Subsurface Eddy Facilitates Retention of Simulated Diel Vertical Migrators in a Biological Hotspot

Diel vertical migration (DVM) is common in zooplankton populations worldwide. Every day, zooplankton leave the productive surface ocean and migrate to deepwater to avoid visual predators and return to the surface at night to feed. This behavior may also help retain migrating zooplankton in biological hotspots. Compared to fast and variable surface currents, deep ocean currents are sluggish, and can be more consistent. The time spent in the subsurface layer is driven by day length and the depth of the surface mixed layer. A subsurface, recirculating eddy has recently been described in Palmer Deep Canyon (PDC), a submarine canyon in a biological hotspot located adjacent to the West Antarctic Peninsula. Circulation model simulations have shown that residence times of neutrally buoyant particles increase with depth within this feature. We hypothesize that DVM into the subsurface eddy increases local retention of migrating zooplankton in this feature and that shallow mixed layers and longer days increase residence times. We demonstrate that simulated vertically migrating zooplankton can have residence times on the order of 30 days over the canyon, which is five times greater than residence times of near‐surface, nonmigrating zooplankton within PDC and other adjacent coastal regions. The potential interaction of zooplankton with this subsurface feature may be important to the establishment of the biological hotspot around PDC by retaining food resources in the region. Acoustic field observations confirm the presence of vertical migrators in this region, suggesting that zooplankton retention due to the subsurface eddy is feasible. Diel vertical migration (DVM) is considered the world's largest migration by biomass. Zooplankton migrate into the sea's surface waters to feed at night when risk of predation by visual predators is low. During the day, zooplankton migrate to deeper waters when risk of predation by predators like seabirds and fish is highest. This behavior may also retain zooplankton in areas of high biological activity, or hotspots. Migration between a rapidly moving surface layer and a sluggish subsurface layer may reduce the net horizontal movement of organisms. Since this behavior is modulated by light intensity, more daylight hours may increase the time spent in the slower subsurface layer and help retain zooplankton in these hotspots. We used simulated zooplankton in a numerical model over Palmer Deep Canyon to test how DVM behavior, and the factors that control the time spent in the subsurface layer, affects zooplankton retention within biological hotspots. Retention was highest for zooplankton when migrations were deepest, days were long, and surface mixed layer was shallow. Performing migrations also increased retention relative to near‐surface nonmigrating zooplankton. Acoustic observations within our study site suggest that the magnitudes of DVM simulated are feasible in this system. Simulated diel vertical migrating zooplankton have increased residence time compared to nonmigrators near a biological hotspotShallow surface layers and long days lead to longer residence times of simulated diel vertical migrating zooplanktonField acoustic measurements show the presence of zooplankton diel vertical migration consistent with simulated migration Simulated diel vertical migrating zooplankton have increased residence time compared to nonmigrators near a biological hotspot Shallow surface layers and long days lead to longer residence times of simulated diel vertical migrating zooplankton Field acoustic measurements show the presence of zooplankton diel vertical migration consistent with simulated migration