The Biological Carbon Pump seen through the “eyes” of multiple high resolution underwater gliders

The ocean biological carbon pump (BCP) is an important component of the carbon cycle; but despite its importance, our understanding of the BCP at the right scales is limited due to logistical constrains. The importance of variability in particulate organic carbon (POC) export on timescales from days to months has been recently highlighted and the long endurance and high-resolution capabilities of gliders make them the ideal platform to fill some of the gaps left by classical observations.

The first set of underwater glider deployments of the GOCART (Gauging Ocean organic Carbon fluxes using Autonomous Robotic Technologies) project took place NW of South Georgia in late 2017 in parallel with the COMICS program. Waters around South Georgia are characterised by large and recurrent phytoplankton blooms fuelled by pulses of iron coming from the islands nearby. As these blooms are associated with the high estimates of carbon export in the Southern Ocean, understanding the physical controls of primary production and carbon export in this key region is critical to understanding the role of the Southern Ocean in carbon uptake.

Gliders observed the transition from winter to spring and captured the onset of stratification, bloom initiation and consequent carbon export to the mesopelagic. Phytoplankton bloom dynamics were initially controlled by recurrent thermal re-stratification events tightly linked to wind forcing while the later bloom decline was caused by nutrient limitation, as recorded by parallel traditional ship measurements. We then linked bloom dynamics to carbon export using optical backscattering (bbp) as a proxy for particle concentration in the water column. Post bloom, background chlorophyll and bbp gradually increased at depth, where both small and large particle were found to be sinking to deeper depths in the mesopelagic. These particle dynamics are then used to determine estimates of carbon flux to the twilight zone and inform models aimed to understand the carbon budget in the mesopelagic.