Results of studies of species diversity of phytoplankton are presented for the waters of Argentine Islands. Water samples were collected during 7^th (2002-2003) and 10^th (autumn 2005) Ukrainian antarctic expeditions. Seasonal variations in the community species composition have been described; the dominant species and taxa, and the time periods of their vegetation were identified for the first time in the waters of Argentine islands. A total of 151 microalgal species (157 taxa, including subspecies and varieties) were recorded, which represented 8 taxonomic divisions. Bacillariophyta (95 spp., 101 taxa) and Dinophyta (34 spp.) were the most species-rich divisions while the others were represented by 1 to 8 taxa only. The highest species diversity was registered in spring and early summer. Three main peaks of the phytoplankton abundance occurred annually during the spring-summer period, corresponding with population maxima of Phaeocystis pouchetii (Chrysophyta) in October, the diatom genera Fragilariopsis, Achnanthes and Corethron in November and early December, and a bloom of Cryptomonas sp., Pyramimonas sp. and other small flagellates in the middle January. Changes in the list of the dominant species, their abundance and size distribution were in accordance with the I to III stages of the spring-summer succession. In autumn and early polar winter when the vegetation was extinct, phytoplankton abundance and species diversity were low.

Keywords: phytoplankton, species diversity, abundance, biomass, Argentine Islands, Antarctic

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