| Summary: | Lake Thingvallavatn is a unique groundwater dependent ecosystem. The water originates from glacier melt water and precipitation falling over the basin, located north of the lake. The basin stretches approximately 1300 km2, largely consisting of wilderness that reaches Langjökull and Thórisjökull glacier in the north. The objectives of the study were to investigate the diversity of microbial communities of Lake Thingvallavatn and two fissures in its basin by differentiating the patterns of phylogenic composition and to establish a microbiological baseline that will enhance the understanding of this unusual ecosystem. In addition, one of the objectives was to evaluate environmental risks of anthropocentric activity to the lake by measuring potential faecal contamination. The results of this study indicate that microbial diversity in Lake Thingvallavatn is not as homogenous as one might think. Cell count and microbial diversity range significantly between sampling locations, showing differences among Lake Thingvallavatn and groundwater fissures in its basin. Furthermore, seasonal variation occurs in the lake but less variation was observed between different depth intervals. Diversity indices were used to estimate biodiversity at the sampling stations. Silfra shows the greatest diversity of all sampling stations in this study, followed by Flosagjá and Lake Thingvallavatn. Lake Thingvallavatn does not seem to follow a global distribution in microbial diversity compared to other freshwater lakes. The reasons behind this might be that Lake Thingvallavatn is a sub-artic lake ecosystem characterized by extreme conditions with especially low and stable water temperature that originates from the lake basin. Sequencing Archaea from Lake Thingvallavatn ecosystem proofed very difficult but members of Crenarcheaota and Euryarcaeota were found. Results show that faecal contamination is present in the lake. One location within Thingvellir National Park came back positive for both E. coli and Enterococcus spp. According to the ...
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