In Saanich Inlet, a fjord located in southern Vancouver Island, British Columbia, dense aggregations of euphausiids exhibit diel vertical migration behavior and their capability of generating turbulence has been suggested. Despite decades of research on diel vertical migration of zooplankton, its variability has not been well studied. In addition, the physical oceanographic environment in Saanich Inlet has not been thoroughly quantified, which raises the possibility of previously observed turbulent bursts of O(10^-5 – 10^-4 W kg^-1) having physical (rather than biological) origin. This work characterizes variability of diel vertical migration behavior using a moored 200-kHz echosounder, complemented by plankton sampling. Physical properties such as barotropic, baroclinic and turbulent signals are described, and the relationship between turbulence and internal waves/scattering layer examined. A two-year high-resolution biacoustic time-series provided by the Victoria Experimental Network Under the Sea (VENUS) cabled observatory allowed quantification of the seasonal variability in migration timing of euphausiids. During spring – fall, early dusk ascent and late dawn descent relative to civil twilight occur. During winter, late dusk ascent and early dawn descent occur. Factors regulating the seasonal changes in migration timing are light availability at the daytime depth of the scattering layers, and size-dependent visual predation risk of euphausiids. Instead of the traditional view of diel vertical migration timing correlated solely with civil twilight, euphausiids also adapt their migration timing to accommodate changes in environmental cues as well as their growth. The pre-spawning period (February – April) is an exception to this seasonal pattern, likely due to the higher energy demands for reproduction. Turbulence and internal waves in Saanich Inlet are characterized based on a one-month mooring deployment. Average dissipation rates are nearly an order of magnitude larger than previously reported values and higher dissipation rates of O(10^-7 – 10^-6 W kg^-1) are occasionally observed. A weak correlation is observed between turbulent dissipation rates and baroclinic velocity/shear. To examine the possibility of biological generation of turbulence, an echosounder at the VENUS cabled observatory is used to simultaneously measure the intensity of the euphausiid scattering layer and its vertical position. Turbulent bursts of the sort previously reported are not observed, and no relation between diel vertical migration and turbulent dissipation rates is found. Physical forcing at the main channel remains as a possible cause of the turbulent bursts. / Graduate / 0416 / 0415

Natural cycles of environmental variability and long-term deoxygenation in the ocean impose oxygen deficiency (hypoxia) on marine communities. My research exploits a naturally occurring hypoxia cycle in Saanich Inlet, British Columbia, Canada where I combined spatial surveys with remotely operated vehicles, ecological time-series from the subsea cabled observatory VENUS, and lab-based respirometry experiments to examine the influence of seasonally variable oxygen conditions on epibenthic communities. In situ oxygen thresholds established for dozens of fish and invertebrate species in this system show they naturally occur in lower oxygen levels than what general lethal and sublethal thresholds would predict. Expansion of hypoxic waters induced a loss of community structure which was previously characterized by disjunct distributions among species. Communities in variable hypoxia also have scale-dependent structure across a range of time scales but are primarily synchronized to a seasonal oscillation between two phases. Time-series revealed timing of diurnal movement in the slender sole Lyopsetta exilis and reproductive behavior of squat lobster Munida quadrispina in the hypoxia cycle. Hypoxia-induced mortality of sessile species slowed the rate of community recovery after deoxygenation. The 10-year oxygen time-series from VENUS, revealed a significant increase in the annual low-oxygen period in Saanich Inlet and that deoxygenation has occurred in this system since 2006. Differences in the critical oxygen thresholds (O2crit) and standard metabolic rates of key species (spot prawn Pandalus platyceros, slender sole, and squat lobster) determined the lowest in situ oxygen at which populations occurred and explained disproportionate shifts in distributions and community respiration. Finally, a meta-analysis on global O2crit reported for crustaceans showed that hypoxia tolerance differs among major ocean basins. Long-term trends of deoxygenation suggest a future regime shift may occur when the duration at which a system remains below critical oxygen levels exceeds the time needed for communities to recover. Species-specific traits will determine the critical threshold and the nature of the community response in systems influenced by variable states of oxygen deficiency. However, oceanographic and evolutionary history provides context when determining the regional response of benthic communities influenced by rapidly changing environments. / Graduate / 0329 / 0416 / 0433 / jwfchu@gmail.com