8th International Nannoplankton Association Conference
Elisa Malinverno, Patrizia Ziveri, Cesare Corselli:
Water-samples for coccolithophorid analysis were collected in November-December, 1997, from the photic zone (0-200m) of nine stations along two ideal W-E and SW-NE transects in the Ionian Basin; CTD data were simultaneously collected. An objective of this work was to improve the understanding of the ecology of coccolithophorids, major contributors to the carbonate sediment of the eastern Mediterranean, including spatial and vertical distribution, total production and species distribution. Sampling depths were planned to obtain a regular sample grid: sampling was finer in the upper photic zone, usually characterised by greater variability, and coarser at depth, where usually both a minor coccolithophorid concentration and species variability are present. For each sampling depth, 6 to 12l of sea-water were immediately filtered on board, on Millipore polycarbonate and cellulose acetate filters (47mm diameter; 0.45µm pore-size), using a vacuum filtration system. The filters were oven-dried and stored in plastic boxes. The cellulose acetate filters have been used for polarised light optical microscope (LM) analysis, and the polycarbonate filters for scanning electron microscopy (SEM; only samples that showed high coccosphere concentrations were analysed with the SEM).
Coccosphere concentration (coccospheres l-1) is usually higher in the upper 50m, often peaking at 25m, while at depth this concentration gradually decreases and at 200m it approaches zero values.
Coccolithophorid species distribution is strongly related to the physical and chemical characteristics of the water-column. The depth of the thermocline coincides well with the halocline and, in the sampling period (late winter), is usually located between 50 and 80m. The upper layers are well mixed, due to strong wind action in this season. Water transparency is lower in the upper 50-80m due to the higher amount of suspended particles of both biogenic and terrestrial origin, although it rapidly increases below the thermo-halocline.
At all stations, the upper photic zone is dominated by E. huxleyi (generally 50-60 upto 80% of the total assemblage). In surface waters (0-25m), diversity is usually high, with high relative abundances of R. xiphos, R. clavigera var. stylifera, P. vandelii, U. tenuis, many species of Syracosphaera (upto 16 per station) and, to a lesser extent, Acanthoica spp., Alisphaera spp. and D. tubifera. Holococcolithophorids are sometimes very abundant, with several species, but at some stations are poorly represented. A. brasiliensis is present in the upper layer, although it usually reaches higher concentrations near 50m depth.
Below the thermocline, new species are present that constitute upto 70% of the assemblage. Among these, F. profunda, G. flabellatus and A. robusta are the most important, and make up the typical 'deep assemblage' in the 80-150m depth-interval. Besides these, other species are typical of the lower photic zone: A. bimurata, P. lepida, O. antillarum, sometimes M. elegans and, among the Syracosphaeraceae, S. anthos, S. lamina and Syracosphaera type K (sensu Kleijne).
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