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

• Published in: Biogeosciences
• Main Authors: C. Schlosser, K. Schmidt, A. Aquilina, W. B. Homoky, M. Castrillejo, R. A. Mills, M. D. Patey, S. Fielding, A. Atkinson, E. P. Achterberg
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Antarctic; Southern Ocean; Antarc*; Antarctic Krill; Euphausia superba
• Online Access: https://doi.org/10.5194/bg-15-4973-2018; https://doaj.org/article/5a5747e922a1468c80ec9f8bfb9d7ca8

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 ...