| Summary: | In this thesis I used paleolimnological methods to study changes in two lakes in Kilpisjärvi region, Somas and Vallijärvi, during the last c. 400 years. I examined the remains of cladocera preserved in sediments to find out species composition and population size in the past. The objective of this study is to reveal if the cladoceran species compositions have changed in the lakes and what is has been driving these changes. In addition, I will use cladoceran resting eggs from the sediment of lake Somas to examine the applicability of resting eggs as a new paleolimnological method. In April 2002 I cored sediment sequences from the lakes and sliced them into 0,5-1 cm subsamples. The samples were dated with led isotope, 210Pb, to reveal the sedimentation rate. From the sediments I determined the organic carbon content, counted the proportions of different cladoceran species, the absolute numbers of individuals and the yearly accumulation of cladocera to the sediment. In addition I calculated the relative share of resting eggs. I examined relationships between the samples and between the samples and measured environmental variables with a set of multivariate methods (DCA, DCCA, PCA, RDA). I tested a previously created cladoceran-water depth model to Vallijärvi data in order to reconstruct past water level development in the lake. In the turn of the last century a distinct interface between two different species compositions can be seen in both lakes. Also the accumulation of cladocera into the sediments has increased in both lakes, and the planktic species have out paced the littoral ones during the last century indicating an increase in productivity. Also sedimentological processes have develop similarly in the lakes. Due to the location of the lakes, their differing morphology and history the changes in biological and sedimentological processes have been stronger in the lake Vallijärvi. The model on past water levels seemed to perform well in the lake Vallijärvi. Water level reconstruction was not, however, problem ...
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