Connectivity Between Surface and Subsurface Phytoplankton Blooms from High-Resolution Ocean Glider Transects in Southern Drake Passage

The southern boundary of the Antarctic Circumpolar Current (ACC) in Drake Passage is a site of strong eddy generation. Coherent mesoscale eddies have been identified as one mechanism for the export of biologically relevant tracers, including chlorophyll, past the Southern ACC Front (SACCF). However, the vertical structure of these tracers remains poorly resolved. Two Seagliders were deployed in this region for a period of four months during austral summer and autumn (December 2014 to April 2015) as part of the ChinStrAP (Changes in Stratification at the Antarctic Peninsula) project. The gliders describe V-shaped dives spanning roughly five kilometers horizontally and four hours temporally. Combined, the gliders provided four months of fluorescence and backscatter measurements down to 200 meters depth.

One glider, collecting sixteen cross-shelf transects, sampled the initiation and development of a surface phytoplankton bloom along the shelf break west of the Shackleton Fracture Zone (SFZ). At bloom initiation, no subsurface chlorophyll maxima were evident. As the bloom developed, the glider observed regions of elevated chlorophyll seaward of the SACCF at 70 m depth, near the base of the mixed layer. This suggests a shift from a light-limited to a nutrient-limited regime. Physical properties in our transects indicate that subsurface blooms may result from isopycnal mixing between surface waters and a subsurface colder Winter Water layer. We calculate potential vorticity along the glider transects and discuss the role of eddy stirring and submesoscale instabilities in this mixing process. Using insight gained from these in situ measurements, along with OceanColor satellite observations spanning longer time periods, we discuss the impact of eddy-driven subduction on primary production in this region.