Beschreibung:
<p>Thingvallavatn lies at 64°10′N and 21°10′W in the subarctic 30 to 40 km from the southwest coast of Iceland at 100.5 m a.s.l. Annual mean air temperature was 2.8°C for the period 1974-1988. The annual range at Thingvellir is 12.5°C. Mean annual precipitation on the lake itself is 1300-1400 mm. Global radiation is 313× 10<sup>7</sup> kJ m<sup>-2</sup> yr<sup>-1</sup>. The basin is formed by tectonic subsidence and glacial erosion, but has since the glacial recession been strongly modified by volcanic activity. The catchment area is characterized by lava flows, of which four entered the lake, and one dammed it. One eruption occurred within the lake. Fault movements have resulted in horizontal and vertical displacement. Soils around the lake are of aeolian origin, well drained, with a low content of clay and a mineral fraction consisting mainly of volcanic ash, susceptible to water and wind erosion. Vegetation is characterized by absence of trees. Contemporary woodland around the lake is mostly confined to protected areas. Plant communities of the open rangelands of the area are moss heath, dwarf shrub heath, graminoid heath and wetlands. The lake is 83 km<sup>2</sup>, with a mean depth of 34 m, and a maximum depth of 114 m. The catchment area is 1000 km<sup>2</sup>, of which 50% is postglacial lava. Interglacial lavas and subglacially formed mountains rising to 800-1100 m, glaciers and glacial deposits account for another 50%. The mean discharge of the last 50 yr is 100 m<sup>3</sup> s<sup>-1</sup>. The precipitation, estimated at 2000-2500 mm yr<sup>-1</sup> in the catchment area, is mostly percolated; hence the lake is 90% spring-fed. The main spring areas in the north show a constant temperature of 2.8-3.5°C. Warmer ground water (7.5-10.7°C) enters the lake in the southwest owing to the nearby Hengill geothermal area. The water level has been regulated since 1959. The lake is dimictic and the temperature commonly reaches 10-11°C in august with a gradient of 2-3°C through the thermocline. On an average, the ice-cover lasted 95 d during the period 1974 to 1990. The nutrients N, P and Si enter the lake at fluxes of 1.4, 0.8 and 240 mg m<sup>-2</sup> yr<sup>-1</sup>. Phytoplankton production is N-limited. Two main energy pathways characterize the lake. The pelagic community is simple with few species (48 phytoplankton, three crustacean zooplankton and one fish species), while the benthic community is more complex with ca. 150 phytobenthic algae, ca. 60 zoobenthic and two fish species. The euphotic layer varies between 20 m in spring and 40 m in summer. Phytoplankton production is bimodal with a spring and early autumn peak of diatoms and a summer minimum of mainly Chrysophyceae. Mean annual production is 95 g C m<sup>-2</sup> (4000 kJ m<sup>-2</sup>). Average biomass is 50 kJ m<sup>-2</sup> resulting in an annual P/B̄ ratio of 64. Zooplankton biomass averages 50 kJ m<sup>-2</sup> and net production amounts to 210 kJ m<sup>-2</sup> yr<sup>-1</sup> giving an annual P/B̄ ratio of 4.2. Net production of planktivorous arctic charr feeding mainly on zooplankton, but also on emerging benthic insects is 28 kJ m<sup>-2</sup> yr<sup>-1</sup> with an average biomass of 50 kJ m<sup>-2</sup> and annual P/B̄ ratio of 0.54. The phytobenthic communities showed a distinct zonation with more algal zone in the stony littoral to 10 m depth with various fast growing herbivorous zoobenthos assemblages adapted. In the sublittoral from 10-25 m depth a Nitella opaca stand supports a small population of the filtrering Chironomus islandicus. The phytobenthos covers ca. 30% of total lake area. The profundal zone is dominated by the slow growing detritivorous Tubifex tubifex. Phytobenthos gross production in the littoral and sublittoral zones averages on a lakewide basis 2800 kJ m<sup>-1</sup>. Average biomass is 500 kJ m<sup>-2</sup> with an annual P/B̄ ratio of 2.6. Zoobenthos (including micro- and meiofauna) production is 100 kJ m<sup>-2</sup> yr<sup>-1</sup>. Annual P/B̄ ratio decreased from 3.6 in the littoral zone to 1.4 in the profundal zone. Three benthic morphs of arctic charr, and threespine stickleback, feed mainly on zoobenthos. Their annual production averaged 3.4 kJ m<sup>-2</sup> yr<sup>-1</sup> with an annual P/B̄ ratio of 0.43. Possible interactions between energy flow in the pelagic and benthic foodchains are pointed out.</p>