Abstract
In Lake Thingvalla, Iceland at 64.degree.15''N and 21.degree.7''W gross phytoplankton production averaged 90 g C/m per yr. Chemical variables, chlorophyll a, seston concentration and sediment composition point out Lake Thingvalla as a medium oligotrophic lake, allochthonous inputs being negligible. In the early cycle 2 periods can be distinguished. The spring, when a limited pool of nutrients (calculated to 7 g N m2 per yr) was converted into organic matter, of which 2-3% sedimented per d [day], diminished the nutrient pool and generating N depleted growth. From late August 1979 the nutrient pool was successively reestablished by faster mixing of the lake. This created new growth conditions, and the beginning of a period where pelagic mineralization rather than sedimentation predominated. Net sedimentation was 0.5 mm/yr or 0.5 g N/m2 per yr.