Philippe Tortell


Relevant Thesis-Based Degree Programs


Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Improved estimates of net community production in the Subarctic Pacific and Canadian Arctic Ocean using ship-based autonomous measurements and computational approaches (2021)

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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On the light and iron dependent coupling of carbon fixation and photosynthetic electron transport in Arctic and Subarctic marine phytoplankton (2016)

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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Temporal trends and biogeochemical controls on methane and nitrous-oxide distributions in coastal waters of the subarctic Pacific Ocean (2016)

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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Concentrations and Turnover Rates of Reduced Sulfur Compounds in Polar and Sub-Polar Marine Waters: Field Application of Novel Analytical and Experimental Techniques (2015)

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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A multi-tracer study of the role of sea ice in the Arctic Ocean carbon cycle (2014)

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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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Contrasting distributions and cycling of reduced sulfur compounds in saline and estuarine waters of the coastal NE subarctic Pacific (2021)

The trace gas dimethylsulfide (DMS) is considered to be one of the most important sulfur compounds in the marine environment. Research on this volatile sulfur compound has been stimulated by its potential role in regulating regional and global climate, and its importance, along with the related sulfur compounds, dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO), as carbon and sulfur sources for microbes in the marine environment. The northeast subarctic Pacific (NESAP) is one of the global DMS hotspots, with significant spatial and temporal variability in DMS production. The difference in nutrient supply between coastal and offshore waters in this region drives significant variability in phytoplankton community structure, primary productivity, and thus sulfur cycling. The goal of this thesis is to characterize the patterns of sulfur cycling in two hydrographically distinct regimes in the NESAP, and to provide insights into the relative contribution of various DMS production pathways. Chapter 2 presents new measurements of DMS, DMSP and DMSO (DMS/P/O) concentrations and turnover rate constants made in the coastal NESAP, as well as ancillary hydrographic and satellite data that help explain the underlying factors influencing DMS/P/O distributions and cycling. A strong linear relationship was demonstrated between DMS and DMSO concentrations, confirming similar ratios found in previous studies. Turnover rate constants for net DMS production from DMSO were comparable to those for DMSP, indicating DMSO reduction as an important pathway for marine DMS production. Similar rate constants for DMSP cleavage and DMSO reduction between the two regimes were found, although the lower average of kDMSPcleav in the continental shelf waters may indicate higher bacterial sulfur demand in the more productive shelf waters. Our findings provide insights into marine sulfur dynamics in adjacent but contrasting marine waters, and highlight the significant contribution of DMSO to DMS production. This result suggests a need for improved understanding of marine DMSO cycling. In addition to the main research presented in the body of this thesis, the Appendices present supplementary tests of various DMSP preservation methods, and a detailed protocol for DMSO reduction by the TiCl3 method. These methodological details will be useful for future sulfur studies.

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Distribution and dynamics of biogenic sulfur in the northeast Subarctic Pacific: insights from new and refined analytical techniques (2019)

The northeast subarctic Pacific (NESAP) is a globally important source of the climate-active gas dimethylsulfide (DMS), yet the processes driving DMS variability across this region are poorly understood. This thesis aims to provide insight into the distribution and cycling of DMS and related sulfur compounds dimethylsulfoxide (DMSO) and dimethylsulfoniopropionate (DMSP) by examining new concentration data, together with biological cycling rates and related oceanographic variables.Chapter 2 examines the distribution of DMS at various spatial scales across contrasting oceanographic regimes of the NESAP. We present a new data set of high spatial resolution DMS measurements across hydrographic frontal zones, together with key environmental variables and biological rate measurements. We combine these new data with existing observations to produce a revised summertime DMS climatology for the NESAP. Our results suggest the presence of two distinct DMS cycling regimes corresponding to microphytoplankton-dominated waters along the continental shelf, and nanoplankton-dominated transitional waters. In all areas, DMS consumption appeared to be an important control on concentration gradients, with higher DMS consumption rate constants associated with lower DMS concentrations. Based on our compiled observations, we estimated that this region emits 0.30 Tg of sulfur to the atmosphere during the summer season.Chapter 3 presents results from two cruises examining DMSO distributions and cycling across the NESAP. We measured DMSO concentrations and turnover rates across a range of hydrographic regions, and quantified rates of DMSO reduction, DMSP cleavage and DMS oxidation. Our results show high concentrations and rapid turnover rates of DMSO across the NESAP. Across our survey, DMSO reduction exceeded DMSPd cleavage at nearly all stations, while the rates of DMSO reduction exceeded those of DMS oxidation at four stations where both these rates were measured. These results suggest that DMSO reduction was an important net source of DMS. A Lagrangian survey showed a significant decrease in DMSO concentrations during periods of peak irradiance, in conjunction with markers of oxidative stress. Our findings highlight the significant contribution of DMSO to DMS production in the NESAP, and its potential physiological importance as an anti-oxidant in phytoplankton assemblages.

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The Distribution of Reduced Sulphur Compounds in Polar Environments: Insights from Observations and Climatologies (2018)

Dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) are key components in the marine reduced sulphur cycle, where they play several roles in the ecology of bacteria and phytoplankton. Upon emission to the atmosphere, DMS plays a role in atmospheric sulphur budgets and radiative balance, having potentially climate-cooling effects. This thesis aims to provide insight into the distribution of these compounds in polar marine waters. This is done by constructing a revised climatology of DMS budgets in the Southern Ocean and by presenting new DMS/P data in the Arctic Ocean. Chapter 2 presents a revised summertime climatology of DMS distributions and fluxes in the Southern Ocean, based on the inclusion of a significant number of high-resolution measurements (~700 000) made in recent years. Based on the climatology written by Lana et al in 2011, the revised climatology shows notable differences in DMS budgets. In particular, we find increased DMS concentrations and sea–air fluxes south of the Polar Frontal zone (between 60 and 70°S), and increased sea–air fluxes in mid-latitude waters (40–50°S). These changes are attributable to both the inclusion of new data and the use of region-specific parameters (e.g. data cut-off thresholds and interpolation radius) in our objective analysis. DMS concentrations in the Southern Ocean exhibit weak though statistically significant correlations with several oceanographic variables, including ice cover, mixed-layer depth and chlorophyll-a. Chapter 3 presents new DMS and DMSP measurements made in the Canadian sector of the Arctic Ocean on the 2015 GEOTRACES expedition, as well as estimates of sea-air fluxes and hydrographic data that presents some potential explanations for these distributions. Across the full sampling transect, we find weak relationships between DMSP:chl a ratios and known diatom marker pigments and elevated DMS/P in partially ice-covered areas. Our high spatial resolution measurements allowed us to examine DMS variability over small scales, and to document DMS concentration gradients across surface hydrographic frontal features. Together, these two chapters help to fill out the understanding of the distribution and cycling of reduced sulphur in polar marine waters, and can serve to provide a baseline for future reduced sulphur work in these regions.

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Methane and nitrous oxide distributions across the North American Arctic Ocean during summer, 2015 (2016)

We collected Arctic Ocean water column samples for methane (CH₄) and nitrous oxide (N₂O) analysis on three separate cruises in the summer and fall of 2015, covering a ~10,000 km transect from the Bering Sea to Baffin Bay. Our sampling program provides a large-scale, three-dimensional view of methane and nitrous oxide concentrations across contrasting hydrographic environments, from the deep oligotrophic waters of the deep Canada Basin and Baffin Bay, to the productive continental shelf regions of the Bering and Chukchi Shelves. Percent saturation relative to atmospheric equilibrium ranged from 30-800% and 75-145% for CH₄ and N₂O, respectively, with the highest concentrations of both gases occurring in waters overlying the continental shelf in the northern Chukchi Sea. Nitrification and denitrification in the sediments of the Bering and Chukchi Shelves constituted a major source of N₂O to the water column, and the resulting high N₂O concentrations were transported across the entire North American Arctic. Methane sources to the water column were more spatially heterogeneous, reflecting a greater variety of hydrographic influences, including likely inputs from sediments, rivers, and sea ice processes. Localized regions of high CH₄ concentrations were observed at various locations across our sampling transect, but unlike N₂O, CH₄ was rapidly consumed through microbial oxidation, as shown by the ¹³C enrichment of CH₄ at low concentrations. High CH₄ signatures were thus more localized across our sampling region. For both CH₄ and N₂O, surface super-saturation and sea-air fluxes were generally low across the region, with valuess of 1.3 ± 1.2 mol m-² d-¹ and -0.52 ± 1.0 µmol m-² d-¹, for CH₄ and N₂O, respectively. Low surface water concentrations were at least partially attributable to dilution by low CH₄ and N₂O fresh water. Our results provide insight into the factors controlling the distribution of CH₄ and N₂O in the North American Arctic Ocean, and an important baseline data set against which future changes can be assessed.

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Assessing the "Ballast" Hypothesis for Carbon Transport in the Ocean: Global Sediment Trap Data Analysis and Simulation in an Earth System Model (2010)

Sediment trap particle flux data analysis and development of a model representation of mineral “ballast”mechanism for carbon transport in the ocean is presented in this study. The validity of several classical POCremineralization models as well as recently hypothesized “ballast” mechanism based POC remineralizationmodels were tested by analyzing data from selected 79 sediment traps at >1500 m from around the world. POCflux modelled with different variations of model representations at each sediment trap site was statisticallycompared with corresponding measured POC flux in order to evaluate the overall predictability of each model atthe global scale. A CaCO₃ single-mineral-ballast model could explain up to —79% of the global F variability atdepth >1500 m and suggests that CaCO₃ may potentially be the mineral type that has dominant control on thevertical transport of Fc,c from sea surface to depth in the open ocean. In addition, ai assessment of the impact ofreduced CaCO₃ production (as a result of ocean surface acidification) on the marine carbon cycle andimplications for future atmospheric CO₂ concentration under the assumption of mineral ballasting of POC ispresented. A CaCO₃ single-mineral-ballast model derived from the data analysis is incorporated into GENIE-i, acomputationally efficient carbon-climate Earth System Model of intermediate complexity. Simulation results froma “business as usual” future carbon emissions scenario in GENIE-i suggest that, by year 2300, calcificationresponse of marine calcifying organisms to increased atmospheric CO₂ concentrations in a CaCO₃-ballastingocean is —63% weaker compared to that in a non-ballasting ocean. With the “ballast” effect in operation, the neteffect of climate feedback and calcification feedback is a global decrease in POC export production, except for insome high latitude regions where enhanced POC production due to decreased sea-ice coverage overrides. If the“ballast” hypothesis is true, a CaCO₃-ballasting mechanism could completely counter the reduction inatmospheric CO₂ concentration by calcification feedback alone in an ocean where no ballasting mechanism ispresent.

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