Abstract. In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia, most likely due to an enhanced supply of the limiting micronutrient iron. Shallow sediments and atmospheric dust deposition are believed to be the main iron sources. However, their relative importance is still unclear and in the South Georgia region have yet not been ascertained because iron measurements are very few. In this study, we use austral summer dissolved iron (dFe) data around South Georgia (January and February 2008) with a coupled regional hydrodynamic and biogeochemical model to investigate natural iron fertilization around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north, i.e., downstream, of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of ~0.02 nM and ~0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and the very shallow shelf-sediments (depths < 20 m) while dust deposition plays a minor role, with almost no effects on surface chlorophyll a concentrations. Iron sources such as run-off not represented explicitly in the model, but that likely contribute to the iron plumes observed around South Georgia, are also discussed together with the potential effects their temporal variability may have on the system.
Distribution and recurrence of phytoplankton blooms around South Georgia, Southern Ocean