Sourcing the iron in the naturally fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics P. van der Merwe, A. R. Bowie, F. Quéroué, L. Armand, S. Blain, Fanny Chever, D. Davies, F. Dehairs, Frédéric Planchon, Géraldine Sarthou, et al. To cite this version: P. van der Merwe, A. R. Bowie, F. Quéroué, L. Armand, S. Blain, et al.. Sourcing the iron in the natu- rally fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics. Biogeosciences, European Geosciences Union, 2015, 12 (3), pp.739-755. 10.5194/bg-12-739-2015. hal-01207931 HAL Id: hal-01207931 https://hal.sorbonne-universite.fr/hal-01207931 Submitted on 1 Oct 2015 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License Biogeosciences, 12, 739–755, 2015 www.biogeosciences.net/12/739/2015/ doi:10.5194/bg-12-739-2015 © Author(s) 2015. CC Attribution 3.0 License. Sourcing the iron in the naturally fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics P. van der Merwe1, A. R. Bowie1,2, F. Quéroué1,2,3, L. Armand4, S. Blain5,6, F. Chever3,7, D. Davies1, F. Dehairs8, F. Planchon3, G. Sarthou9, A. T. Townsend10, and T. W. Trull1,11 1Antarctic Climate and Ecosystems CRC, University of Tasmania, TAS 7004, Australia 2Institute for Marine and Antarctic Studies, University of Tasmania, Battery Point, TAS 7004, Australia 3Laboratoire des Sciences de l’Environnement Marin (LEMAR), Université de Bretagne Occidentale, CNRS,IRD, UMR6539, IUEM, Technopole Brest Iroise, Place Nicolas Copernic, 29280 Plouzané, France 4Department of Biological Sciences and Climate Futures, Macquarie University, North Ryde, NSW 2109, Australia 5Sorbonne Universités, UPMC Univ Paris 06, UMR7621, Laboratoire d’Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France 6CNRS, UMR7621, Laboratoire d’Océanographie Microbienne, Observatoire Océanologique, 66650 Banyuls/mer, France 7IFREMER/Centre de Brest, Département REM/EEP/Laboratoire Environnement Profond, CS 10070, 29280 Plouzané, France 8Vrije Universiteit Brussel, Analytical, Environmental and Geo-Chemistry & Earth System Sciences research group, Brussels, Belgium 9CNRS, Université de Brest, IRD, Ifremer, UMR6539 LEMAR, IUEM ; Technopôle Brest Iroise, Place Nicolas Copernic, 29280 Plouzané, France 10Central Science Laboratory, University of Tasmania, Sandy Bay, TAS 7005, Australia 11Commonwealth Scientific and Industrial Research Organisation, Oceans and Climate Flagship, GPO Box 1538, Hobart, Tasmania, Australia Correspondence to: P. van der Merwe ([email protected]) Received: 26 August 2014 – Published in Biogeosciences Discuss.: 18 September 2014 Revised: 15 December 2014 – Accepted: 8 January 2015 – Published: 6 February 2015 Abstract. The KEOPS2 project aims to elucidate the role of Fe levels were only elevated by a factor of ∼ 2. Over the natural Fe fertilisation on biogeochemical cycles and ecosys- Kerguelen Plateau, ratios of pMn = pAl and pFe = pAl resem- tem functioning, including quantifying the sources and pro- ble basalt, likely originating from glacial/fluvial inputs into cesses by which iron is delivered in the vicinity of the Ker- shallow coastal waters. In downstream, offshore deep-waters, guelen Archipelago, Southern Ocean. The KEOPS2 pro- higher pFe = pAl, and pMn = pAl ratios were observed, sug- cess study used an upstream high-nutrient, low-chlorophyll gesting loss of lithogenic material accompanied by retention (HNLC), deep water (2500 m), reference station to compare of pFe and pMn. Biological uptake of dissolved Fe and Mn with a shallow (500 m), strongly fertilised plateau station and and conversion into the biogenic particulate fraction or ag- continued the observations to a downstream, bathymetrically gregation of particulate metals onto bioaggregates also in- trapped recirculation of the Polar Front where eddies com- creased these ratios further in surface waters as the bloom monly form and persist for hundreds of kilometres into the developed within the recirculation structure. While resus- Southern Ocean. Over the Kerguelen Plateau, mean partic- pension of shelf sediments is likely to be one of the impor- ulate (1–53 µm) Fe and Al concentrations (pFe D 13.4 nM, tant mechanisms of Fe fertilisation over the plateau, fluvial pAl D 25.2 nM) were more than 20-fold higher than at an off- and glacial sources appear to be important to areas down- shore (lower-productivity) reference station (pFe D 0.53 nM, stream of the island. Vertical profiles within an offshore re- pAl D 0.83 nM). In comparison, over the plateau dissolved circulation feature associated with the Polar Front show pFe Published by Copernicus Publications on behalf of the European Geosciences Union. 740 P. van der Merwe et al.: Kerguelen Plateau: particulate trace metal sources and pMn levels that were 6-fold and 3.5-fold lower, respec- a small change in the efficiency estimate could result in dif- tively, than over the plateau in surface waters, though still ferent conclusions as to the efficacy, for instance, of artificial 3.6-fold and 1.7-fold higher respectively than the reference Fe fertilisation as a means of mitigating rising atmospheric station. Within the recirculation feature, strong depletions of concentrations of anthropogenic CO2. pFe and pMn were observed in the remnant winter water Dissolved Fe (dFe < 0.2 µm) includes colloidal and (temperature-minimum) layer near 175 m, with higher values nanoparticulate Fe, which may only be partially bioavail- above and below this depth. The correspondence between the able, as well as soluble Fe (sFe < 0.02 µm) which is highly pFe minima and the winter water temperature minima im- bioavailable (de Baar and de Jong, 2001). As a result, the plies a seasonal cycle is involved in the supply of pFe into larger particulate fraction (> 0.2 µm) is often less studied due the fertilised region. This observed association is indicative to the perception that it has low bioavailability. However, the of reduced supply in winter, which is counterintuitive if sedi- particulate fraction can yield important information for sev- ment resuspension and entrainment within the mixed layer is eral reasons; firstly the dissolved fraction is constantly in a the primary fertilising mechanism to the downstream recircu- state of change with uptake, particle scavenging and rem- lation structure. Therefore, we hypothesise that lateral trans- ineralisation occurring simultaneously and at varying rates port of pFe from shallow coastal waters is strong in spring, depending on many factors including complexation with or- associated with snow melt and increased runoff due to rain- ganic ligands (Johnson et al., 1997) and the biological com- fall, drawdown through summer and reduced supply in win- munity present (Sunda, 2001). Thus, interpretation of dFe ter when snowfall and freezing conditions predominate in the data is difficult without a rarely obtained perspective on the Kerguelen region. time varying aspects of the dFe distribution. Secondly, as a fraction of the total Fe, the major sources of Fe into fer- tilised regions (e.g. weathering products delivered by fluvial and glacial processes, resuspension of sediments and pore- 1 Introduction waters, atmospheric and extra-terrestrial dust) are small par- ticles (> 0.2 µm), with the concentration being more stable Small scale fertilisation experiments have now clearly estab- over weeks to months, due to its abundance and relatively lished that Southern Ocean primary production is limited by slow biological uptake. The particulate fraction is primarily the availability of the micronutrient iron (Fe) (Boyd et al., lost from surface waters through sinking, either directly or 2007; de Baar, 2005). This limitation on the biological pump via adhesion to bioaggregates (Frew et al., 2006). However, means that the Southern Ocean does not realise its full po- there is a constant transfer of dissolved Fe to particulate Fe, tential in transferring atmospheric CO2 into the ocean inte- either via biological uptake or precipitation and, particulate rior; a result illustrated in Antarctic continental ice records Fe to dissolved Fe, via dissolution and biologically medi- over geological timescales and supported by modelling stud- ated processes (Moffett, 2001). Thus, the particulate fraction ies (Barnola et al., 1987; Bopp et al., 2003; Martin, 1990; that is small enough to avoid sinking out of the water col- Watson et al., 2000). Less well understood is the overall sys- umn rapidly (0.2–5 µm) can be considered as a significant tem response to the addition of Fe as efficiency estimates source of dissolved Fe, with the rate of supply into surround- (defined here as the amount of carbon exported relative to ing waters dependent on the dissolution and leaching rate. Fe added above baseline conditions) can vary by an order of Furthermore, there is growing evidence that particles in this magnitude (Blain et al., 2007; Pollard et al., 2009; Savoye size fraction are readily produced