| Summary: | In the Arctic region, global warming progresses quickly. Arctic freshwater indicator species were used as model organisms to study environmental change. The objective of the thesis was to evaluate the severity of warming and acidification for fauna in Arctic freshwaters. Freshwater acidification was confirmed in northern Finland. Precipitation was acidic and water pH was an alarmingly low 4.9 in small ponds. This measured low pH was also the mean precipitation pH in northern Finland. Increasing acidic precipitation together with warming are major threats to acid sensitive Arctic species especially in ponds in northern Finland, due to their low water volume and their limited buffering capacity. The new method to calculate climate risk for selected species predicts physiologically harmful temperatures in Lapland. The Arctic indicator species have not experienced these predicted high temperatures during the last 60-years. The findings highlight that the lethal effect of heat stress caused by global changes need to be studied in a species- perspective to allow predictions for Arctic populations. Behavioural patterns, such as sexual reproduction, cannibalism and colonisation can help the species to survive in the severe and fast changing Arctic. The result shows that these beneficial behavioural patterns were present in the Arctic tadpole shrimp populations in the High Arctic Svalbard. Colonisation after the Last Glacial Maximum (LGM) happened from refugia in the South-West resulting in diverse invertebrate fauna in Arctic archipelagos Svalbard, Franz Josef Land and Novaya Zemlya. Together, the results highlight populations where the possible management actions should be carried out.
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