Physical conditions controlling the development of a regular phytoplankton bloom north of the Crozet Plateau, Southern Ocean

Venables, H.J.; Pollard, R.T.; Popova, E.E. ORCID: 2007 Physical conditions controlling the development of a regular phytoplankton bloom north of the Crozet Plateau, Southern Ocean. Deep-Sea Research II, 54 (18-20). 1949-1965.

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A phytoplankton bloom occurs north of the Crozet Plateau annually from September to January. The area, bounded to the north by the Sub-Antarctic Front, is the most northerly of the areas of regular high productivity in the otherwise high-nutrient low-chlorophyll Southern Ocean. Chlorophyll concentrations are at background values to the south and only slightly enhanced over the shallow plateau, producing three contrasting productivity regimes. The CROZet natural iron bloom and EXport experiment (CROZEX) project was aimed at testing the hypothesis that the bloom is caused by natural iron fertilisation from the sediments and islands of the Crozet Plateau. In this paper, the temporal and spatial progression of the bloom and the contrasting productivity regimes are investigated using SeaWiFS and MODIS chlorophyll-a (chl-a), photosynthetically available radiation (PAR) from SeaWiFS, QuikSCAT wind-stress data, Argo float profiles and data from three research cruises to the region. Comparison of satellite chl-a data against in situ measurements showed that the satellite data were lower by a factor of approximately 2. Iron, light and grazing are all important in explaining the different productivity regimes. To the north, light is dominant in controlling the timing and location of the bloom initiation, but the spatial distribution of the peak chl-a values obtained through the season is associated with the flow pattern and their proximity to the plateau and so are likely driven by nutrient availability. Mixed-layer depths in Argo float density profiles show a shallowing of the mixed-layer depth to the north of 10.8±1.0 m degree−1 across the study area. This gradient, together with a latitudinal gradient in PAR, is sufficient to explain the observed spatial progression of the bloom.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 0967-0645
Additional Keywords: SeaWiFS; MODIS; Argo; Sub-Antarctic Front (SAF); Phytoplankton bloom; Mixed-layer depth
Date made live: 12 Nov 2007 +0 (UTC)

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