Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area

Mosseri, Julie; Quéguiner, Bernard; Armand, Leanne K.; Cornet-Barthaux, Véronique

doi:10.1016/j.dsr2.2007.12.003

Cited by 101 publications

(130 citation statements)

References 85 publications

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“…Studies conducted during KEOPS1 showed a decoupling of the seasonal use of nitrate and silicic acid stocks over the Kerguelen Plateau (Mosseri et al, 2008), although Si : NO 3 assimilation ratios were close to 1, in accordance with what is expected for non-limiting iron conditions (Takeda, 1998; Park et al (2014). The inset picture gives indication about surface chl a (colour scale), surface velocity fields (arrows), and the polar front (PF, black line).…”

Section: A J Cavagna Et Al: N Cycling In the Vicinity Of Kerguelenmentioning

confidence: 70%

“…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). A Si limitation of diatom growth during this period cannot be ruled out either, since measured silicic acid concentrations (∼ 1 µmol L −1 ) are significantly below measured half-saturation constants for Si uptake (5-15 µmol L −1 ; Mosseri et al, 2008). However, F v /F m ratios (ratio of variable fluorescence F v to the maximum fluorescence F m , quantitatively related to the efficiency of photochemistry; Falkowski and Raven, 1997) over the plateau in late summer remained high, indicating that the phytoplankton community was relieved from nutrient stress, including iron stress .…”

Section: High Primary Production In Naturally Iron-fertilized Bloomsmentioning

confidence: 96%

“…Hutchins and Bruland, 1998). Mosseri et al (2008) attributed this peculiar decoupling to the capacity of diatoms to grow on ammonium that could result from high heterotrophic activity at the end of the productive season. These observations highlight the potential importance of nitrogen recycling in the surface layer.…”

Section: A J Cavagna Et Al: N Cycling In the Vicinity Of Kerguelenmentioning

confidence: 99%

“…Once nitrate has been transported into the cell, it has to be further reduced into ammonium before it can be assimilated (Mulholland and Lomas, 2008). At the end of summer, the f ratio over the plateau decreases rapidly, from 0.6 to 0.2, indicating that the system evolved from a nitrate-based to an ammonium-based primary production (Mosseri et al, 2008;P. Raimbault, KEOPS 1 database).…”

Section: Upper-ocean N Cycling: High F Ratios and Nitrification Ratesmentioning

confidence: 99%

“…Net primary production during summer over the Kerguelen Plateau (January-February, KEOPS 1; Mosseri et al, 2008;Lefèvre et al, 2008) (Table 1). This seasonal pattern is in agreement with satellite Chl-a distribution data showing that the bloom starts in early November, reaches its maximum level in late November-early December, and collapses in January-February (Mongin et al, 2008).…”

Section: High Primary Production In Naturally Iron-fertilized Bloomsmentioning

confidence: 99%

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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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Section: A J Cavagna Et Al: N Cycling In the Vicinity Of Kerguelenmentioning

confidence: 70%

Section: High Primary Production In Naturally Iron-fertilized Bloomsmentioning

confidence: 96%

Section: A J Cavagna Et Al: N Cycling In the Vicinity Of Kerguelenmentioning

confidence: 99%

Section: Upper-ocean N Cycling: High F Ratios and Nitrification Ratesmentioning

confidence: 99%

Section: High Primary Production In Naturally Iron-fertilized Bloomsmentioning

confidence: 99%

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

et al. 2015

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Instrumented elephant seals reveal the seasonality in chlorophyll and light‐mixing regime in the iron‐fertilized Southern Ocean

Blain

Renaut

Xing

et al. 2013

Geophysical Research Letters

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[1] We analyze an original large data set of concurrent in situ measurements of fluorescence, temperature and salinity provided by sensors mounted on the elephant seals of Kerguelen Island. Our results were mainly gathered in regions of the Southern Ocean where the typical iron limitation is relieved by natural iron fertilization. Thus the role of light as the proximal factor of control of phytoplankton can be examined. We show that self-shading, and consequently stratification, are major factors controlling the integrated biomass during the bloom induced by iron fertilization. When the mixed layer was the shallowest, the maximum Chl ML achievable by the given light-mixing regime was however not reached, most likely due to silicic acid limitation. We also show that a favorable light-mixing regime prevails after the spring equinox and is maintained for roughly seven months (October-April). Citation: Blain, S., S. Renaut, X. Xing, H. Claustre, and C. Guinet (2013), Instrumented elephant seals reveal the seasonality in chlorophyll and light-mixing regime in the ironfertilized Southern Ocean, Geophys. Res. Lett., 40,[6368][6369][6370][6371][6372]

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Significant mixed layer nitrification in a natural iron‐fertilized bloom of the Southern Ocean

Fripiat

Elskens

Trull

et al. 2015

Global Biogeochemical Cycles

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Nitrification, the microbially mediated oxidation of ammonium into nitrate, is generally expected to be low in the Southern Ocean mixed layer. This paradigm assumes that nitrate is mainly provided through vertical mixing and assimilated during the vegetative season, supporting the concept that nitrate uptake is equivalent to the new primary production (i.e., primary production which is potentially available for export). Here we show that nitrification is significant (~40-80% of the seasonal nitrate uptake) in the naturally iron-fertilized bloom over the southeast Kerguelen Plateau. Hence, a large fraction of the nitrate-based primary production is regenerated, instead of being exported. It appears that nitrate assimilation (light dependent) and nitrification (partly light inhibited) are spatially separated between the upper and lower parts, respectively, of the deep surface mixed layers. These deep mixed layers, extending well below the euphotic layer, allow nitrifiers to compete with phytoplankton for the assimilation of ammonium. The high contributions of nitrification to nitrate uptake are in agreement with both low export efficiency (i.e., the percentage of primary production that is exported) and low seasonal nitrate drawdown despite high nitrate assimilation.

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Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area

Cited by 101 publications

References 85 publications

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

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

Instrumented elephant seals reveal the seasonality in chlorophyll and light‐mixing regime in the iron‐fertilized Southern Ocean

Significant mixed layer nitrification in a natural iron‐fertilized bloom of the Southern Ocean

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