution O₂/N₂ sampling and computations with a one-dimensional gas model to derive a new NCP tracer, ΔO₂/N₂'. Using numerical simulations and field data, ΔO₂/N₂' is shown to be nearly equivalent to ΔO₂/Ar throughout the study region. The tools developed in this thesis can improve the accuracy and coverage of NCP observations, facilitating improved evaluations of the climate-dependent sensitivity of NCP and its relationship to other ocean ecosystem processes.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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