Stephanie Waterman

Zooplankton are a key constituent of oceanic food webs, thus, oceanographic processes that influence distributions of zooplankton can be important in defining biological hotspots. There are gaps in understanding these bio-physical interactions due to limited observations during stormy, winter seasons; few studies that consider multiple physical factors, under varying seasonal and environmental conditions; and scarcity of ocean turbulence observations (where turbulence is important for zooplankton). Therefore, this work has three goals: 1. to investigate the mechanisms that influence zooplankton distributions in the vicinity of a submarine canyon during winter downwelling; 2. to evaluate turbulence parameterizations applied to glider-collected data, to be used in assessing turbulence-zooplankton relationships; 3. to investigate the influence of physical oceanographic factors on zooplankton distributions in a shelf basin in different years and seasons. To address these goals underwater gliders were used to sample co-located hydrographic properties, zooplankton acoustics, and/or turbulence intensity for extended periods at high spatiotemporal resolutions.A multi-glider deployment in the vicinity of a submarine canyon on the West Coast of Vancouver Island during a stormy, winter period showed that canyon downwelling had a minimal influence on the distribution of zooplankton. Water mass composition played a more significant role in influencing the distribution of zooplankton, with Pacific Equatorial Water associated with higher abundance and non-migrating zooplankton.An evaluation of the finescale parameterization and Thorpe scale method for estimating turbulence intensity, by comparing to co-located microstructure observations, found that despite overestimating turbulence intensity, the Thorpe scale method better captured the spatiotemporal features compared to the finescale parameterization, even though the finescale parameterization better estimated the magnitude of turbulence intensity. Consequently, the Thorpe scale method was deemed more appropriate for examining turbulence-zooplankton relationships.A multi-year glider survey of Roseway Basin on the Scotian Shelf during fall and early winter showed that: 1. turbulence did not appear to have any influence on zooplankton distributions; 2. zooplankton abundance was at times correlated with the proportion of Warm Slope Water, primarily when Cabot Strait Subsurface water dominated; and 3. the depth of non-migrating zooplankton layers, when present, correlated with the pycnocline depth and/or depth of the 1026 kg m⁻³ isopycnal.

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In this thesis, I use a novel set of hydrography and turbulence measurements from the southeastern Beaufort Sea toi. compare estimates of the turbulent kinetic energy dissipation rate, ε, obtained independently from shear and temperature microstructure measurements;ii. characterize turbulence and mixing in the Amundsen Gulf region of the southeastern Beaufort Sea; andiii. describe the characteristics of tracer diffusion in an oceanic flow as it transitions between fully turbulent and nearly-laminar.I collected the measurements over 10 days in 2015 using an ocean glider measuring temperature, conductivity, and pressure on O(10)-cm scales and shear and temperature on O(1)-mm turbulent scales.The two independent ε estimates agree within a factor of 2 when ε exceeds 3 × 10⁻¹¹ W kg⁻¹, but diverge by up to two orders of magnitude at smaller values. I identify the noise floor of the shear measurements as the primary reason for this divergence and, therefore, suggest that microstructure temperature measurements are preferable for estimating ε in low energy environments like the Beaufort Sea.I find that turbulence is typically weak in Amundsen Gulf: ε has a geometric mean value of 2.8 × 10⁻¹¹ W kg⁻¹ and is less than 1 × 10⁻¹⁰ W kg⁻¹ in 68% of observations. Turbulent dissipation varies over five orders of magnitude, is bottom enhanced, and is primarily modulated by the M2 tide. Stratification is strong and frequently damps turbulence, inhibiting diapycnal mixing in up to 93% of observations. However, a small number of strongly turbulent mixing events disproportionately drive net buoyancy fluxes. Heat fluxes are modest and nearly always below 1 W m⁻².Finally, I use the turbulence measurements to demonstrate how tracer diffusion in the ocean transitions continuously between turbulent diffusion and near-molecular diffusion as turbulence weakens and stratification strengthens. I use the buoyancy Reynolds number, ReB, to quantify the relative energetic contributions of potential and kinetic energy to the flow dynamics and find that present models for tracer diffusion are accurate to within a factor of 3 when ReB > 10. However, contrary to expectations, I find that significant enhanced tracer diffusivity at turbulent scales remains present when ReB is below unity.

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The Arctic Ocean is a unique oceanographic environment that sits at the frontier of the impacts of climate change. In light of the ongoing dramatic changes observed in the Arctic Ocean, there has been a growing interest in improving our understanding of turbulent ocean mixing rates, which play an integral role in setting numerous oceanographic properties. However, scarcity of direct turbulence measurements in the Arctic Ocean inhibits our ability to robustly quantify the space-time variability of mixing in this region and understand the mechanisms that underpin it. This thesis addresses this issue by employing a finescale parameterization of turbulent dissipation to estimate turbulent mixing metrics from three unique Arctic Ocean datasets that span a wide range of distinct space and time scales. Key results include the following. First, estimated internal wave-driven dissipation rates span multiple orders of magnitude, both across large geographic domains and temporally on local scales. Despite this wide variability, dissipation rates display distinct regional differences, with estimated turbulent metrics that are consistently higher on the Canadian Arctic shelf than in the central basins. Dissipation rate time series also vary systematically at key tidal frequencies and on seasonal time scales, but exhibit no interannual trends on periods of up to 16 years. Additionally, a characterization of mixing regimes reveals large-scale spatial structure in the distribution of turbulent, non-turbulent, and marginal mixing regimes. Non-turbulent conditions are most prevalent, but wide variability implies that turbulent mixing occurs in all regions at least some of the time. Finally, dissipation rate estimates from each dataset provide consistent, statistically-significant evidence that tidal forcing and stratification strength modulate turbulence more strongly than wind speed, topographic roughness, or sea ice cover; however, the correlations between each of these metrics and turbulence are generally weak. Overall, the primary contribution of this thesis is the provision of an improved statistical characterization of turbulent metrics in the Arctic Ocean on unprecedented spatial and temporal scales. This characterization puts more limited mixing measurements into a broader context and further provides a valuable observational baseline that can be used to inform Arctic Ocean modelling studies.

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