Instrumented elephant seals reveal the seasonality in chlorophyll and light-mixing regime in the iron-fertilized Southern Ocean

We analyze an original large data set of concurrent in situ measurements of fluorescence, temperature and salinity provided by sensors mounted on the elephant seals of Kerguelen Island. Our results were mainly gathered in regions of the Southern Ocean where the typical iron limitation is relieved by natural iron fertilization. Thus the role of light as the proximal factor of control of phytoplankton can be examined. We show that self‐shading, and consequently stratification, are major factors controlling the integrated biomass during the bloom induced by iron fertilization. When the mixed layer was the shallowest, the maximum ChlML achievable by the given light‐mixing regime was however not reached, most likely due to silicic acid limitation. We also show that a favorable light‐mixing regime prevails after the spring equinox and is maintained for roughly seven months (October–April). Key Points Simultaneous determination of chlorophyll and mixed layer depth Importance of the light‐mixing regime when iron limitation is relieved Role of self‐shading in the control of chlorophyll in iron‐fertilized regions