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

Abstract

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.

Unveiling the Si cycle using isotopes in an iron-fertilized zone of the Southern Ocean: from mixed-layer supply to export

“…This latter one is significantly heavier than the Si isotopic composition of the fertilized area measured a few days before (1.99 ± 0.03 ‰, A3-1). As already proposed by Closset et al (2014) and Lasbleiz et al (2014), this suggests that a development of diatoms could have occurred before our sampling, consuming a fraction of the DSi standing stock and increasing the δ 30 Si DSi of surface waters. The same evidence of surface production has also been deduced by Dehairs et al (2015), who observed a slight nitrate depletion and enrichment of δ 15 N-NO − 3 .…”

“…In the ML, the plateau and HNLC areas exhibited significantly different silicon isotopic composition of both seawater and particles, reflecting the different degrees of Si utilization by diatoms. The HNLC reference station (R2) displayed low chlorophyll a and BSi and very low Si uptake rates, consistent with its iron-depleted condition and the dominance of non-siliceous organisms (see details in Lasbleiz et al, 2014, andClosset et al, 2014). In surface waters of the HNLC area, the concentration of biogenic silica was the lowest measured in the surface during KEOPS-2 (0.30 ± 0.03 µmol L −1 ) and its silicon isotopic composition was low (0.73 ± 0.04 ‰) and similar to A3-1 ( Fig.…”

“…The low iron concentration that prevails at this station in early spring might have been sufficient to trigger a short phytoplankton growth (dominated by nanophytoplankton; Lasbleiz et al, 2014) as soon as light conditions became favourable. Then, both the high silica dissolution to production ratio (D : P > 1) observed in the ML (Closset et al, 2014) and the high barium excess measured between 200 and 400 m (Jacquet et al, 2015) suggest that this material was exported and remineralized by the time we visited the station. This could be confirmed by the clear δ 30 Si BSi maximum observed between 100 and 200 m (Fig.…”

Nitrogen uptake and nitrification in the subarctic North Atlantic Ocean

“…These phytoplankton groups tend to dominate the autotrophic assemblage later in the growing season (Lochte et al 1993;Dandonneau et al 2004;Barton et al 2013) and are often associated with increased DON (e.g., urea) uptake (Berg et al 1997). This is in contrast to larger diatoms that bloom earlier in the season in response to high NO -3 and silicate inputs (Lochte et al 1993;Alkire et al 2014;Cetinić et al 2015).…”

“…The region is characterized by some of the largest phytoplankton blooms on Earth, which begin in the spring as the mixed layer shoals and light levels rise. Rapidly growing diatoms typically dominate the spring bloom phytoplankton community, driving high rates of NO -3 uptake and export production (Rynearson et al 2013;Alkire et al 2014;Cetinić et al 2015). As silicate concentrations become limiting, diatom growth slows and smaller phytoplankton such as dinoflagellates and coccolithophores become dominant under the nutrient-deplete conditions characteristic of the late summer (Tarran et al 2001;Dandonneau et al 2004; Barton et al 2013).…”