Composition of diatom communities and their contribution to plankton biomass in the naturally iron-fertilized region of Kerguelen in the Southern Ocean

Abstract

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.

Impact of the natural Fe-fertilization on the magnitude, stoichiometry and efficiency of particulate biogenic silica, nitrogen and iron export fluxes

“…Importance of elemental composition on export fluxes 4.3.1. The silica ballast efficiency dependent on diatom communities The relative contribution of each diatom is described with great detail by Lasbleiz et al 2016. Briefly, at the low and moderately productive stations (R-2 and E stations), the live cell abundance was dominated by large and highly silicified diatom species such as Fragilariopsis kerguelensis (18% at R-2 and E-1% and 6% at E-5) and Thalassionema nitzschioides (21% at R-2 and E-1% and 17% at E-5).…”

“…PN export fluxes are multiplied by 6.6 to examine directly the variations of the C:N Redfield ratio. a primary production data (mmol C m À 2 d À 1 ) from Cavagna et al (2015); b dFe concentrations (nmol L À 1 ) from Quéroué et al (2015); c N uptake data (mmol N m À 2 d À 1 ) from Cavagna et al (2015); d Si uptake data (mmol Si m À 2 d À 1 ) from Closset et al (2014); e total mesozooplankton biomass data (g C m À 2 ) from Carlotti et al (2015); f bacterial respiration rates (mmol C L À 1 d À 1 ) from Christaki et al (2014); g taxonomic data from Lasbleiz et al, 2016; h POC export fluxes (mmol m À 2 d À 1 ) from Planchon et al (2015). 0.66 mmol C L À 1 d À 1 at other stations (Christaki et al, 2014).…”

Understanding Diatoms’ Past and Future Biogeochemical Role in High‐Latitude Seas

“…Both studies demonstrated diatom biomass was, in many cases, most of the bulk POC (Armand et al, ; Lasbleiz et al, ). Additionally, when considering all plankton groups, their POC sum aligned well with bulk POC measurements (Lasbleiz et al, ). Applying the cold‐water diatom allometry to these data result in unrealistically high diatom contributions, where they alone could exceed bulk POC measurements; thus, our hypothesis that all polar studies applying diatom allometry have systematically underestimated their abundance is disproved.…”

Mesoscale Variability of Conditions Favoring an Iron‐Induced Diatom Bloom Downstream of the Kerguelen Plateau

“…Satellite images of surface chlorophyll concentrations show that the Kerguelen plume is highly heterogeneous, displaying strong gradients even within the plume itself, in response to eddies and frontal structures (Figure c)). The community structure of the bloom's primary producers has been studied during ship‐based oceanographic campaigns, which have focused mainly on differentiating diatoms types and on pigment and size analyses (Armand et al, ; Lasbleiz et al, ; Mosseri et al, ; Quéguiner, 2013; Trull et al, ; Uitz et al, ). However, in‐situ observations are limited in spatial extent by the large size of the plume and the remoteness of the area.…”

“…PHYSAT ocean color re‐analyses are available on the GlobColour platform or on PHYSAT's official webpage (http://log.cnrs.fr/Physat-2?lang=fr). Pigment in situ observations can be obtained requesting an access to (http://keops2.obs-vlfr.fr/) or from the publication by Lasbleiz et al ().…”