The biogeochemical Structure of Southern Ocean cyclonic eddies from Ship-based observation

Mesoscale eddies play a key role in modulating global ocean biogeochemical cycles. Satellite observations are an excellent tool to understand physical-biogeochemical interaction in mesoscale eddies, however these are limited to the surface ocean. Composite studies combining satellite and profiling float observations have described the mean three-dimensional subsurface structure of eddies, however they are limited by the inherent assumption that each eddy has the same vertical distribution of properties. Here, we use hydrographic data from two voyages, conducted in the Austral summer and autumn, to document the three-dimensional biogeochemical structure of two mesoscale cyclonic eddies and to illustrate their seasonal variability. Our study demonstrates that the macronutrient distribution is largely driven by eddy dynamics, yielding identical eddy structure below the surface mixed layer in both seasons. However, the distribution of phytoplankton and particulate organic matter differs between the two eddies. We found that biomass concentration in the eddy core was high in summer and low in autumn, despite the eddy cores being replete in nutrients in both seasons with respect to surrounding waters. These results suggest that eddies not only influence the vertical supply of macronutrients but also the coupling between zoo and phytoplankton, and the iron supply. We also found a layer of high ammonium concentration at the base of the mixed layer, directly below a low biomass concentration in the surface layer, suggesting an interplay between eddy induced stratification and biological production.