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Spatial and seasonal variability of the transport of biogenic compounds in the Southern Ocean

Antarctica phase III (1993-1997)

Spatial and seasonal variability of the transport of biogenic compounds in the Southern Ocean

Promotor

Dr Frank Dehairs
Vrije Universiteit Brussel
Laboratorium voor Analytische Scheikunde (ANCH)
Pleinlaan, 2
B-1050 BRUSSELS
Phone: +32 (0)2 629 32 60
Fax: +32 (0)2 629 32 74
E-mail: fdehairs@vub.ac.be

Topics

This study focused on the investigation of two intensely related fluxes in the Southern Ocean:

  • the nutrient uptake regime, with main emphasis on the uptake of nitrogen sources, and
  • 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 distinct scenarios for the seasonal evolution of the uptake regime and the phytoplankton community structure were observed:

  • 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.
  • 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, diatoms predominantly consumed 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 potentialfor nitrate excess in oligotrophic Antarctic waters.

Export flux as reflected by mesopelagic stocks of particulate Ba-barite, was 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 N-W Weddell Sea and the Prydz Bay area. Although in these environments algal growth was high 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 and lower algal biomasses. 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. Furthermore, an empirical relationship was observed between mesopelagic Ba-barite stocks and oxygen consumption. This enabled us to write a transfer function relating mesopelagic particulate Ba concentrations and the fraction of exported carbon respired in the mesopelagic water column (Dehairs et al., 1996). This function was 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.

Comparison of export production rates calculated from mesopelagic Ba stocks with rates calculated from sediment trap Ba fluxes (the latter from Nurnberg et al. 1995) was possible for the PFZ of the Atlantic sector. Both approaches produced similar results. For the Indian sector (60° E) export production estimated from mesopelagic Ba ranges between 20 mg C·m-2·d-1 for the Prydz Bay shelf region to 80 mg C·m-2·d-1 just south of the Polar Front, representing respectively between 1 and 41 % of primary production.

References:

Dehairs F., Shopova D., Ober S., Veth C. and Goeyens L. (1996): Particulate barium stocks and oxygen consumption in the Southern Ocean mesoplegaic water column during spring and early summer: Relationship with export production. Deep-Sea Research, in press.

Goeyens L., Semeneh M., Elskens M., Shopova D., Baumann M.E.M. and F. Dehairs (accepted for publication): Phytoplanktonic nutrient utilization and nutrient signature in the Southern Ocean, Journal of Marine Systems

Kamykowski D., and Zentara S.J. (1989): Circumpolar plant nutrient covariation in the Southern Ocean: patterns and processes. Mar. Ecol. Prog. Ser., 58: 101-111.

Kamykowski D., and Zentara S.J. (1985): Nitrate and silicic acid in the world ocean: patterns and processes. Mar. Ecol. Prog. Ser., 26: 47-59.

Nürnberg C.C. (1995): Bariumfluss und Sedimentation im südlichen Südatlantik - Hinweise auf Produktivitätsänderungen im Quartär, Geomar Report 38, 105 pp.

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