Thermal stratification in Paanajärvi, northern Republic of Karelia, Russia

Six visits between July 1996 and July 1997 were made from Finland to measure temperatures at depths between 0-80 m at five points using a combined oxygen temperature meter (YSI model 57). Paanajärvi lies in a tectonic depression (width 0,6-1,3 km, mid-line length 24,5 km, maximum depth 128 m) close to the Arctic Circle with an ice-cover of about six months and its main inflow (from the west) at the opposite end from the outlet (in the east). Except for spring and autumn turnovers, the thermal stratification in the lake is very clear all year round. The greatest changes are connected with the spring turnover followed by rapid warming of the 10-15 m epilimnion, which develops more quickly near the inlet end and spreads gradually towards the outlet end. The dominance of the western inlet waters, which make up over nine-tenths of all the water flowing through the lake, is to be seen in winter in the form of a cold "epilimnetic river", 10 metres deep, one result of which is that the hypolimnion at the outlet end is 1 °C colder than that at the inlet end. Thermal stratification in Paanajärvi is much more clearly defined than in normal Finnish lakes, and resembles that in alpine lakes rather than lakes typical of the Fennoscandian Shield