Abstract: The South Shetland Islands are located in the south side of the Scotia Sea and separated from the Antarctic Peninsula by Bransfield Strait. The region has been a focus of oceanographic research and has become an important fishing ground for Antarctic krill (Euphausia superba) recently. The euphotic zone is the most productive zone in the ocean and occurs the highest concentration of marine biota. Understanding of the characteristics and dynamics of water masses in the euphotic zone can provide basic information for oceanographic research, and also provide scientific support for the distribution and habitat of pelagic organisms, such as the Antarctic krill. In this study, we analyzed temperature and salinity profile data collected from the euphotic zone along five transects around the South Shetland Islands between January and March 2013. Results indicate a clear vertical water mass structure in the deeper areas in the north part of the South Shetland Islands. Antarctic Summer Surface Water, Winter Water and Circumpolar Deep Water occur from surface down, respectively. The Bransfield Strait, which locates in the south of the South Shetland Islands, is almost entirely dominated by the Transitional Zonal Water with Weddell Sea influence (TWW). The water mass structure is complex in its southwestern basin of the Bransfield Strait; the TWW, the Transitional Zonal Water with Bellingshausen Sea influence, and water from the Drake Passage occur in this region. Elucidating the complicated water mass structure around the South Shetland Islands is scientifically important for further study on biophysical interactions in the Southern Ocean.

Key words: South Shetland Islands, water mass, Antarctic Circumpolar Current, Circumpolar Deep Water, Bransfield Strait, euphotic zone