| Summary: | Real-time coupled physical and biological forecasting was conducted prior to and during the CROZet natural bloom and EXport experiment (CROZEX) programme in the Southern Ocean (SO) between November 2004 and January 2005. The programme was aimed at investigating, through both measurements and modelling, the origin and fate of an intense and long-lasting phytoplankton bloom that is unusual for the otherwise high-nutrient low-chlorophyll conditions that characterise the SO. It was hypothesised that dissolved iron supplied to the system from shallow sediments accumulates downstream of the Crozet Islands during winter and creates conditions that favour phytoplankton production. Regular 14-day forecasts and reanalysis of the biogeochemical fields were issued on a weekly basis using a 3D primitive equation model coupled with an 11-component ecosystem model, along with assimilation of remotely sensed and in situ data. Real-time modelling assisted not only in optimisation of the cruise strategy, but also provided an ongoing synthesis of observations and test of the working hypotheses proposed to explain bloom dynamics in the study area.The model results indicated that a complex interplay between light, iron, and silicate limitation on primary production and grazing control determined the spatial extent and dynamics of the phytoplankton bloom. The model suggested that natural fertilisation of the area by dissolved iron was insufficient on its own to initiate a phytoplankton bloom. The persistence of deep mixing in the southern part of the iron-enriched waters was delaying, or in some areas preventing, the development of the bloom.
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