Quantifying the influence of sub-mesoscale dynamics on the supply of iron to Southern Ocean phytoplankton blooms

Abstract

Southern Ocean phytoplankton growth is limited by iron. Episodes of natural iron fertilisation are pivotalto triggering phytoplankton blooms in this region, the Kerguelen Plateau bloom being one prominentexample. Numerous physical mechanisms that may supply iron to the euphotic zone in the KerguelenPlateau region, and hence trigger a phytoplankton bloom, have been identified. However, the impact ofsub-mesoscaleflows in delivering iron has been omitted. With a scale of order 10 km, sub-mesoscalefilaments and fronts can dramatically increase vertical velocities and iron transport.An innovative technique is developed to investigate the role of vertical advection associated with sub-mesoscale features on the supply of iron to the photic zone. First, Lagrangian trajectories are calculatedusing three dimensional velocityfields from high resolution numerical simulations; iron concentration isthen computed along these Lagrangian trajectories. The contribution of mesoscale- (1/20°resolution)and sub-mesoscale-resolving models (1/80°resolution) is compared, thereby revealing the sensitivity ofiron supply to horizontal resolution. Ironfluxes are clearly enhanced by a factor of 2 with the resolution,thus showing that the vertical motion induced by the sub-mesoscales represents a previously neglectedprocess to drive iron into the photic waters of the Kerguelen Plateau. Show more