bonney upwelling

Blue whales feed in cool upwelled waters off southern Australia (between Bass Strait and the Great Australian Bight) between November and April or May. This coincides with the strongest incidence of south-easterly winds during the year, associated with the passage of high pressure systems to the south of the continent. In brief, ‘upwelling’ winds drive a surface current parallel with the north-west-trending coast. The rotation of the Earth (Coriolis effect) makes this surface current drift to the left, away from the coast. It is replaced with colder, oxygen-rich, nutrient-rich water that originated in the deep Southern Ocean (Antarctic Intermediate Water). This wonderful phenomenon occurs between the west coast of Tasmania, and the eastern Great Australian Bight, and is now known as the Great Southern Australian Upwelling System. During upwelling events, this cool water is drawn onto the continental shelf and toward the surface. Where the shelf is narrowest, such as between Portland (VIC) and Robe (SA), upwelling is deflected to the surface after coming into contact with the shelf, and a distinctive ‘blue’ (colour-enhanced) surface plume is visible in satellite images. Upwelled nutrients are exposed to sunlight and become available for phytoplankton, the minute plants of the sea, and life blooms profusely. Krill swarms gather to feed on the phytoplankton, and everything from barracouta to blue whales gather to feed on krill.

Upwelling varies from year to year due to climate variability, but we still haven’t found a direct relationship between upwelling variability and blue whale abundance, probably due to blue whales’ ability to choose between widely separated feeding areas which may be affected in different ways by climate variability.