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Fabio C. De Leo, Ocean Networks Canada, University of Victoria, Canada
Bruno Ogata, Universidade Federal Paulista, São Vicente, Brazil
Akash Sastri, Ocean Networks Canada, University of Victoria, Canada
Steve Mihaly, Ocean Networks Canada, University of Victoria, Canada
Martin Heesemann, Ocean Networks Canada, University of Victoria, Canada
Moira Galbraith, Fisheries and Oceans Canada, Institute of Ocean Sciences, Canada
Do Barkley Canyon trap overwintering populations of Neocalanus plumchrus (Copepoda, Calanidae) at depth? Insights into particulate organic carbon flux to deep-sea sediments based on video imagery from a seafloor cabled observatory

It is well documented in the literature that submarine canyons may enhance local productivity by topographically constraining surface ocean currents and therefore triggering local upwelling, as well as by trapping populations of diel-vertically migrating zooplankton, which in turn act as trophic subsidies for larger top predators such as seabirds and marine mammals. Little is known however, about how submarine canyon water column productivity is exported down to deep-sea sediments considering the high-level of topographic heterogeneity found among different canyon systems, and the varying levels of connectivity between canyons and respective adjacent continental shelves. Here we report for the first time, to the best of our knowledge, video observations of dense deep scattering layers composed primarily of ontogenetic migrating copepods (Neocalanus spp.) from various seafloor cameras installed in a deep-sea cabled observatory, around 1000 m of depth in Barkley Canyon. We developed an automated video image analysis protocol for estimating copepod abundances at the benthic boundary layer, and tested the protocol robustness in reproducing the natural ontogenetic migration cycle of Neocalanus spp (most likely N. plumchrus). We discuss the use of our proposed video imaging technique as a complementary measure to traditional estimates of particulate organic carbon (POC) flux from sediment traps, which seldom capture the contribution from larger zooplankton. We analyzed videos from May 2013 to January 2015 (~575 days) from three high-definition cameras installed in Barkley Submarine Canyon, at its axis (890 m), at the northern canyon wall (890 m), and in a control area in the nearby slope (400 m). Videos were captured for 5 minutes at two-hour intervals.  A total of 33,486 still images were extracted from 1,674 videos and used in a computer-automated image analysis protocol implemented in the software Image J. The protocol followed several steps, which included image transformation to 8-bit, sharpening, and thresholding, and finally a particle count analysis that fitted an ellipsoid function to any particle below a threshold of maximum diameter size fitting Neocalanus spp. size range distribution. Our results showed a seasonal distribution of copepod abundances at the benthic boundary layer in Barkley Canyon that tightly mirrored the ontogenetic cycle of Neocalanus spp. From spring to early summer, particle counts were lower as copepods are dominating the upper ocean mesozooplankton biomass, being part of the diets of juvenile salmon, myctophids, baleen whales, and seabirds. However, copepod abundances were statistically significantly higher during late summer through autumn, and winter, matching the descent of Neocalanus spp. populations of non-feeding fifth-stage (C5) copepodites to deep waters. Between autumn and early winter, abundances remained high, representing the molting and non-feeding adult stages, which then spawn at or near the overwintering depth, and remain at depth until they die. We believe our observations using seafloor cameras connected to a cabled observatory are capturing a major missing link in the carbon flux estimates from the upper ocean to the deep-sea in the NE Pacific. We plan to further improve our video analysis protocols to continue monitoring this carbon export flux throughout the life span of the observatory, building a multi-decadal data set.

 

 

Special Session 2: Interdisciplinary studies in Barkley Canyon
Oral Presentation
Barkley Canyon, overwintering copepods, Neocalanus plumchrus, POC flux, cabled observatory, video imagery, automated video analysis