Biological productivity regime and associated N cycling in the vicinity of Kerguelen Island area, Southern Ocean

Abstract

Abstract. Although the Southern Ocean is considered a High Nutrient Low Chlorophyll area (HNLC), massive and recurrent blooms are observed over and downstream 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 (NO3− and NH4+), and nitrification rates were measured at 8 stations in austral spring 2011 (October–November) during the KEOPS2 cruise in the Kerguelen area. Iron fertilization stimulates primary production, with 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 is mainly sustained by nitrate uptake, with f ratio (corresponding to NO3− uptake/(NO3− uptake + NH4+ uptake)) lying in the upper end of the observations for the Southern Ocean (up to 0.9). Unexpectedly, we report unprecedented rates of nitrification (up to ~3 mmol C m−2 d−1, with ~90% of them <1 mmol C m−2 d−1). It appears that nitrate is assimilated in the upper part of the mixed layer (coinciding with the euphotic layer) and regenerated in the lower parts. We suggest that such high contribution of nitrification to nitrate assimilation is driven by (i) a deep mixed layer, extending well below the euphotic layer, allowing nitrifiers to compete with phytoplankton for the assimilation of ammonium, (ii) extremely high rates of primary production for the Southern Ocean, stimulating the release of dissolved organic matter, and (iii) an efficient food web, allowing the reprocessing of organic N and the retention of nitrogen into the dissolved phase through ammonium, the substrate for nitrification.

Dissolved greenhouse gases (nitrous oxide and methane) associated with the naturally iron-fertilized Kerguelen region (KEOPS 2 cruise) in the Southern Ocean

“…The fact that the western and eastern sections showed high Fe levels (Quéroué et al, 2014) relative to the central section of the TEW, and that these sections had high CH 4 levels, suggests that Fe stimulates CH 4 production. A similar situation occurs at the A3 stations with high T Chl a levels and PP rates, as shown by Cavagna et al (2014). For example, A3-2 and TEW-7 (maximum T Chl a) had the highest integrated primary production rates (up to 3380 mg m −2 d −1 ) and the lowest C export level of around 2-3 % ); this suggests an intense level of PP supported by regenerated N sources.…”

Nitrogen cycling in the Southern Ocean Kerguelen Plateau area: evidence for significant surface nitrification from nitrate isotopic compositions

“…A direct result of this condition is that estimates of New or Exportable Production which are based on the assumption that all surface water nitrate results from nitrification in the deep ocean and vertical supply to the surface waters, are too high. Correcting New Production for the effect of nitrification would bring closer the estimates of exportable production and Export production during KEOPS 2, as measured with the 234 Th methodology (see papers by Cavagna et al, 2014;Planchon et al, 2014, both …”

“…4.5). Information on nitrate, ammonium uptake experiments and C, N Export flux via the 234 Th method is given in the contributions by Cavagna et al (2014) and Planchon et al (2014). As part of the analysis protocol for assessing carbon export via the 234 Th method, we also analysed δ 15 N of suspended particulate nitrogen in the size fractions 1 to 53 and > 53 µ m, as sampled with large-volume in situ pumps .…”

The relative importance of phytoplankton aggregates and zooplankton fecal pellets to carbon export: insights from free-drifting sediment trap deployments in naturally iron-fertilised waters near the Kerguelen Plateau

“…Figure 2 shows water column properties and biomass at each site. The HNLC reference station R-2 located outside the fertilised area was characterised by a relatively deep mixed layer (96 m), low net primary productivity (NPP) (euphotic zone 1 % PAR-integrated NPP = 135 ± 6 mg C m −2 d −1 ; Cavagna et al, 2014), low surface chlorophyll (chlorophyll a mixed layer average = 0.6 µg Chl a L −1 ), and biomass (mixed-layer-integrated POC = 4.7 g C m −2 ). Stations E-1, E-3 and E-5 were located in an eddy-like, bathymetrically trapped recirculation feature in deep waters east of the Kerguelen Islands (stationary meander of the polar front), with a mixed layer depth varying from 33 (E-3) to 70 m (E-1).…”