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.

Coastal salt marshes store large amounts of carbon but the magnitude and patterns of greenhouse gas (GHG; i.e., carbon dioxide (CO) and methane (CH)) fluxes are unclear. Information about GHG fluxes from these ecosystems comes from studies of sediments or at the ecosystem-scale (eddy covariance) but fluxes from tidal creeks are unknown. We measured GHG concentrations in water, water quality, meteorology, sediment CO efflux, ecosystem-scale GHG fluxes, and plant phenology; all at half-hour time-steps over one year. Manual creek GHG flux measurements were used to calculate gas transfer velocity () and parameterize a model of water-to-atmosphere GHG fluxes. The creek was a source of GHGs to the atmosphere where tidal patterns controlled diel variability. Dissolved oxygen and wind speed were negatively correlated with creek CH efflux. Despite lacking a seasonal pattern, creek CO efflux was correlated with drivers such as turbidity across phenological phases. Overall, night-time creek CO efflux (3.6 ± 0.63 µmol/m/s) was over two times higher than night-time marsh sediment CO efflux (1.5 ± 1.23 µmol/m/s). Creek CH efflux (17.5 ± 6.9 nmol/m/s) was four times lower than ecosystem-scale CH fluxes (68.1 ± 52.3 nmol/m/s) across the year. These results suggest that tidal creeks are potential hotspots for CO emissions and could contribute to lateral transport of CH to the coastal ocean due to supersaturation of CH (>6000 µmol/mol) in water This study provides insights for modelling GHG efflux from tidal creeks and suggests that changes in tide stage overshadow water temperature in determining magnitudes of fluxes.