Canyon Downwelling and Water Mass Influences on Winter Zooplankton Distributions in the Coastal Northeast Pacific

Abstract

AbstractSubmarine canyons are considered biological hotspots due to enhanced mixing, upwelling, and other dynamic processes. There have been few observational surveys of canyon processes or zooplankton‐watermass relations during stormy downwelling seasons. In winter 2017, three autonomous underwater gliders provided highly‐resolved hydrographic and zooplankton backscatter observations in and around a canyon in the Northeast Pacific. While a glider's slow speed typically confounds spatial and temporal signals, the concurrent use of multiple gliders, sampling a canyon transect repeatedly, allowed us to explore the canyon system both spatially (i.e., canyon influence) and temporally (i.e., changing water type fractions). Canyon downwelling displaced the pycnocline by about 30 m depth over the scale of the canyon, which is ∼30% of the total pycnocline depth deviation observed over the course of the study. Diel vertically migrating (DVM) zooplankton generally stayed above the pycnocline and mean zooplankton volume scattering strength (Sv) in the upper water column (above the pycnocline) was higher when the pycnocline was deeper. At mid‐depths (150–200 m depth), zooplankton backscatter and DVM behavior were correlated with water type fractions, with higher Sv being associated with higher proportions of Pacific Equatorial Water and stronger DVM being associated with higher proportions of Pacific Subarctic Upper Water. Winter zooplankton distributions appear to be influenced by multiple, interacting drivers including canyon dynamics and water mass composition. In our study, the latter was found to have the stronger influence on zooplankton distribution.