Basing on the analysis of major water mass boundaries and their thermochaline characteristics, T,S diagrams' typification has been realized for hydrological stations carried out in the area of South Orkney Isls and Shetland Isls in January-February since 1970. Three types of years have been singled out for the area of South Orkney Isls. The first type was characterized by presence of intermediate cold layer. In some years of this type (1 97 1, 1972, 1978, 1982) the layer had positive temperature, while in other years (1980, 1981) the layer temperature was negative. The second year type (1979, 1984, 1987) was characterized by rather narrow range of water temperature from the surface to the bottom, being observed at the same time, higher salinity. The third type (1970, 1976, 1984, 1989) was characterized by higher degree of principal water mass mixing within the area and lack of distinct boundaries between the same. Two year types were revealed in the area of South Orkney Isls. Availability of a warmed up layer of summer modification and a cold intermediate layer was characteristic of first year type. Lack of surface water of summer modification was characteristic of second year type (1 970, 1979, 198 1, 1985, 1989).
As water structure within those areas was considered to be the consequence of hydrodynamics processes and an indirect index of hydrodynamic situation observed, the results of typification could be used when analysing krill aggregations' distribution and density for the period of years under examination.