Sub-mesoscale structure and the development of an eddy in the Subantarctic Front north of the Crozet Islands

Two stations only 20 km apart were observed to have quite different biological and biogeochemical characteristics. The first site had low concentrations of chlorophyll and sufficient nutrients to support phytoplankton growth. The second site had high concentrations of chlorophyll, depleted nutrient concentrations and significant export of phytodetrital material had taken place. The two sites were located in the Polar Frontal Zone of the Southern Ocean to the northwest of the Crozet Islands. The main physical difference between the two sites was in the depth of the mixed layer. At the first site, the mixed layer was deep and well mixed, whereas the second site had a shallow, stratified layer; otherwise the horizontal gradients of physical properties were weak. Survey data from the surrounding area showed that the productive site was located on the edge of a filament of water drawn into a developing meander of the Subantarctic Front. Remotely sensed data provided a history of the growth of the meander in the Subantarctic Front and its development into an eddy in the Polar Frontal Zone. The dynamics associated with the filament in the meander were clearly important in driving the primary productivity, as an intrusion of saline water into adjacent fresher water generated a shallow mixed layer and conditions suitable for phytoplankton growth. The dynamics promoting conditions favourable to phytoplankton growth continued to operate as chlorophyll was enhanced in the eddy after the main bloom had died away. Later measurements suggested that the surface layer had changed from diatom dominated to a coccolithophore or calcite-rich community.