The ocean contributes to regulating atmospheric CO2 levels, partly via variability in the fraction of primary production (PP) which is exported out of the surface layer (i.e., the e ratio). Southern Ocean studies have found that contrary to global‐scale analyses, an inverse relationship exists between e ratio and PP. This relationship remains unexplained, with potential hypotheses being (i) large export of dissolved organic carbon (DOC) in high PP areas, (ii) strong surface microbial recycling in high PP regions, and/or (iii) grazing‐mediated export that varies inversely with PP. We find that the export of DOC has a limited influence in setting the negative e ratio/PP relationship. However, we observed that at sites with low PP and high e ratios, zooplankton‐mediated export is large and surface microbial abundance low suggesting that both are important drivers of the magnitude of the e ratio in the Southern Ocean.

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.

In the naturally iron-fertilized surface waters of the northern Kerguelen Plateau region, the early spring diatom community composition and contribution to plankton carbon biomass were investigated and compared with the high nutrient, low chlorophyll (HNLC) surrounding waters. The large iron-induced blooms were dominated by small diatom species belonging to the genera Chaetoceros (Hyalochaete) and Thalassiosira, which rapidly responded to the onset of favorable light-conditions in the meander of the Polar Front. In comparison, the iron-limited HNLC area was typically characterized by autotrophic nanoeukaryote-dominated communities and by larger and more heavily silicified diatom species (e.g. Fragilariopsis spp.). Our results support the hypothesis that diatoms are valuable vectors of carbon export to depth in naturally iron-fertilized systems of the Southern Ocean. Furthermore, our results corroborate observations of the exported diatom assemblage from a sediment trap deployed in the iron-fertilized area, whereby the dominant Chaetoceros (Hyalochaete) cells were less efficiently exported than the less abundant, yet heavily silicified, cells of Thalassionema nitzschioides and Fragilariopsis kerguelensis Our observations emphasize the strong influence of species-specific diatom cell properties combined with trophic interactions on matter export efficiency, and illustrate the tight link between the specific composition of phytoplankton communities and the biogeochemical properties characterizing the study area.