| Summary: | The autotrophic component on Antarctic ice is mainly constituted by diatoms among which the pennates are the most conspicuous in the ice. Benthic diatoms dominate the platelet and bottom communities whereas planktonic species are generally found in the superficial ice layers. The species composition of each of these layers has to be considered as a record of the diatom assemblages, which were present when the freezing process was started. Floristic investigations on pack ice are relatively scarce. In this work we have studied the species composition and distribution of the diatom assemblages in different layers and in the platelet ice of an ice core sampled the 7th of November 1999 in Terra Nova Bay (Antarctica). Floristic changes in diatom assemblages were analysed, only for bottom and platelet ice, starting from ice cores sampled in the next three weeks (till 3rd of December). Benthic species (eg. Berkeleya adeliense, Entomoneis sp., Navicula spp, Nitzschia lecontei, N. stellata) colonize the lower layers and the bottom of the ice-cores. Planktonic species: Asteromphalus parvulus, Thalassiosira gravida and T. maculata are found only in the first layers of the ice-cores, whereas T. gracilis, T. lentiginosa and several species of are distributed along the entire ice-cores. Fragilariopsis dominated pelagic spring blooms; however each of the species of this genus represents only a few percent of the sea ice diatom assemblage. In addition, F. curta, considered the most representative planktonic species of Terra Nova Bay, contributes only 2% of the total ice community. At the beginning of the Austral spring the population was constituted by the presence of benthonic and planktonic diatoms both in the layer below of pack ice (bottom ice) and underneath platelet ice. With the passing of time, a microalgal bloom developed both in bottom ice (1.30 to 432 μg/l Chla) and in the platelet ice (90 to 382 μg/l Chla). The benthonic communities characterized the bottom ice, Entomoneis sp. (2.2 x 10-7 cell/l) and Nitzschia ...
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