An Autonomous Underwater Vehicle (AUV) carrying 600 kHz RDI Acoustic Doppler Current Profilers (ADCPs) was deployed at four locations over the West Spitsbergen outer shelf in July 2010. The backscatter signal recorded by the ADCPs was extracted and analysed to document the vertical distribution and patchiness of the pelagic scattering layer during midnight sun. At the northernmost locations (Norskebanken and Woodfjorden), fresher and colder water than at the southernmost locations (Kongsfjordbanken and Isfjordbanken) prevailed in the surface layer (0-20 m) and scatterers (interpreted as zooplankton) were distributed over the entire sampling depth (0-150 m). In contrast, more saline and warmer Atlantic Water dominated the surface layer at Kongsfjordbanken and Isfjordbanken and scatterers were concentrated in the top 20 m, above the pycnocline. Scatterers formed patchy aggregations at all locations, but patchiness generally increased with the density of organisms and decreased at depths >80 m. This study contributes to our understanding of the vertical distribution of pelagic organisms in the Arctic and the spatial coverage of the AUV has extended early acoustic studies from 1-dimensional observations limited to Arctic fjords to a broader, more general offshore coverage. No synchronised or unsynchronized vertical migrations were detected, but the short time scale associated with AUV deployments resulted in high variance and autonomous vehicles are therefore not as effective as long-term mooring deployments to study vertical migrations. AUV-based acoustic surveys of the pelagic communities are nonetheless highly complementary to Eulerian studies, for instance by providing spatial measurements of patchiness.
|Publication status||Published - 9 May 2016|