MEPS 249:1-14 (2003)  -  doi:10.3354/meps249001

Mesoscale surface distribution of biogeochemical characteristics in the Crozet Basin frontal zones (South Indian Ocean)

M. Fiala1,*, B. Delille2, C. Dubreuil3, E. Kopczynska4, K. Leblanc3, J. Morvan5, B. Quéguiner3, S. Blain6, C. Cailliau6, P. Conan1, R. Corvaisier6, M. Denis3, M. Frankignoulle2, L. Oriol1, S. Roy3

1Laboratoire Arago, Université P & M Curie, UMR-CNRS 7621, 66651 Banyuls-sur-mer Cedex, France
2Unité d¹Océanographie Chimique, Institut de Physique, Université de Liège, 4000 Sart Tilman, Belgium
3Laboratoire d¹Océanographie et de Biogéochimie, Université de la Méditerranée, UMR-CNRS 6535, Parc Scientifique et Technologique de Luminy, Case 901, 13288 Marseille Cedex 9, France
4Department of Antarctic Biology, Polish Academy of Sciences, Ustrzycka 10, 02141 Warszawa, Poland
5Ecole Normale Supérieure de Chimie de Rennes, Avenue du Général Leclerc, 35700 Rennes-Beaulieu, France
6Laboratoire des Sciences de l¹Environnement Marin, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, UMR-CNRS 6539, 29280 Plouzané, France

ABSTRACT: A mesoscale study was conducted in January and February 1999 in the Crozet Basin frontal zones (43°50¹ to 45°20¹S; 61°00¹ to 64°30¹E) within the southernmost and easternmost convergence area of the Antarctic Circumpolar Current (ACC) and the Agulhas Return Current (ARC). Distribution of biogeochemical parameters was strongly linked to the merged Subtropical (STF) and Subantarctic (SAF) Fronts which mark the border between the cold and less saline subantarctic waters and the warm and more saline subtropical waters. This survey took place during a post-bloom period. Chlorophyll a concentrations were low throughout the study area ranging from 0.2 µg l-1 in the Polar Frontal Zone (PFZ) to 0.4 µg l-1 in the Subtropical Zone (STZ). Maximum chlorophyll a values (0.8 µg l-1) associated with an increase in biogenic silica concentration (from 0.03 to 0.34 µM) and a diatom peak (1.2 x 105 cells l-1) were encountered in the northeastern part of the STF edge. Despite northwardly decreasing concentrations of nitrates from 14 µM in the PFZ to 6 µM in the STZ, they were not the main factor limiting phytoplankton growth. Low silicic acid (mean = 0.6 µM) could have limited diatom development in the PFZ and the STZ where diatom numbers were low. In STZ waters, where average diatom numbers were highest, various species of Nitzschia and Thalassiothrix were common, but Pseudonitzschia spp. were dominant. Throughout the survey area, pico- and nano-sized cells dominated the phytoplankton assemblage, and their number was the highest in the STZ. Cyanobacteria, only present in subtropical waters >12.5°C, were the major component of the picoplankton size-fraction. While dinoflagellate numbers were low in the Subantarctic Zone (SAZ), their abundance and species numbers increased in the STZ, where Oxytoxum laticeps became dominant and several further large-size species of Prorocentrum, Ceratium and Gymnodinium appeared in addition to those at the STF. The distribution of different biogeochemical parameters suggests that the Crozet Basin frontal region is a non-exporting system at the end of summer. During this post-bloom period, biological activity is low and phytoplankton growth severely limited. This is evidenced by the weak dependence of the partial pressure of carbon dioxide (pCO2) on biological activity and the importance of the air-sea exchange in maintaining pCO2 close to saturation.

KEY WORDS: Frontal zones · Nutrients · Biogenic silica · Chlorophyll a · pCO2 · Phytoplankton · Bacteria

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