MICROPHYTE PIGMENTS AND RESTING SPORES AT THE WATER-SEDIMENT INTERFACE IN THE SUB-ANTARCTIC DEEP-SEA (INDIAN SECTOR OF THE SOUTHERN-OCEAN)

During the ANTARES I cruise (April-May 1993) in the Indian Sector of t he Southern Ocean, deep-sea sediments were collected using a multiple gravity corer in the Polar Frontal Zone (PFZ) and ''frontal zone'' (be tween Crozet and Kerguelen Islands) and in the Permanently Open Ocean Zone (POOZ, south-west of Kerguelen and Heard Islands). Microphyte bio mass indicators (Chi a, b, c and related phaeopigments measured by spe ctrofluorometry, tests on natural fluorescence, and detection of encys ted diatoms observed by scanning electron microscopy SEM) were carried out on both sediment and overlying ''fluff'', whenever present. Evalu ation of the phytoplankton biomass in the overlying surface water mass es also was performed at each sampling station. The phytoplankton biom ass was, on the whole, very low and decreased from north to south (0.2 5-0.10 mu g l(-1) Chl a, with a maximum on the ''frontal zone''), wher eas the sediment-water interface of the southern sampling stations (PO OZ) was pigment-enriched. A consistent bioclastic ''fluffy'' layer was only present in the POOZ stations (up to 13 mu g l(-1) Chi a, 157 mu g l(-1) Phaeo a). Biodeposition of this type was very thin or absent n orth of the PF (Polar Front), illustrating stronger advection in the n orthern part of the study area. Hydrodynamic studies of the Crozet Bas in and the presence of benthic pennate coastal diatoms at the sediment interface of these northern stations further suggest increased advect ive activity in the north. Surficial sediments (first 5 mm) also were enriched with pigments in the POOZ (up to 0.15 mu g g(-1) DW Chi a, 3. 5 mu g g(-1) DW Phaeo a), whereas in the PFZ the Chi a concentrations were insignificant. Pigment gradients in the sediments, particularly w ell marked in the POOZ, indicate the absence of a secondary intense re working such as bioturbation or resuspension. Nevertheless, also in th e POOZ, an unusual and well preserved megafaunal faecal cast was sampl ed and analysed. Sediments at southern stations (50-55 degrees S, 56-7 5 degrees E; 3600-4700 m depth) were the most enriched with encysted m icrophytic cells (mainly diatoms belonging to the genus Chaetoceros). This was related primarily to the presence of a ''fluffy'' layer, comp osed mainly of well preserved detritic frustules of various diatom gen era, including robust Nitzschia and Thalassiosira as well as chains of delicate Chaetoceros, Corethron and Rhizosolenia. Well preserved cocc oliths and silicoflagellates also were noticeable in the fluffs. Reviv al tests (cultures on F/2 medium, initiated with samples from the wate r-sediment interface) were positive for the southern stations, leading to low diversity diatom assemblages dominated by the genera Chaetocer os and Nitzschia. (C) 1997 Elsevier Science Ltd.