Finland has about 130 000 lakes with a surface area of more than 0.04 km~2, but only 47 of these lakes are large (i.e. with an area over 100 km~2; Mannio et al. 2000). Of these large lakes five are situated in North Karelia, the easternmost province of Finland. Their water chemistry and phytoplankton have been included in both regional and national monitoring programmes from the 1960s, and since 2000 they have been encompassed by the Finnish Eurowaternet (Nie-mi et al. 2001). The aim of this work is to give an overview of two of these lakes, Lake Koitere and L. Hoytiainen, and to discuss the long-term data on water quality and phytoplankton in respect to paleolim-nological data.
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