The Lake Mývatn Ecosystem, Iceland

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Abstract
Lake Myvatn lies at 63.degree.35''N and 17.degree.00''W and at 278 m above sea level in northern Iceland. The climate is more continental than maritime. But in some years the proximity to the Arctic Circle causes the climate to be cold, wet and foggy. Lake Myvatn has 2 basins: the north basin and the south basin. Volcanic activity since 1975 has caused a land rise of 34 cm at the northeast coast of the north basin. Water enters the lake mainly from cold and warm springs along the east shore (range 5.2-23.1.degree. C) and is discharged, to the west, into the River Laxa. The drainage basin is at least 1000 km2. Retention time in the north basin was 14 days and in south basin, 24 days. Temperature records reveal large variations in response to climatic factors. Air temperatures correlate well with water temperature. The annual amplitude of water temperature was 18.degree. C with a daily amplitude of 3.degree. C in south basin. Wind-induced water level fluctuations are 0.7 m in the north basin and 0.4 m in the south basin. Average ice-cover is 190 days yr-1. Resuspension is high and varies with depth. It was 3 times higher in the north than the south basin. The resuspension provides renewal of nutrients from the bottom, but P seems limiting during calm periods with high demands. N-fixation ensures sufficient N. The N:P ratio in the outlet is 8:1. An inverse thermocline develops in winter and O2 is depleted from the bottom water, but oxygenated conditions are re-established in Feb.-March due to production of benthic diatoms. Phytoplankton gross production was 118 g C m-2 yr-1 in the south basin but only 75 in the north basin. Seasonal succession was unimodal. High production due to Anabaena flos-aquae coincided with high radiation, high temperature and time of ice-break. Production was low at other seasons despite considerable biomass of diatoms and chrysophyceans. Net production in the south basin was calculated to 600 kcal m-2 yr-1. A net production of benthic diatoms amounted to 2220 kcal m-2 yr-1. A dim-light adapted Cladophora aegagropila forms massive phytobenthos in the south basin with 50 g ash-free dry weight m-2. Calculated production was 500 kcal m-2 yr-1. Macro- and epiphytes are important in the north-basin. A living substratum of Cladophora and macrophytes stabilizes the sediment and acts as a sieve for resuspended organic matter. A close relation was found between substrate type and zoobenthos abundance and production. In the south basin, with mud and Cladophora, chironomids dominate numerically (average of total zoobenthos 75,500 individuals m-2) and in terms of production (200 kcal m-2 yr-1). In the north basin, with diatoms, average values are 9750 m-2 and 32 kcal m-2 yr-1. Benthic Crustacea produce approximately 10 kcal m-2 yr-1 in both basins. Zooplankton produced on an average 43 and 15 kcal m-2 yr-1 in the south and north basin, respectively. Fish production, calculated from catch data, was 3 kcal m-2 yr-1 and ducks produced 0.5 kcal m-2 yr-1. Thus energy flow is mainly channeled through phyto- and zoobenthos.