Microphyte pigments and resting spores at the water-sediment interface in the Subantarctic deep sea (Indian sector of the Southern Ocean)

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During the ANTARES I cruise (April-May 1993) in the Indian Sector of the Southern Ocean, deep-sea sediments were collected using a multiple gravity corer in the Polar Frontal Zone (PFZ) and 'frontal zone' (between Crozet and Kerguelen Islands) and in the Permanently Open Ocean Zone (POOZ, south-west of Kerguelen and Heard Islands). Microphyte biomass indicators (Chl a, b, c and related phaeopigments measured by spectrofluorometry, tests on natural fluorescence, and detection of encysted diatoms observed by scanning electron microscopy SEM) were carried out on both sediment and overlying 'fluff', whenever present. Evaluation of the phytoplankton biomass in the overlying surface water masses also was performed at each sampling station. The phytoplankton biomass was, on the whole, very low and decreased from north to south (0.250.10 μg l-1 Chl a, with a maximum on the 'frontal zone'), whereas the sediment-water interface of the southern sampling stations (POOZ) was pigment-enriched. A consistent bioclastic 'fluffy' layer was only present in the POOZ stations (up to 13 μg l-1 Chl a, 157 μg l-1 Phaeo a). Biodeposition of this type was very thin or absent north of the PF (Polar Front), illustrating stronger advection in the northern part of the study area. Hydrodynamic studies of the Crozet Basin and the presence of benthic pennate coastal diatoms at the sediment interface of these northern stations further suggest increased advective activity in the north. Surficial sediments (first 5 mm) also were enriched with pigments in the POOZ (up to 0.15 μg g-1 DW Chl a, 3.5 μg g-1 DW Phaeo a), whereas in the PFZ the Chl a concentrations were insignificant. Pigment gradients in the sediments, particularly well marked in the POOZ, indicate the absence of a secondary intense reworking such as bioturbation or resuspension. Nevertheless, also in the POOZ, an unusual and well preserved megafaunal faecal cast was sampled and analysed. Sediments at southern stations (50-55°S, 56-75°E; 3600-4700 m depth) were the most enriched with encysted microphytic cells (mainly diatoms belonging to the genus Chaetoceros). This was related primarily to the presence of a 'fluffy' layer, composed mainly of well preserved detritic frustules of various diatom genera, including robust Nitzschia and Thalassiosira as well as chains of delicate Chaetoceros, Corethron and Rhizosolenia. Well preserved coccoliths and silicoflagellates also were noticeable in the fluffs. Revival tests (cultures on F/2 medium, initiated with samples from the water sediment interface) were positive for the southern stations, leading to low diversity diatom assemblages dominated by the genera Chaetoceros and Nitzschia.

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Deep-Sea Research Part II: Topical Studies in Oceanography