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

Abstract

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.

A subsurface eddy associated with a submarine canyon increases availability and delivery of simulated Antarctic krill to penguin foraging regions

“…Since this previous study also assumed that zooplankton are passive in the horizontal and only swim in the vertical, we assume that these particle simulations also represent simulated krill. Therefore, the residence times presented by Hudson et al (2022) are the same for our simulated krill, and we present these data here for comparison to residence times in the absence of PDC. We extend the findings of Hudson et al ( 2022) by calculating the simulated krill distribution metrics described below within known penguin foraging regions.…”

“…Calculated residence times were averaged from December to February to cover the chick-rearing period. Residence times for simulated zooplankton performing DVM within PDC have been reported previously (Hudson et al 2022). Since this previous study also assumed that zooplankton are passive in the horizontal and only swim in the vertical, we assume that these particle simulations also represent simulated krill.…”

“…Model-estimated residence times of non-vertically migrating simulated particles increase with depth within this feature, ranging from ~20 d at 50 m to ~130 d at 300 m within the subsurface eddy (Hudson et al 2021). Simulated zooplankton performing DVM from 10 to 300 m were retained for nearly 30 d within the canyon (Hudson et al 2022). This suggests that this subsurface feature has the potential to retain critical food resources while penguins rear their chicks during the breeding season near PDC.…”

“…Based on these observations, DVM was simulated for migrations between 10 and 50, 10 and 150, and 10 and 300 m at a vertical swimming speed of 0.03 m s −1 . Previous simulations illustrated that swimming speed had little to no effect on residence times of vertically migrating simulated zooplankton (Hudson et al 2022), so only 1 vertical swimming speed was considered here.…”

“…In the presence of PDC, flow below the mixed layer (~50 m; Hudson et al 2021Hudson et al , 2022 has a strong barotropic component, with flow following isobaths around the canyon, forming a closed, recirculating eddy (Fig. 2a−e; Hudson et al 2021).…”