Abstract
Abstract. The island of South Georgia is situated in the iron (Fe)-depleted Antarctic Circumpolar Current of the Southern Ocean. Iron emanating from its shelf system fuels large phytoplankton blooms downstream of the island, but the actual supply mechanisms are unclear. To address this, we present an inventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore waters, and the water column in the vicinity of South Georgia, alongside data on zooplankton-mediated Fe cycling processes, and provide estimates of the relative dissolved Fe (DFe) fluxes from these sources. Seafloor sediments, modified by authigenic Fe precipitation, were the main particulate Fe source to shelf bottom waters as indicated by the similar Fe ∕ Mn and Fe ∕ Al ratios for shelf sediments and suspended particles in the water column. Less than 1 % of the total particulate Fe pool was leachable surface-adsorbed (labile) Fe and therefore potentially available to organisms. Pore waters formed the primary DFe source to shelf bottom waters, supplying 0.1–44 µmol DFe m−2 d−1. However, we estimate that only 0.41±0.26 µmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical diffusive and advective mixing. Other trace metal sources to surface waters included glacial flour released by melting glaciers and via zooplankton egestion and excretion processes. On average 6.5±8.2 µmol m−2 d−1 of labile particulate Fe was supplied to the surface mixed layer via faecal pellets formed by Antarctic krill (Euphausia superba), with a further 1.1±2.2 µmol DFe m−2 d−1 released directly by the krill. The faecal pellets released by krill included seafloor-derived lithogenic and authigenic material and settled algal debris, in addition to freshly ingested suspended phytoplankton cells. The Fe requirement of the phytoplankton blooms ∼ 1250 km downstream of South Georgia was estimated as 0.33±0.11 µmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep winter mixing, and aeolian dust estimated as ∼0.12 µmol m−2 d−1. We hypothesize that a substantial contribution of DFe was provided through recycling of biogenically stored Fe following luxury Fe uptake by phytoplankton on the Fe-rich shelf. This process would allow Fe to be retained in the surface mixed layer of waters downstream of South Georgia through continuous recycling and biological uptake, supplying the large downstream phytoplankton blooms.
Highlights
The Southern Ocean is the largest “high-nitrate, lowchlorophyll” (HNLC) region of the global ocean (Buesseler et al, 2004), as a consequence of low iron (Fe) supply and reduced phytoplankton growth (Buesseler et al, 2004; Tsuda et al, 2009)
We present an inventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore waters, and the water column in the vicinity of South Georgia, alongside data on zooplankton-mediated Fe cycling processes, and provide estimates of the relative dissolved Fe (DFe) fluxes from these sources
Authigenic particle include surface-scavenged trace metals and secondary minerals, such as amorphous FeO(OH), that are formed in seawater and because of their age are insoluble to weak acid leaches (e.g. 25 % acetic acid solution)
Summary
The Southern Ocean is the largest “high-nitrate, lowchlorophyll” (HNLC) region of the global ocean (Buesseler et al, 2004), as a consequence of low iron (Fe) supply and reduced phytoplankton growth (Buesseler et al, 2004; Tsuda et al, 2009). Despite the overall HNLC status of the Southern Ocean, regions in the wake of islands feature large seasonal phytoplankton blooms; the Fe sources to these blooms are poorly constrained (de Jong et al, 2012; Planquette et al, 2007; Pollard et al, 2009). As a consequence of the Fe fertilization, the waters in the vicinity of South Georgia support extensive phytoplankton blooms and a large biomass of zooplankton, fish, seabirds, and marine mammals, some of which are exploited commercially (Atkinson et al, 2001; Murphy et al, 2007)
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