Surface phytoplankton samples were studied quantitatively and qualitatively in February 1996 - November 1998 and January 2003 - November 2005 at the shore and in the center of Admiralty Bay, King George Island. Phytoplankton assemblages showed spring-summer peaks (maxima 4.0-5.2×106 cells l-1) associated with small variations in low atmospheric pressure, and low velocity winds. They were dominated by nano-sized (<20 µm) flagellates and picoplankton (~2 µm). The prevalent nanoflagellates were either Prasinophyceae, Cryptophyceae, or Prymnesiophyceae. Diatoms were next in abundance. Of the seven spring-summer diatom blooms, five had initiated at the shore (maximum 9.8×105 cells l-1; November 1998). They were significantly greater than in the open water, and did not spread into the bay centre. Two observed open water blooms did not reach the shore. Diatoms formed up to 44% of the total cells in the period 1996-98; they only formed <5% in 2003-05. Shore and open water populations differed by diatom dominance structure. Pennates (Fragilariopsis spp., F. cylindrus, Pseudo-nitzschia spp.), and benthic species were prevalent at the shore; centrics (Thalassiosira spp., Chaetoceros socialis) were most common offshore. In 2003-05 diatoms were relatively impoverished in Chaetoceros spp. and the larger (>20 µm) Fragilariopsis spp. Nano-sized Thalassiosira spp. were the winter dominants. Diatom species dominance structure may change at each of the two sites within a month (e.g. shore site: F. cylindrus dominant in October ’98; T. gravida in November ’98). Dinoflagellates showed summer increases associated with diatom blooms. Variations in phytoplankton cell concentrations, the species structures between the shore and open waters, and between seasons appear to be related to physical factors: changes in wind velocity and direction, inflow of waters from the Bransfield Strait, ice melting and changes in atmospheric pressure.