| Summary: | Climate change, caused by anthropogenic emissions of carbon dioxide (CO2) and other greenhouse gasses into the atmosphere, is producing profound impacts on ecosystems. The Arctic Ocean is one of the most vulnerable regions of the world and is experiencing the most substantial effects of climate change. This region is characterized by strong seasonality with highly productive episodes, called phytoplankton blooms, that together with the entrance of ice-melt, reduce the CO2 concentration in seawater (from about 370 to 130 μatm). During spring and summer, the low CO2 concentrations might limit the photosynthesis of marine plants and algae. However, during these periods the increase of atmospheric and oceanic CO2 may stimulate the primary productivity in the Arctic Ocean. Increased atmospheric CO2 is directly related with Arctic warming and the consequent acceleration of glaciers and sea-ice melting. During spring and summer, the ice cover retreat increases the light irradiance in submersed areas and ice-free areas become more frequent. An increase of ice-free waters may probably favour the expansion of marine vegetation into the Arctic and the migration of subarctic species to northern regions, characterized by long day length in summer. Since pelagic and benthic ecosystems contribute largely to the primary productivity of the Arctic Ocean, in this thesis I investigate the experimental effects of two abiotic factors: increased CO2 and long day length. These effects have been tested on planktonic communities in surface waters and benthic macrophytes in subarctic and Arctic ecosystems. In parallel, I investigate the relationship between planktonic primary production rates estimated with three different methods: the O2 mass balance, the 18O method and the 14C method. This comparison of methods has been carried out for the first time in the Arctic Ocean, specifically in the north and northwest of the Svalbard shelf, and we conclude that the 14C method, with incubations of 24 h, underestimate the gross primary ...
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