Lake Paajarvi is located approximately 100 km north from Helsinki in Finland, a well-known lake country having about 60,000 lakes. This small lake is frozen for nearly five months of the year, though snow accumulation is not heavy. It is deeper than many other lakes in Finland, but the deepest portion occupies only a small part. It is surrounded by simple but beautiful vegetation, mainly northern coniferous forest of pine and spruce. In the fall, broadleaf trees growing mixed among conifers come ablaze with autumnal tints of yellow and gold, offering the typical scenic beauty of northern Europe.

The concentration of dissolved oxygen in the lake water is high both at the surface and along the bottom, while the pH value is somewhat acidic presumably owing to acid rain which is becoming a serious problem in Scandinavian countries. The concentration of chlorophyll indicates a low level of productivity. Population density in the drainage basin is extremely low, and the lake is mainly used for swimming, sport.fishing and sailing. However, the progress of eutrophication by nutrient loading from fertilized crop fields and forests is alarming. The lake is therefore under continuous study by the Lammi Biological Station near the lake, as an area for reference in the comparative study of eutrophication (1, 2).