The subsurface biological structure of Southern Ocean eddies revealed by BGC-Argo floats

Mesoscale eddies play several key roles in the Southern Ocean by redistributing momentum, potential vorticity, heat and salt. However, the role of eddies in Southern Ocean biogeochemistry has received less attention. Previous studies have been based on satellites or models. In this study, Southern Ocean eddies were matched with BGC-Argo float profiles to investigate the subsurface biological structure of Southern Ocean eddies. We focused on four frontal regions in the Indian sector of the Southern Ocean where data density is greater than in the other basins. The seasonality and regional variability of subsurface chlorophyll and backscatter structures were revealed by calculating anomalies in eddies relative to outside of eddies, in each season and frontal region. Chlorophyll anomalies in the mixed layer were mostly positive for both cyclones and anticyclones in all regions in spring, summer and autumn. Anomalies in winter were close to zero. Eddy pumping was the most likely mechanism sustaining high chlorophyll in cyclones. Eddy-induced Ekman pumping, in combination with deep vertical mixing was likely the driver of enhanced chlorophyll in anticyclones. Backscatter anomalies were mostly consistent with chlorophyll anomalies. Chlorophyll to backscatter ratios (chlorophyll:b_bp) were also calculated to help understand the role of light acclimation in the phytoplankton. There were strong subsurface maxima for in-eddy chlorophyll:b_bp in spring and summer. Such structures were weak in autumn and winter, but chlorophyll:b_bp in the upper 30 m in autumn and winter was mostly higher than spring and summer in the same region. Photo-acclimation to low light at depth was evident for both cyclones and anticyclones in spring and summer in the euphotic zone, while in autumn and winter it was observed throughout the upper water column. The majority of positive chlorophyll anomalies were due to both an increase of biomass and photo-acclimation while few cases were due to photo-acclimation only.