Mixed‐layer depth (MLD) has been widely linked to phytoplankton dynamics in Antarctica's coastal regions; however, inconsistent definitions have made intercomparisons among region‐specific studies difficult. Using a data set with over 20,000 water column profiles corresponding to 32 Slocum glider deployments in three coastal Antarctic regions (Ross Sea, Amundsen Sea, and West Antarctic Peninsula), we evaluated the relationship between MLD and phytoplankton vertical distribution. Comparisons of these MLD estimates to an applied definition of phytoplankton bloom depth, as defined by the deepest inflection point in the chlorophyll profile, show that the maximum of buoyancy frequency is a good proxy for an ecologically relevant MLD. A quality index is used to filter profiles where MLD is not determined. Despite the different regional physical settings, we found that the MLD definition based on the maximum of buoyancy frequency best describes the depth to which phytoplankton can be mixed in Antarctica's coastal seas.

Bathymetric depressions (canyons) exist along the West Antarctic Peninsula shelf and have been linked with increased phytoplankton biomass and sustained penguin colonies. However, the physical mechanisms driving this enhanced biomass are not well understood. Using a Slocum glider data set with over 25,000 water column profiles, we evaluate the relationship between mixed layer depth (MLD, estimated using the depth of maximum buoyancy frequency) and phytoplankton vertical distribution. We use the glider deployments in the Palmer Deep region to examine seasonal and across canyon variability. Throughout the season, the ML becomes warmer and saltier, as a result of vertical mixing and advection. Shallow ML and increased stratification due to sea ice melt are linked to higher chlorophyll concentrations. Deeper mixed layers, resulting from increased wind forcing, show decreased chlorophyll, suggesting the importance of light in regulating phytoplankton productivity. Spatial variations were found in the canyon head region where local physical water column properties were associated with different biological responses, reinforcing the importance of local canyon circulation in regulating phytoplankton distribution in the region. While the mechanism initially hypothesized to produce the observed increases in phytoplankton over the canyons was the intrusion of warm, nutrient enriched modified Upper Circumpolar Deep Water (mUCDW), our analysis suggests that ML dynamics are key to increased primary production over submarine canyons in the WAP.

The biological pump supplies carbon to the oceans’ interior, driving long-term carbon sequestration and providing energy for deep-sea ecosystems^1,2. Its efficiency is set by transformations of newly formed particles in the euphotic zone, followed by vertical flux attenuation via mesopelagic processes³. Depth attenuation of the particulate organic carbon (POC) flux is modulated by multiple processes involving zooplankton and/or microbes^4,5. Nevertheless, it continues to be mainly parameterized using an empirically derived relationship, the ‘Martin curve’⁶. The derived power-law exponent is the standard metric used to compare flux attenuation patterns across oceanic provinces^7,8. Here we present in situ experimental findings from C-RESPIRE⁹, a dual particle interceptor and incubator deployed at multiple mesopelagic depths, measuring microbially mediated POC flux attenuation. We find that across six contrasting oceanic regimes, representing a 30-fold range in POC flux, degradation by particle-attached microbes comprised 7–29 per cent of flux attenuation, implying a more influential role for zooplankton in flux attenuation. Microbial remineralization, normalized to POC flux, ranged by 20-fold across sites and depths, with the lowest rates at high POC fluxes. Vertical trends, of up to threefold changes, were linked to strong temperature gradients at low-latitude sites. In contrast, temperature played a lesser role at mid- and high-latitude sites, where vertical trends may be set jointly by particle biochemistry, fragmentation and microbial ecophysiology. This deconstruction of the Martin curve reveals the underpinning mechanisms that drive microbially mediated POC flux attenuation across oceanic provinces.