An oxygen isotope study in a lake-river system in southern Finland

The project 'Nutrients from river basins - experimental and modelling approach' (NUTRIBA) is part of the Baltic Sea Research Programme (BIREME). The programme started in 2003 and runs for three years. The drainage basin of the River Kokemaeenjoki in southern Finland running to the Baltic Sea is selected for study. The River Kokemaeenjoki basin is the fourth largest water system in Finland with a total surface area of 27 000 km2 and an average discharge of 230 m3s-1. In connection to the larger research in the Kokemaeenjoki river system stable isotope studies are carried out in a selected lake-river chain in the upper part of the water system. The site selected for study comprises of five lakes forming a chain. In addition to having a different hydrological position the lakes vary in their trophic status and the size of their drainage areas varies more than one order of magnitude. The theoretical residence time ranges from 0.1 to about 3 years. The surface area of the lakes varies from 1 to about 31.5 km2, the catchment area from 116 to 1171 km2, and the water volume of the lakes from 1.6 to 247x103m3. Water was sampled for isotope studies from the lakes at two depths and their in- and outlet river channels every second week during the ice-free period and monthly during the ice covered period. The results from lake water samples collected during the first year show higher d18O values than those of regional mean precipitation and different deg.ree of evaporation. Stratification formed in the deeper lakes and turn-over in spring and autumn were traced. Variations of d18O values in lake waters during April-November ranged from c. 1 to 2 per mille. Mean seasonal variation found in d18O for the regional precipitation is 4-5 per mille