Lake Thingvalla is the largest lake in Iceland with an area of 83.7 km2, a maximum depth of 114 m and an average depth of 34.1 m. The lake is located about 30 km east from Reykjavik in a geologically active zone and is almost entirely tectonic in its origin. In fact, a fault zone traverses Iceland and cuts through the landscape of Thingvalla, actually the lake basin itself. The yearly cycle of the lake is affected partly by the origin of the lake, and partly by its position in the middle of the Atlantic ocean. Weather is wet and windy throughout the year. The yearly temperature amplitude of the lake is 12deg C. Yearly precipitation ranges from 1,226 mm at the northern to 1,436 mm at the southern end (mean 1975-1980). The mean discharge rate from the lake is about 110 m3 sec-1, and the retention time is about 300 days, i. e. almost one year. Water mainly flows into the lake via an underground spring system within the porous volcanic lava surrounding the lake on its north and east sides. However, water entering the lake can be divided into two categories on the basis of ionic composition and temperature. In the northwestern part water of glacial origin percolates the lava basement during some 30-70 years before entering the lake, mostly through subterraneous conduits. Water from this direction has the same ionic composition as the lake water. Water coming from geothermically active areas at the southwestern end is slightly warmer and with higher chloride and silicon content than the lake water in general. The inflows are very stable throughout the year. Although natural in origin, Lake Thingvalla serves as a natural reservoir for an electric power plant with water intake via a tunnel through the mountain barrier embracing the south end. The water level is slightly regulated and fluctuates around 0.5 m. Both the lake and its surroundings are oligotrophic. A slightly pronounced stratification usually occurs between 10 and 25 m depth in most years. The temperature difference within this layer is 4-4.5deg C. The stratification increases during spring and summer until late August. Cooling of the lake in September starts a gradual, complete mixing of the lake. Recently, symptoms of eutrophication have been observed in the littoral zone of the lake and investigations have been made by responsible authorities (Q, 2, 3, 4).