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Glider observations of physical processes pertaining to zooplankton distributions Howatt, Tara


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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