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Spatial and seasonal variability of new production and export production in the Southern Ocean
Dehairs, F.A.; Semeneh, M.; Elskens, M.; Goeyens, L. (1997). Spatial and seasonal variability of new production and export production in the Southern Ocean, in: Caschetto, S. (Ed.) Belgian research programme on the Antarctic: scientific results of phase III (1992-1996): 1. Marine biochemistry and ecodynamics. pp. A3/03/001/1-83
In: Caschetto, S. (Ed.) (1997). Belgian research programme on the Antarctic: scientific results of phase III (1992-1996): 1. Marine biochemistry and ecodynamics. Belgian scientific research programme on Antarctica. Federal Office for Scientific, Technical and Cultural Affairs: Brussel. 220 pp., more
In: Belgian scientific research programme on Antarctica. Belgian Science Policy Office: Brussel. , more

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  • Dehairs, F.A., more
  • Semeneh, M.
  • Elskens, M., more
  • Goeyens, L., more

    This study focuses on the investigation of two intensely related fluxes in the Southern Ocean: (a) the nutrient uptake regime, with main emphasis on the uptake of nitrogen sources and (b) the type and intensity of export production towards the deeper layers and the sediments. The separation in Southern Ocean provinces of silicate excess at nitrate exhaustion and of nitrate excess at silicate exhaustion, as suggested by Kamykowski and Zentara (1985, 1989), was supported by our investigations of the silicate to nitrate uptake ratios. Oligotrophic Antarctic waters mainly exhibit proportionally higher silicate removal what induces a potential for nitrate excess. The nitrogen uptake regime of such areas is characterised by low absolute as well as specific nitrate uptake rates throughout. Maximal values did not exceed 0.15 µM d-1 and 0.005 h-1, respectively. Corresponding f-ratios ranged from 0.39 to 0.86. This scenario contrasts strikingly to the more fertile ice edge areas. They showed a drastic but short vernal increase in nitrate uptake. Absolute uptake rates reached a maximum value of 2.18 µM d-1 whereas the maximal specific uptake rate was 0.063 h-1. This peak nitrate utilisation during early spring led to the observed potential for silicate excess. With increasing seasonal maturity the nitrate uptake became inhibited by the presence of enhanced ammonium availability (up to 8 % of the inorganic nitrogen pool), however, and after a short period of intensive nitrate consumption the uptake rates drop to very low levels, which are comparable to the ones observed in the area of nitrate excess at silicate exhaustion. The nitrogen uptake by phytoplankton was also studied in relation to the biomass and structure of the community in the Atlantic and Indian sectors of the Southern Ocean. Two scenarios for the seasonal evolution of the uptake regime and the phytoplankton community structure are discussed. In the Marginal Ice Zone of the Scotia-Weddell Confluence area, the transition from a predominantly nitrate based system to a predominantly ammonium based one was paralleled by the disappearance of diatom dominance and the concomitant development of a dense flagellate dominated phytoplankton community. On the other hand, in the Coastal and Continental Shelf Zone and Open Oceanic Zone of the Indian sector, the shift in the nitrogen uptake regime occurred without significant change in the phytoplankton community structure. Diatoms dominated the assemblage throughout and about 80% of the phytoplankton biomass was in the > 10 µm size fraction. Unlike the first scenario, diatom growth was largely depending on ammonium. Thus, in areas of persistent water column stability and less selective grazing pressure, a shift in the uptake regime can occur without changes in the community structure. The dominance of diatoms under regenerated production provides, furthermore, physiological supports for the observed potential tor nitrate excess in oligotrophic Antarctic waters. Export fluxes, as traced by mesopelagic stocks of particulate Ba-barite, were found to be strongly dependent on the type of production. In environments with predominant regenerated production, export production did not sustain significant Ba-barite accumulation. This appeared to be the characteristic situation for environments having shallow mixed layers as a result of melt water input, such as the NW Weddell Sea and the Prydz Bay region. Although in these environments algae growth was high (large nitrate depletions and uptake rates at the beginning of the growth season) grazing pressure was also high, as witnessed by high subsurface ammonium. Open ocean areas, on the contrary, had larger export fluxes despite their lower surface productivities (lower integrated nitrate depletions) and algae biomass. The latter properties resulted from deeper mixed surface layers and relatively reduced grazing pressures (poor ammonium build-up). The Polar Front region appeared to be an intermediate system. Additionally, a transfer function was used to calculated the fraction of exported carbon respired in the subsurface and intermediate waters (i.e. the mesopelagic water column). This function was first validated to the ANTARKTIS X/6 Ba data set and thereafter applied to the whole of our Southern Ocean data set. Export production obtained from mesopelagic Ba accumulation was subsequently compared to sediment trap Ba fluxes.

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