Mechanisms of dissolved and labile particulate iron supply to shelf waters and phytoplankton blooms off South Georgia, Southern Ocean

Abstract

<p><strong>Abstract.</strong> 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<span class="thinspace"></span><span class="inline-formula">∕</span><span class="thinspace"></span>Mn and Fe<span class="thinspace"></span><span class="inline-formula">∕</span><span class="thinspace"></span>Al ratios for shelf sediments and suspended particles in the water column. Less than 1<span class="thinspace"></span>% 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<span class="thinspace"></span><span class="inline-formula">µ</span>mol<span class="thinspace"></span>DFe<span class="thinspace"></span><span class="inline-formula">m</span><span class="inline-formula">⁻²</span><span class="thinspace"></span>d<span class="inline-formula">⁻¹</span>. However, we estimate that only <span class="inline-formula">0.41±0.26</span><span class="thinspace"></span><span class="inline-formula">µ</span>mol<span class="thinspace"></span>DFe<span class="thinspace"></span><span class="inline-formula">m</span><span class="inline-formula">⁻²</span><span class="thinspace"></span>d<span class="inline-formula">⁻¹</span> 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 <span class="inline-formula">6.5±8.2</span><span class="thinspace"></span><span class="inline-formula">µ</span>mol<span class="thinspace"></span><span class="inline-formula">m</span><span class="inline-formula">⁻²</span><span class="thinspace"></span>d<span class="inline-formula">⁻¹</span> of labile particulate Fe was supplied to the surface mixed layer via faecal pellets formed by Antarctic krill (<i>Euphausia superba</i>), with a further <span class="inline-formula">1.1±2.2</span><span class="thinspace"></span><span class="inline-formula">µ</span>mol<span class="thinspace"></span>DFe<span class="thinspace"></span><span class="inline-formula">m</span><span class="inline-formula">⁻²</span><span class="thinspace"></span>d<span class="inline-formula">⁻¹</span> 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.</p> <p>The Fe requirement of the phytoplankton blooms <span class="inline-formula">∼ 1250</span><span class="thinspace"></span><span class="inline-formula">km</span> downstream of South Georgia was estimated as <span class="inline-formula">0.33±0.11</span><span class="thinspace"></span><span class="inline-formula">µ</span>mol<span class="thinspace"></span><span class="inline-formula">m</span><span class="inline-formula">⁻²</span><span class="thinspace"></span>d<span class="inline-formula">⁻¹</span>, with the DFe supply by horizontal/vertical mixing, deep winter mixing, and aeolian dust estimated as <span class="inline-formula">∼0.12</span><span class="thinspace"></span><span class="inline-formula">µ</span>mol<span class="thinspace"></span><span class="inline-formula">m</span><span class="inline-formula">⁻²</span><span class="thinspace"></span>d<span class="inline-formula">⁻¹</span>. 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.</p>