On the aggregation of orca (Orcinus orca) in the western Great Australian Bight, Australia: where do the nutrients come from? (#188)
Abstract: For so-far unknown reasons, pods of orca aggregate along the upper continental slope of the western Great Australian Bight near the Bremer Canyon group every year in austral summer. There are no marked surface phytoplankton blooms in the region and most of the region is classified as oligotrophic (nutrient low). Hydrocarbon seeps within submarine canyons were suggested as a potential nutrient source, but this mechanism cannot explain the seasonality of the orca aggregation. Here we discuss another potential upwelling mechanism that has not been considered before: upwelling induced by the interaction of ocean currents and shelf-incised submarine canyons, recently mapped during the maiden CAPSTAN voyage. Preliminary findings using a three-dimensional hydrodynamic shelf-sea model (COHERENS) seem to confirm this hypothesis. The simulated nutrient enrichment along the shelf break of the continental margin remains a sub-surface feature, undetectable by satellite measurement, where it can trigger sub-surface phytoplankton blooms as a prime food source for the orca. Furthermore, the analysis of both satellite and tidal-gauge sea level data indicate that this upwelling can only develop during November-March, when the influence of the Leeuwin Current, that otherwise suppresses canyon upwelling, fades in the region.