| Summary: | This interdisciplinary thesis uses active and passive acoustics to study the polar marine ecosystems. The polar oceans are some of the most remote, harshest and least well studied environments of the planet, and also the regions where climate change and the associated changes in marine ecosystems happen fastest. The four papers that comprise this thesis are based on acoustic data from moored and vessel-mounted instruments and cover different aspects of the Arctic and Antarctic marine ecosystems. Vessel-mounted Acoustic Doppler Current Profiler (ADCP) data was used to map circulation patterns in the highly dynamical Fram Strait region. Such unstructured ADCP data is relatively seldom used in regional studies due to challenges associated with the interpretation of temporal and spatial variability. This was addressed by compiling a large data set, binning, different spatial interpolation methods and discussion of individual sections. The analysis showed that the Yermak Pass Branch can be as important as the Svalbard Branch in transporting Atlantic Water into the Arctic Ocean. The ADCP data was thereafter combined with vessel-mounted echosounder data and numerical modelling to investigate the impact of Atlantic Water circulation on plankton and fish distribution within four major troughs that cut into the Svalbard shelf. The Hinlopen Trough received the strongest and most direct Atlantic Water inflow and showed stronger acoustic backscatter from fish and zooplankton than the shelf, shelf break, and deep ocean. These results suggest that the balance between throughflow and retention creates favourable habitats in the trough for fish, benthic organisms and marine mammals. Passive acoustic data was used to study the sources and seasonal variation of ambient sound in the deep Southern Ocean. Passive acoustic monitoring has the advantage of autonomously and non-invasively gathering data over large spatial and temporal scales. The Southern Ocean, one of the last acoustically pristine oceans due to its remoteness and lack ...
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