Physical Drivers of Biogeochemical Variability in the Polar Front Meander

The Southern Ocean plays a vital role in global ocean circulation, and the Polar Front (PF) is one of its most important physical features. The PF meander south of Tasmania, around 153 degrees E, 55 degrees S, is a very dynamic region which spawns mesoscale eddies, and influences local biogeochemistry and sea-air interaction. By using voyage and ancillary data, we investigated the unusually strong spring bloom in the vicinity of the PF meander in 2018. We infer that the upwelling of deep water at the front and in eddies, brings macronutrients and dissolved iron (dFe) to the surface. Chlorophyll concentration peaked at over 0.6 mg m(-3), which is anomalously high for this area. With reduced iron limitation, the physiological characteristics of phytoplankton in the northern, downstream part of the study area also changed. The photochemical efficiency was improved and released this area from its usual high-nutrient low-chlorophyll (HNLC) status. This was mainly indicated by the increase in the dawn Fv/Fm maximum (indictor of photochemical efficiency) from 0.2 to over 0.5. With the biomass increase and healthier community status, we observed consumption of surface dissolved inorganic carbon and increased particulate organic carbon production to about 40 mu mol L-1, forming a weak local carbon sink. Through the investigation of multiple years, a weak positive correlation between mixed layer depth shoaling and phytoplankton growth was found, but there was significant interannual variability in this relationship, likely caused by variable eddy conditions and dFe delivery.

Automated summary

The Polar Front in the Southern Ocean is a significant factor in global ocean circulation and impacts biogeochemistry and sea-air interaction. A study of the region in 2018 revealed an unusually strong spring bloom, potentially due to upwelling bringing nutrients and dissolved iron. This led to changes in the phytoplankton community and an increase in carbon sink activity.