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

Christaki, Urania; Lefèvre, D.; Georges, C.; Colombet, Jonathan; Catala, Philippe; Courties, Claude; Sime-Ngando, Télesphore; Blain, S.; Obernosterer, Ingrid

doi:10.5194/bg-11-6739-2014

Cited by 58 publications

(86 citation statements)

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“…Maiti et al (2013) demonstrated an inverse relationship between primary production and export efficiency in the Southern Ocean, contrasting with the positive relationship between primary production and export prevailing in most of the global ocean (Laws et al, 2011). These so called high biomass, low export regimes (HBLE, Lam and Bishop, 2007;Lam et al, 2011) appear to be characterized by enhanced heterotrophic microbial activity Christaki et al, 2014) and intense reprocessing and fragmentation of particulate matter by zooplankton. On the Kerguelen Plateau for example, net community production (NCP) was estimated to be 6.6 mol m À 2 yr À 1 (Jouandet et al, 2008), although only a small fraction of this NCP was exported out of the mixed layer (Rembauville et al, 2015b).…”

Section: Introductionmentioning

confidence: 94%

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

Rembauville

Manno

Tarling

et al. 2016

Deep Sea Research Part I: Oceanographic Research Papers

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a b s t r a c tBiogeochemical and diatom export fluxes are presented from two bathypelagic sediment trap deployments in the Antarctic Zone of the Southern Ocean. One of the sediment traps was deployed in very productive, naturally iron-fertilized waters downstream of South Georgia (P3, 2000 m) and compared to a deployment in moderately productive waters upstream of the island system (P2, 1500 m). At both sites significant diatom export events occurred in spring (November) and contained mostly empty cells that were associated with low particulate organic carbon (POC) fluxes. A summer export pulse occurred one month later at P2 (end February/March) compared to P3 (end January). Diatom fluxes at P3 were one order of magnitude higher than at P2, a difference mainly attributed to the short and intense export of resting spores from Chaetoceros Hyalochaete and Thalassiosira antarctica species. Aside from these resting spores, diatom export assemblages at both sites were dominated by empty Fragilariopsis kerguelensis frustules. The fraction of diatoms exported as empty frustules was considerably lower at P3 (52%) than P2 (91%). This difference was related to the flux of intact diatom resting spores at P3 and may partially explain the lower Si:C export stoichiometry observed at P3 (1.1) compared to P2 (1.5). Through the enumeration of full diatom frustules and subsequent biomass calculations we estimate that diatom resting spores account for 42% of annual POC flux in the productive waters downstream of South Georgia. At both sites the contribution of diatom vegetative stages to POC fluxes was considerably lower (o 5%). From these analyses we conclude that resting spore export contributes towards the slightly higher bathypelagic (POC) flux at P3 (40.6 mmol m À 2 y À 1 ) compared to P2 (26.4 mmol m À 2 y À 1 ). We compared our sediment trap records with previously published diatom assemblage data from the mixed layer and surface sediments (3760 m) around South Georgia. The relative proportion of diatom resting spores within diatom assemblages increases as a function of depth and is explained by selective preservation of their robust frustules. Our study highlights the significance of diatom resting spore export as a carbon vector out of the mixed layer. Furthermore, the contribution or resting spores to POC flux in the bathypelagic ocean and sediments suggests they play a particularly important role in sequestering biologically fixed CO 2 over climatically relevant timescales.

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Section: Introductionmentioning

confidence: 94%

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

Rembauville

Manno

Tarling

et al. 2016

Deep Sea Research Part I: Oceanographic Research Papers

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“…Despite the much higher primary production for the fertilized area in comparison with the HNLC area, the export of organic matter remains relatively small during spring and summer . Low export efficiency suggests that the products of primary production are mainly recycled within the mixed layer by an efficient microbial loop (Sarthou et al, 2008;Obernosterer et al, 2008;Christaki et al, 2014;Malits et al, 2014). The carbon sequestration efficiency induced by iron fertilization remains proportionally relatively low (Jacquet et al, , 2015.…”

Section: High Primary Production In Naturally Iron-fertilized Bloomsmentioning

confidence: 99%

“…In early February (KEOPS 1), primary production and growth rate rapidly decreased in the euphotic layer from about 14 down to 2 µmol C L −1 d −1 and from ∼ 0.3 down to 0.01 d −1 respectively. Associated with these changes, the mixed layer production system evolved from being autotrophic, characteristic for spring, to heterotrophic with respiration exceeding photosynthesis Christaki et al, 2014). With nitrate and phosphate at non-limiting concentrations, a top-down control of phytoplankton development (mostly diatoms; Georges et al, 2014;Sackett et al, 2014) is the most likely cause of this change during the decay of the bloom in late summer (KEOPS 1; Carlotti et al, 2008;Brussaard et al, 2008;Sarthou et al, 2008).…”

Section: High Primary Production In Naturally Iron-fertilized Bloomsmentioning

confidence: 99%

Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean

et al. 2015

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Abstract. Although the Southern Ocean is considered a high-nutrient, low-chlorophyll (HNLC) area, massive and recurrent blooms are observed over and downstream of the Kerguelen Plateau. This mosaic of blooms is triggered by a higher iron supply resulting from the interaction between the Antarctic Circumpolar Current and the local bathymetry. Net primary production, N uptake (NO − 3 and NH + 4 ), and nitrification rates were measured at eight stations in austral spring 2011 (October-November) during the KEOPS 2 cruise in the Kerguelen Plateau area. Natural iron fertilization stimulated primary production, with mixed layer integrated net primary production and growth rates much higher in the fertilized areas (up to 315 mmol C m −2 d −1 and up to 0.31 d −1 respectively) compared to the HNLC reference site (12 mmol C m −2 d −1 and 0.06 d −1 respectively). Primary production was mainly sustained by nitrate uptake, with f ratios (corresponding to NO − 3 -uptake / (NO − 3 -uptake + NH + 4 -uptake)) lying at the upper end of the observations for the Southern Ocean (up to 0.9). We report high rates of nitrification (up to ∼ 3 µmol N L −1 d −1 , with ∼ 90 % of them < 1 µmol N L −1 d −1 ) typically occurring below the euphotic zone, as classically observed in the global ocean. The specificity of the studied area is that at most of the stations, the euphotic layer was shallower than the mixed layer, implying that nitrifiers can efficiently compete with phytoplankton for the ammonium produced by remineralization at low-light intensities. Nitrate produced by nitrification in the mixed layer below the euphotic zone is easily supplied to the euphotic zone waters above, and nitrification sustained 70 ± 30 % of the nitrate uptake in the productive area above the Kerguelen Plateau. This complicates estimations of new production as potentially exportable production. We conclude that high productivity in deep mixing system stimulates the N cycle by increasing both assimilation and regeneration.

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“…During KEOPS 2 the incorporation of 3 Hleucine was used to estimate bacterial production. BP data are described in Christaki et al (2014). In Fig.…”

Section: Mesopelagic Ba Xs and Bacterial Productionmentioning

confidence: 99%

“…The same area was visited earlier in 2005 during summer at a late stage of the bloom (KEOPS 1; January-February 2005), offering a unique opportunity to estimate the main carbon fluxes over most of the growth sea- son. Mesopelagic C remineralization estimates are compared to particle and biological parameters as reported in other papers included in this issue Christaki et al, 2014;Dehairs et al, 2014;Laurenceau-Cornec et al, 2015;Planchon et al, 2014;van der Merve et al, 2015) and in Blain et al (2007), Christaki et al (2008), Jacquet et al (2008a), Park et al (2008) and Savoye et al (2008).…”

Section: Introductionmentioning

confidence: 99%

Early spring mesopelagic carbon remineralization and transfer efficiency in the naturally iron-fertilized Kerguelen area

et al. 2015

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Abstract. We report on the zonal variability of mesopelagic particulate organic carbon remineralization and deep carbon transfer potential during the Kerguelen Ocean and Plateau compared Study 2 expedition (KEOPS 2; OctoberNovember 2011) in an area of the polar front supporting recurrent massive blooms from natural Fe fertilization. Mesopelagic carbon remineralization (MR) was assessed using the excess, non-lithogenic particulate barium (Ba xs ) inventories in mesopelagic waters and compared with bacterial production (BP), surface primary production (PP) and export production (EP). Results for this early season study are compared with the results obtained during a previous study (2005; KEOPS 1) for the same area at a later stage of the phytoplankton bloom. Our results reveal the patchiness of the seasonal advancement and of the establishment of remineralization processes between the plateau (A3) and polar front sites during KEOPS 2. For the Kerguelen plateau (A3 site) we observe a similar functioning of the mesopelagic ecosystem during both seasons (spring and summer), with low and rather stable remineralization fluxes in the mesopelagic column (150-400 m). The shallow water column (∼ 500 m), the lateral advection, the zooplankton grazing pressure and the pulsed nature of the particulate organic carbon (POC) transfer at A3 seem to drive the extent of MR processes on the plateau. For deeper stations (> 2000 m) located on the margin, inside a polar front meander, as well as in the vicinity of the polar front, east of Kerguelen, remineralization in the upper 400 m in general represents a larger part of surface carbon export. However, when considering the upper 800 m, in some cases, the entire flux of exported carbon is remineralized. In the polar front meander, where successive stations form a time series, two successive events of particle transfer were evidenced by remineralization rates: a first mesopelagic and deep transfer from a past bloom before the cruise, and a second transfer expanding at mesopelagic layers during the cruise. Regarding the deep carbon transfer efficiency, it appeared that above the plateau (A3 site) the mesopelagic remineralization was not a major barrier to the transfer of organic matter to the seafloor (close to 500 m). There, the efficiency of carbon transfer to the bottom waters (> 400 m) as assessed by PP, EP and MR fluxes comparisons reached up to 87 % of the carbon exported from the upper 150 m. In contrast, at the deeper locations, mesopelagic remineralization clearly limited the transfer of carbon to depths of > 400 m. For sites at the margin of the plateau (station E-4W) and the polar front (station F-L), mesopelagic remineralization even exceeded Published by Copernicus Publications on behalf of the European Geosciences Union. S. H. M. Jacquet et al.: Early season mesopelagic carbon remineralizationupper 150 m export, resulting in a zero transfer efficiency to depths > 800 m. In the polar front meander (time series), the capacity of the meander to transfer carbon to depth > 800 ...

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

Cited by 58 publications

References 58 publications

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

Strong contribution of diatom resting spores to deep-sea carbon transfer in naturally iron-fertilized waters downstream of South Georgia

Production regime and associated N cycling in the vicinity of Kerguelen Island, Southern Ocean

Early spring mesopelagic carbon remineralization and transfer efficiency in the naturally iron-fertilized Kerguelen area

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