Phytoplankton productivity and community structure in the vicinity of the Crozet Plateau during austral summer 2004/2005

The CROZet natural iron bloom and EXport experiment (CROZEX) was carried out around the Crozet Plateau (Southern Ocean) in late austral spring-early summer 2004/2005. The primary aim of this study was to test the hypothesis that natural Fe fertilisation from the islands enhanced primary production to the north of the plateau, leading to elevated carbon export. During the 12-week shipboard study (November–January), size-fractionated chlorophyll-a (chl-a), primary productivity and phytoplankton pigments were determined at 20 stations around the Crozet Plateau, both inside and outside the expected bloom region. Satellite-derived chl-a data from August 2004 to April 2005 revealed that the cruise period covered the declining stage of a large phytoplankton bloom north of the Crozet Plateau. Productivity derived from satellite chl-a data showed a strong north–south gradient, with productivity reaching a maximum of 1.9 g C m−2 d−1 in the north (late October), 0.6 over the plateau (late March) and 0.4 in the south (early December). From November to early December, measured chl-a and productivity were still relatively high northwest of the plateau (54–74 mg chl-a m−2 and 0.5–1.1 g C m−2 d−1), the assemblage being dominated by Phaeocystis sp. and microplankton representing ∼50% of total chl-a. Declining biomass and productivity to the northwest in mid-December was associated with a picoplankton (∼52%) dominated community (19–38 mg chl-a m−2 and 0.1–0.4 g C m−2 d−1). In contrast, at the north eastern edge of the plateau, a mixed diatom/Phaeocystis dominated bloom (229 mg chl-a m−2 and 3 g C m−2 d−1) developed in mid-January. South of the plateau in the control high-nutrient low-chlorophyll (HNLC) region, biomass and productivity were always significantly lower, with only a small chl-a peak in December, and a community composed of microflagellates, cyanobacteria and some giant diatoms. Particulate organic carbon (POC):chl-a ratios provided physiological evidence for the impact of Fe fertilisation north of the plateau. The assimilation rate (i.e. integrated productivity normalised to integrated chl-a, ∫P/∫chl-a) did not show significant differences between the north and south, probably due to the late sampling period relative to the bloom peak. Overall, measurements of primary productivity during CROZEX supported the hypothesis of enhanced biomass and production due to natural iron enrichment of the region north of the Crozet Plateau, while south of the plateau, low biomass and productivity resulted primarily from Fe limitation.