Palmer Deep Canyon is one of the biological hotspots associated with
deep bathymetric features along the Western Antarctic Peninsula. The
upwelling of nutrient-rich Upper Circumpolar Deep Water to the surface
mixed layer in the submarine canyon has been hypothesized to drive
increased phytoplankton biomass productivity, attracting krill, penguins
and other top predators to the region. However, observations in Palmer
Deep Canyon lack a clear in-situ upwelling signal, lack a physiological
response by phytoplankton to Upper Circumpolar Deep Water in laboratory
experiments, and surface residence times that are too short for
phytoplankton populations to reasonably respond to any locally upwelled
nutrients. This suggests that enhanced local upwelling may not be the
mechanism that links canyons to increased biological activity. Previous
observations of isopycnal doming within the canyon suggested that a
subsurface recirculating feature may be present. Here, using in-situ
measurements and a circulation model, we demonstrate that the presence
of a recirculating eddy may contribute to maintaining the biological
hotspot by increasing the residence time at depth and retaining a
distinct layer of biological particles. Neutrally buoyant particle
simulations showed that residence times increase to upwards of 175 days
with depth within the canyon during the austral summer. In-situ particle
scattering, flow cytometry, and water samples from within the subsurface
eddy suggest that retained particles are detrital in nature. Our results
suggest that these seasonal, retentive features of Palmer Deep Canyon
are important to the establishment of the biological hotspot.