The fate of added iron during a mesoscale fertilisation experiment in the Southern Ocean

The first Southern Ocean Iron RElease Experiment (SOIREE) was performed dur ing February 1999 in Antarctic waters south of Australia (61 degreesS, 140 degreesE), in order to verify whether iron supply controls the magnitude of phytoplankton production in this high nutrient low chlorophyll (HNLC) regi on. This paper describes iron distributions in the upper ocean during our 1 3-day site occupation, and presents a pelagic iron budget to account for th e observed losses of dissolved and total iron from waters of the fertilised patch. Iron concentrations were measured underway during daily transects t hrough the patch and in vertical profiles of the 65-m mixed layer. High int ernal consistency was noted between data obtained using contrasting samplin g and analytical techniques. A pre-infusion survey confirmed the extremely low ambient dissolved (0.1 nM) and total (0.4 nM) iron concentrations. The initial enrichment elevated the dissolved iron concentration to 2.7 nM. The reafter, dissolved iron was rapidly depleted inside the patch to 0.2-0.3 nM , necessitating three re-infusions. A distinct biological response was observed in iron-fertilised waters, rela tive to outside the patch, unequivocally confirming that iron limits phytop lankton growth rates and biomass at this site in summer. Our budget describ ing the fate of the added iron demonstrates that horizontal dispersion of f ertilised waters (resulting in a quadrupling of the areal extent of the pat ch) and abiotic particle scavenging accounted for most of the decreases in iron concentrations inside the patch (31-58% and 12-49% of added iron, resp ectively). The magnitude of these loss processes altered towards the end of SOIREE, and on days 12-13 dissolved (1.1 nM) and total (2.3 nM) iron conce ntrations remained elevated compared to surrounding waters. At this time, t he biogenic iron pool (0.1 nM) accounted for only 1-2% of the total added i ron. Large pennate diatoms ( > 20 mum) and autotrophic flagellates (2-20 mu m) were the dominant algal groups in the patch, taking up the added iron an d representing 13% and 39% of the biogenic iron pool, respectively. Iron re generation by grazers was tightly coupled to uptake by phytoplankton and ba cteria, indicating that biological Fe cycling within the bloom was self-sus taining. A concurrent increase in the concentration of iron-binding ligands on days 11-12 probably retained dissolved iron within the mixed layer. Oce an colour satellite images in late March suggest that the bloom was still a ctively growing 42 days after the onset of SOIREE, and hence by inference t hat sufficient iron was maintained in the patch for this period to meet alg al requirements. This raises fundamental questions regarding the biogeochem ical cycling of iron in the Southern Ocean and, in particular, how bioavail able iron was retained in surface waters and/or within the biota to sustain algal growth. (C) 2001 Elsevier Science Ltd. All rights reserved.