The Influence of Icebergs and Sea Ice to Ocean Acoustics and Whales
Alfred Wegener Institute for Polar and Marine Research, GERMANY
The soundscape of the Southern Ocean is dominated by sea ice, icebergs and marine mammals. Sea ice, with its highly absorbent snow cover creates one of the most quiet acoustic environments on earth. Large table icebergs, on the other hand, weighing up to 100.000.000.000 tons and moved by ocean currents can accumulate kinetic energy in the terajoule range which is occasionally released on collisions with other bergs, the ice shelf or the continental shelf. This creates some of the loudest events in the worlds oceans. However, the largest contribution to the underwater sound budget make marine mammals, blue whales in particular.
The Perennial Acoustic Observatory in the Antarctic Ocean is continuously monitoring the underwater sound below the Eckstroem ice shelf, Dronning Maud Land, Antarctica since 2005. Live streaming is available at www.awi.de/PALAOA. Additionally, an array of moored autonomous recorders deployed throughout the Weddell Sea provides basin wide sampling of acoustic data. We observe a large variability in the acoustics due to the transient nature of all the influences mentioned above: Sea ice is highly dynamic, very large icebergs are singular events and most marine mammals are migratory species - and are again influenced by the ice. So detailed knowledge of the ice situation is essential to understand the acoustics. Apparently simple questions like the population counts of certain marine mammal species are still answered with surprisingly high uncertainty yet, some estimates differ by an order of magnitude. One of the reasons is that animal counts in the polar regions are typically conducted on ship based surveys. Few ships will enter ice covered areas, however, which results in dramatic undersampling of these. And for species, like the Antarctic Minke whale which are known to populate mainly the pack ice this is fatal in terms of prediction. But as most marine mammal species are vocalizing, passive acoustic monitoring is a modern tool for density estimates. Our acoustic recordings in fact contain a wealth of information about these animals. Frequent high resolution ASAR imagery by ENVISAT and RADARSAT provided by the www.PolarView.aq portal enables a precise view on the ice conditions on top of our recorders. So we can determine the influence of ice to the presence of animals. Particulary, comparing densities in ice covered and no ice situations allows to extrapolate from the ship based open water counts to animal numbers in the ice.