Bloom initiation

3.2 The bloom north of Crozet - initiation

Fig. 3.2 SeaWIFS images for November 1998, 1999, 2000 and 2001

We have studied the area around the Crozet Plateau using monthly SeaWIFS satellite imagery, comparing the images with hydrographic measurements conducted during WOCE (Pollard and Read, 2001). These data show a bloom being initiated annually to the north of the plateau in Aug-Sept, peaking in Oct-Nov (Fig. 3.2). At its peak the bloom covers a wide area (400 km x 400 km) and is clearly bounded to the west and north by the S-shaped ACC pathway (Pollard and Read, 2001). Read et al (2000) showed nitrate and silicate were low west and north of this pathway, but were not limiting south and east of it. Their measurements extended just onto the western Crozet Plateau in Jan 1995. Our conceptual picture is that winter mixing draws iron from the sediments around Crozet into the water column. The mixing and circulation pathways result in the iron enrichment of a hydrographically constrained region in which production is most probably light limited in winter. Primary production is enhanced when near-surface stratification begins in spring and the bloom is then terminated by iron or silicate limitation and collapses with some associated export flux. According to this conceptual picture the region which is sufficiently iron replete at the beginning of spring for a bloom to occur is controlled by local circulation patterns, the depths of the fertilizing topography, the seasonal stratification and the stability of the iron in the water column.

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