Dissolved iron in the vicinity of the Crozet Islands, Southern Ocean

Abstract

The annual phytoplankton bloom occurring north of the Crozet Plateau provides a rare opportunity to examine the hypothesis that natural iron fertilisation can alleviate HNLC conditions normally associated with the Southern Ocean. Therefore, during CROZEX, a large multidisciplinary study performed between November 2004 and January 2005, measurements of total dissolved iron (DFe, 0.2 m) were made on seawater from around the islands and atmospheric iron deposition estimated from rain and aerosol samples. waters. Enrichment of dissolved iron (>1 nM) at close proximity to the islands suggests that the plateau and the associated sediments are a source of iron. Waters further north also appear to be affected by this input of coastal and shelf origin, although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of ~ 0.07 nM.km -1 at the time of sampling. Using lateral and vertical diffusion coefficients derived from Ra isotope profiles and also estimates of atmospheric inputs, it was then possible to estimate a DFe concentration of ~ 0.55 nM to the north of the islands prior to the bloom event, which is sufficient to initiate the bloom, the lateral island source being the largest component. A similar situation is observed for other Sub-Antarctic Islands such as Kerguelen, South Georgia, that supply dissolved iron to their surrounding waters, thus, enhancing chlorophyll concentrations.Keywords: Dissolved Iron, Crozet Islands, Southern Ocean, HNLC. Planquette et al., 26/04/07 3 The hypothesis that iron can act as a limiting micro nutrient in High Nutrient Low Chlorophyll (HNLC hereafter) regions is now generally accepted and has been investigated on a number of occasions (Boyd et al., 2000;de Baar et al., 2005). IntroductionThe iron hypothesis originally proposed by Martin (1990) has led to numerous studies which all demonstrate that the addition of iron to HNLC waters causes an increase in phytoplankton productivity. Subsequent investigations into iron's role in phytoplankton physiology have also revealed important findings. Among them, one can cite its role in photosynthetic and respiratory electron transport, nitrate reduction, and chlorophyll synthesis (Sunda and Huntsman, 1995; Sunda and Huntsman, 1997). The broader implication is that in HNLC waters, the presence of iron can increase the efficiency of the biological pump and promote drawdown of atmospheric carbon dioxide (Bakker et al., 2001;Bakker et al., 2005; Boyd et al., 2004; Law et al., 2006;Martin et al., 1990).The Southern Ocean is subjected to these HNLC conditions and is depicted as the largest potential sink of anthropogenic CO 2 in the global ocean (Martin, 1990; Tréguer and Pondaven, 2001) and as a key system in the context of climate change (Sarmiento et al., 1998). However, due to the existence of distinct regional sub systems differing in their physical and biological properties (Arrigo et al., 1998; Tréguer and Jacques, 1992), this ocean should not be viewed as one entity.Ther...

Natural iron enrichment around the Antarctic Peninsula in the Southern Ocean

“…These values indicate that neither dissolved nor particulate iron penetrate far into open ocean in the latitudinal/meridional direction when the main current is parallel to the coast. Our estimate of SL DFe (25 km) is identical to the SL DFe found by Planquette et al (2007) and is comparable to the SL DFe (16 km) found by Johnson et al (1997) for Monterey Bay. However, much higher values for SL DFe were estimated for near-shore waters off the Kerguelen Islands (151 km; Bucciarelli et al, 2001) and near the Galapagos Islands (103 km; Gordon et al, 1998).…”

“…However, much higher values for SL DFe were estimated for near-shore waters off the Kerguelen Islands (151 km; Bucciarelli et al, 2001) and near the Galapagos Islands (103 km; Gordon et al, 1998). These differences were attributed to different water circulation patterns in those coastal systems and the difference between the respective surface areas of the islands (Planquette et al, 2007). Geochemical characteristic of the source area, type of continental shelf and margin, and differences in the biological processes might also play some role.…”

Sedimentary and atmospheric sources of iron around South Georgia, Southern Ocean: a modelling perspective

“…As previously measured around South Georgia (Nielsdóttir et al, 2012, Holeton et al, 2005, but also in other regions of the Southern Ocean (e.g. Ardelan et al, 2010;Bucciarelli et al, 2001;Planquette et al, 2007), the model depicts highest dFe concentrations in near-shore waters (> 5 nM), decreasing with distance from the island. Our model results clearly show that the South Georgia island mass effect can reach regions located far away from the island (> 1000 km), where dFe concentrations may still be above background concentrations.…”

Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau

“…Stations M10 and M5 are located in the PFZ . Altimetry data suggest station M6 might be seasonally influenced by a weakly marked Polar Front (Park et al, 1993;, but the presence of a temperature minimum layer (1.6°C at 200 m) strongly supports its belonging to the AAZ (Pollard et al, 2002;Planquette et al, 2007;Salter et al, 2014). Statistical differences in minimum diameter (d min ) and size-normalized weight (SNW) between the four study sites were tested for three species independently (N. pachyderma, T. quinqueloba, G. bulloides) using a non-parametric Kruskall-Wallis test.…”