Microbial food web dynamics during spring phytoplankton blooms in the naturally iron-fertilized Kerguelen area (Southern Ocean)

U Christaki,D Lefèvre,P Catala,C Courties,T Sime-Ngando,J Colombet,I Obernosterer,C Georges,S Blain

doi:10.5194/bg-11-6739-2014

Abstract

Abstract. Microbial food web dynamics were determined during the onset of several spring phytoplankton blooms induced by natural iron fertilization off Kerguelen Island in the Southern Ocean (KEOPS2). The abundances of heterotrophic bacteria and heterotrophic nanoflagellates, bacterial heterotrophic production, bacterial respiration, and bacterial growth efficiency, were consistently higher in surface waters of the iron-fertilized sites than at the reference site in HNLC (high nutrient low chlorophyll) waters. The abundance of virus-like particles remained unchanged, but viral production increased by a factor of 6 in iron-fertilized waters. Bacterial heterotrophic production was significantly related to heterotrophic nanoflagellate abundance and viral production across all sites, with bacterial production explaining about 70 and 85%, respectively, of the variance of each in the mixed layer (ML). Estimated rates of grazing and viral lysis, however, indicated that heterotrophic nanoflagellates accounted for a substantially higher loss of bacterial production (50%) than viruses (11%). Combining these results with rates of primary production and export determined for the study area, a budget for the flow of carbon through the microbial food web and higher trophic levels during the early (KEOPS2) and the late phase (KEOPS1) of the Kerguelen bloom is provided.

Highlights

The Southern Ocean has a unique geography with major implications for the global ocean circulation and climate system
Variable fractions of primary production are channelled through heterotrophic bacteria, and most of this phytoplankton-derived organic carbon is respired to carbon dioxide (CO2)
The “historical” A3 station situated above the Kerguelen plateau (Blain et al, 2007, 2008) was characterized by a deep ML (ZML, depth of the mixed layer from 150 to 170 m) (Fig. 2)

Summary

Introduction

The Southern Ocean has a unique geography with major implications for the global ocean circulation and climate system It is the largest HNLC (high nutrient low chlorophyll) ocean where iron limits phytoplankton primary production, resulting in a large stock of unused major inorganic nutrients (Martin and Fitzwater, 1990). A pronounced shift to larger phytoplankton cells, in particular diatoms, has been generally observed upon natural (Blain et al, 2007; Pollard et al, 2009) or artificial (Boyd et al, 2007; Smetacek et al, 2012) iron additions. U. Christaki et al.: Microbial food web dynamics during spring phytoplankton blooms studies have suggested regional variability in the ecosystem response to iron addition and in the carbon export (Blain et al, 2007; Pollard et al, 2009; Boyd et al, 2007). The question on how much carbon fixed by primary production is mineralized within the microbial food web, and what part of this particulate organic carbon is made available for higher trophic levels or export, is important in the present context

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