Science Plan

CROZEX Science and Implementation Plan

1. Summary

RRS Discovery cruises 285 and 286 took place from early November 2004 to mid-January 2005, two 5-week legs in the area 40-50°S, 40-60°E in the southwest Indian Ocean sector of the Southern Ocean. The cruises were a component of the George Deacon Division's NERC Core Strategic Proposal BICEP (Biophysical Interactions and Controls over Export Production), with additional support from NERC responsive mode grants.

Fig. 1 SeaWIFS images show the bloom progression from October 1998 to January 1999

The objective of CROZEX was to study both the initiation of, and export from, the annual phytoplankton bloom north of the Crozet Plateau (Figure 1). This bloom is in the generally High Nutrient - Low Chlorophyll (HNLC) region of the Southern Ocean. Experiments in which iron has been added in HNLC regions (SOIREE, Eisenex, Eifex, etc) have proved that iron is the limiting nutrient, so it is likely that the Crozet bloom is initiated by iron supplied from shallow sediments around the Crozet Islands and possibly from runoff from the islands themselves. Data from the cruises is helping us to understand the factors controlling mixing, iron chemistry, phytoplankton and zooplankton response and export production in naturally highly productive regions. In particular we seek to quantify the export from the phytoplankton bloom. Hence the results obtained may serve as a model for the possible consequences of large scale sustained iron fertilization on export production in this region.

In addition to measurements in the bloom north of the Crozet Islands, a site south of the Islands where a bloom is never observed was occupied as a control.