MEPS 190:53-67 (1999)  -  doi:10.3354/meps190053

Sedimentation pathways in the Indian sector of the Southern Ocean during a production regime dominated by regeneration

Caroline Cailliau1, Sauveur Belviso2,*, Madeleine Goutx3, Alain Bedo4, Young Park5, Edwige Charriaud5

1LPCM, UPMC-CNRS-INSU, BP 08, 06230 Villefranche-sur-Mer, France
2LSCE, UMR CEA-CNRS, av. de la Terrasse, 91198 Gif-sur-Yvette, France
3LMM, CNRS-EP 2032, Campus de Luminy, Case 907, 13288 Marseille Cédex 9, France
4LOBE, Station Zoologique, La Darse, 06230 Villefranche-sur-Mer, France
5LOP, 43 rue Cuvier, 75005 Paris, France

ABSTRACT: During summer 1994, the production regime at 2 sites located in the Indian sector of the Southern Ocean, one in the Permanent Open Ocean Zone (POOZ) at 52° S, and a second in the Seasonal Ice Zone (SIZ) at 63° S, was dominated by regeneration (0.3 < f-ratio < 0.4). Two time series, each of about 4 d, were performed over pre-determined time intervals of 4 h using a free-floating sediment trap set at 200 m at the 2 sites. Hourly variations of C, N, chlorophyll a (chl a) and its degradation products, taxon-specific pigments, lipid classes and dimethylsulfoniopropionate (DMSP) were measured simultaneously. Measurements in the water column were done during the sediment trap drifting. Fucoxanthin, a typical diatom pigment, was the major accessory pigment found in the trap material at the 2 stations, whereas, in the water column, the phytoplankton was dominated by flagellates in the POOZ and diatoms in the SIZ. This suggests selective grazing of diatoms by zooplankton and/or mass sinking of diatoms, at least in the POOZ. However, since the set of compounds exhibited strong diel cycles in the POOZ, the export flux appears to mainly result from the zooplankton. The results are ambiguous as to whether the intensified sedimentation at night resulted from vertical migration of euphausiids, since copepods crossing the pycnocline were rare, and/or the nocturnal increase of feeding activity of copepods and microzooplankton. At the SIZ, diatoms dominated in the mixed layer and at the deep phytoplankton maximum (DPM) located at the depth of the temperature minimum (50 to 100 m). However, pigment signature in the trap material suggested the selective sedimentation of nanoflagellates (essentially pelagophytes). Correspondingly, there was high proportions of sterols up to 40% in the trap material. The diel variations somewhat resembling POOZs, the low chl a-to-phaeopigments ratio and the presence of phaeopigments in their most degraded forms were strong indications of the key role played by zooplankton in the export fluxes in the SIZ. Hence, the selective feeding of phytoplankton by zooplankton strongly influenced the sedimentation pathways in the Indian Sector of the Southern Ocean. Additionally, DMSP, which is produced by phytoplankton in superficial waters, is shown to be as labile as chl a when processed by grazers.

KEY WORDS: Antarctic · Austral Ocean · Sediment traps · Diel cycles · Phytoplankton · Pigments · Lipids · DMSP · DMS

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