Living coccolithophores from the Gulf of Sirte during the summer of 2008

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The Gulf of Sirte is a largely unexplored area in the southernmost Mediterranean Sea. We are presenting here 2008 summer data on the distribution pattern of living coccolithophores, a main phytoplankton calcifying group, in 105 samples from 20 stations. The survey includes coastal and offshore stations, enabling us to provide indications on the dynamics of phytoplankton productivity in relation to oceanographic processes. The total coccosphere concentrations show higher values in the offshore stations and lower ones for coastal sites. Umbellosphaera tenuis, Emiliania huxleyi, Florisphaera profunda, Syracosphaera pulchra HOL (Calyptrosphaera oblonga) and Rhabdosphaera spp. dominate the assemblages. The coccolithophore community shows the typical vertical zonation, with K-strategist taxa (among others U. tenuis, Rhabdosphaera spp. and Discosphaera tubifera) in the upper 75 meters depth and Lower Photic Zone taxa, dominated by F. profunda, below. The latter species shows its maximum development in the Deep Chlorophyll Maximum layer, which occurs in the upper part of the Levantine Intermediate Water, where the nutrient content is higher than in the overlying layers. The mesoscale oceanographic circulation significantly affects the spatial and vertical distribution of coccolithophores, with the thermocline and halocline depth shaping the vertical zonation of coccolithophore taxa and resulting in a strong lateral gradient within the gulf: in the eastern sector, under the influence of the Gulf of Sirte anticyclone, the DCM is deeper and so is the occurrence of K-strategist taxa and F. profunda.

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