Seasonality and scale of the Kerguelen plateau phytoplankton bloom: A remote sensing and modeling analysis of the influence of natural iron fertilization in the Southern Ocean

The phytoplankton bloom that develops over the Kerguelen plateau following natural input of iron is analysed on a regional and seasonal scale. The relation between chlorophyll, bathymetry, and surface advection fields is not as obvious as it first appears from large-scale annual mean field. The high chlorophyll biomass does not always correspond with the shallowest water, and there are portions of the plateau, which persistently exhibit low chlorophyll. Despite this complex dynamic, a one-dimensional model calibrated for HNLC (high-nutrient low-chlorophyll) region is able to capture the observed increase in chlorophyll by increasing the deep iron concentration. The elemental budget shows similarity in terms of carbon, nitrogen, and silicon but differences in terms of iron with the budget calculated during the mission. This discrepancy either has its origin in the structure of the iron cycling in the model or in the temporal scarcity of data that could only be collected during the summer months. In the model, flexibility of the Fe/C ratio associated with high Fe export and input fluxes prevents high carbon sequestration efficiency. This first insight with remote sensing data and the model allows the validation of some of the key mechanisms of natural iron fertilization and exposes problems that will need to be solved to have a complete biogeochemical diagnostic of this natural iron fertilization.