Philippe Tortell

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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Marine phytoplankton primary productivity, the photosynthetic conversion of CO₂ into organic carbon by microscopic photosynthetic algae in the surface ocean, plays a fundamental role in ecosystem dynamics and global biogeochemical cycles. Consequently, the ability to accurately measure, monitor and predict environmental influences on this process over a range of spatial and temporal scales is crucial. The work presented in this thesis evaluates the application of fast repetition rate fluorometry (FRRF) for instantaneous, high resolution estimates of phytoplankton primary productivity. Results from both laboratory experiments and field work in Arctic and Subarctic marine waters show that the conversion factor required to derive carbon-based primary productivity estimates from FRRF-derived rates of electron transport in photosystem II (ETR) varies significantly in response to the interacting effects of iron and light availability (Chapter 2), over diurnal cycles (Chapter 3), and in response to nitrogen and light availability under low temperatures (Chapter 4). At a photo-physiological level, a high conversion factor is observed under conditions of excess excitation energy, where the amount of light energy absorbed in the pigment antenna exceeds the capacity for downstream metabolic processes, i.e. carbon fixation. Phytoplankton employ numerous mechanisms to alleviate excess excitation energy after charge separation, and these processes are postulated to be responsible for the increased de-coupling of ETR and carbon fixation. Consistent with this hypothesis, a strong correlation was observed between the derived conversion factor and the dissipation of excess excitation energy before charge separation, which can be estimated as non-photochemical quenching (NPQ). Because NPQ can be estimated from FRRF measurements, it can be used as a proxy for the magnitude and variability of the conversion factor between carbon fixation and ETR, and this approach holds potential to significantly improve carbon-based primary productivity estimates from FRRF measurements. The work presented in this thesis advances our understanding of the coupling between light absorption, photo-chemistry, and carbon fixation in response to various environmental gradients. The experimental approach taken demonstrates how an appreciation of photo-physiological processes of photosynthesis is critical for improved estimates of phytoplankton primary productivity at regional scales.

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This PhD thesis examines the marine cycling of the greenhouse gases methane (CH₄) and nitrous-oxide (N₂O) in coastal British Columbia waters. The primary objectives of the work were to increase spatial and temporal data availability in an under-sampled coastal region, and to examine the processes responsible for CH₄ and N₂O distributions, and their sensitivity to changing environmental conditions (e.g. O₂-availability). Using a novel high-throughput analytical system, based on purge and trap gas chromatography-mass spectrometry (GC-MS), we measured a 6 year time-series of monthly water column CH₄ and N₂O profiles from Saanich Inlet, British Columbia, as well as three years of water column profiles and surface measurements along the West coast of Vancouver Island (WCVI). The physical and biological processes responsible for the observed CH₄ and N₂O distributions were investigated using relationships with ancillary physical data and biological data, including recently available meta-genomic information. The results presented in this thesis document a dominant role for O₂ concentrations in driving spatial and temporal variability in CH₄ and N₂O concentrations over a range of scales. In Saanich Inlet, the seasonal cycle of anoxia and deep water renewal exerts a primary control on water column N₂O and CH₄ accumulation, with additional likely contributions from sedimentary processes and in situ cycling of various nitrogen species and methylated compounds in the upper water column. In both Saanich Inlet and the WCVI, inter-annual variability and longer-term trends are associated with changes in upwelling intensity and El Nino events, and these changes are set against a background of declining O₂ concentrations across the Subarctic Pacific. Results from our work suggest that coastal CH₄ and N₂O concentrations may be responding significantly to these long-term declines in O₂ levels, with significant implications for regional sea-air fluxes of climate-active trace gases.

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Dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP) and dimethyl sulfoxide (DMSO) are ubiquitous in surface marine environments. These reduced sulfur compounds are crucial to the physiological ecology of bacteria and phytoplankton. DMS also has a role in climate regulation, as a source of aerosols that back-scatter incoming solar radiation. This thesis aims to characterize the processes driving DMS/P/O accumulation in polar and sub-polar marine waters. Chapters two and three document DMS/P/O concentrations in surface waters of the Subarctic Northeast Pacific, using automated measurement systems. These studies employed an existing system based on membrane inlet mass spectrometry (MIMS) and a novel automated system for the sequential analysis of DMS/P/O (OSSCAR; see chapter three). DMS/P/O concentrations demonstrate significant spatial variability over a range of scales in both coastal and open ocean waters, revealing relationships with key oceanographic variables. Chapter four describes the first application of a recently developed, stable isotope tracer technique using purge and trap capillary inlet mass spectrometry (PT-CIMS) in Antarctic sea-ice. This chapter documents extremely rapid DMS/P/O turnover in sea-ice brines and demonstrates a previously unrecognized role for DMSO, as well as DMSP, as important sources of DMS in these environments. Chapters five and six use MIMS, PT-CIMS, and OSSCAR in parallel to examine DMS/P/O cycling in the Subarctic Northeast Pacific, and in coastal waters of the Western Antarctic Peninsula (WAP). Chapter five focuses on the spatial distribution of DMS accumulation and net production in the Subarctic Pacific, while chapter six follows the seasonal changes in DMS/P/O concentrations and its production in the WAP, and highlights short-scale temporal variability of DMS/P/O. Results demonstrate strong spatial gradients in DMS production and consumption terms (higher values in near-shore waters) in the Subarctic Pacific, and showed that net DMS production predicts DMS accumulation in surface waters. Over the seasonal cycle in the WAP, zooplankton grazing and DMSP cleavage dominated DMS production, and bacterially mediated DMS consumption controlled the removal of DMS in surface waters.

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Recent Arctic warming and reduced summer sea ice extent have stimulated increased research into the role of sea ice in the high latitude carbon cycle. Using data collected on a number of field expeditions throughout the Arctic Ocean, I apply a multi-tracer approach to investigate the influence of the sea-ice life cycle on the biological and abiotic export of CO₂ into the sub-surface. The results of this study illuminate the role of sea ice in polar carbon cycling across the perennial sea ice region of the central Canada Basin and in the seasonal ice zone of the Canadian Arctic Archipelago. In the perennial sea ice region, lateral exchanges of shelf derived carbon were found to exert the most important control on carbon distribution in the central Canada Basin, both in the surface mixed layer and in the sub-surface halocline. Stable carbon isotope data suggest that surface water particulate organic carbon is derived, to a large extent, from external inputs from Eurasian rivers. Further, results from a suite of geochemical tracers show that sub-surface accumulation of dissolved inorganic carbon in the halocline reflects an organic matter remineralization signature derived from the shelves and transported into the halocline by dense Pacific winter waters. Within the seasonal ice zone, observations over the winter-spring transition illustrated a highly dynamic carbon cycle, and results from this study provide new insight into the biological, physical and chemical factors which contribute to C cycling in different depth horizons of the ice over this period. Physical constraints on inorganic carbon cycling dominated CO₂ distributions in the majority of the ice column early in the season. As the melt period advanced, sea ice melt dilution led to decreasing CO₂ partial pressures in brine, contributing to pCO₂ under-saturation and CO₂ uptake from the atmosphere as the melt period advanced. In contrast, the carbonate system in bottom ice layers was much more closely tied to the flourishing algal community. Our inorganic carbon system measurements within natural sea ice brine samples further reveal the limitations of current thermodynamic constants used to compute carbonate system equilibrium in sea ice systems.

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