Formation, transport and decay of an intense phytoplankton bloom within the High-Nutrient Low-Chlorophyll belt of the Southern Ocean

The blooms associated with South Georgia coincide with the largest predicted carbon sink in the Southern Ocean. A major injection of iron and silica is required to sustain them over a long growth season that exceeds 4 months. The mechanisms for this are still poorly known but a cruise in January 2005 provided an opportunity to sample waters both upstream and downstream of the North Scotia Ridge to examine the processes involved. SeaWiFS imagery gave a misleading impression of bloom progression due to pronounced but highly regionalised subsurface chlorophyll-a (chl-a) maxima. Our combined measurements of oceanography, macronutrients, chl-a and floral composition showed that a single bloom that had seeded over the western shelf of South Georgia two months before the cruise developed into two distinct entities. Water retention and turbulent mixing south of the island led to severe nutrient depletion, low chl-a concentrations in surface waters (and in SeaWiFS images) yet pronounced subsurface maxima. By contrast the bloom over the northern shelf persisted in a shallow mixed layer, was florally different and appeared to be advected downstream into oceanic waters to the north. We found evidence for upwelling of nutrients on the upstream shelf edge of the island where the flow diverges to pass around each side of its shelf. This would provide one mechanism for the injection of iron and silica to sustain these massive South Georgia blooms and seed waters downstream for an extended growing season.