The Bonney Upwelling
Spring winds kick off a remarkable time in the waters off the coast of Portland. Through a combination of dynamic environmental events, these waters become the richest marine feeding area in Australia – the Bonney Upwelling.
From November to May, cold nutrient rich water ‘wells up’ from the deep sea floor onto the continental shelf where sunlight converts the nutrients into food for a huge variety of marine animals.
Krill is one of the most important animals in this food web. Many species including schooling fish, barracouta, arrow squid, short-tailed shearwaters, fairy prions, little penguins, fur seals, dolphins and blue whales eat krill.
Fur seals, dolphins, gannets, albatrosses and a variety of sharks feed on schooling fish and squid, and the rain of dead or dying animals from this food web to the seafloor sustains the rock lobster and giant crab populations.
As the Upwelling season progresses, food becomes more and more abundant, with krill swarms becoming larger and denser, and predators becoming more numerous. The Upwelling not only supports this amazing array of life, but also the fishing industry that is so important to our local community.
Although Upwelling extends continuously from western Bass Strait to the Great Australian Bight west of Kangaroo Island, the most intense upwelling occurs from Cape Nelson west to Robe.
A satellite image showing Sea Surface Temperature (SST) where blue is cold water and red is warm water. The cold plume of the Bonney Upwelling extends from Cape Nelson, near Portland into South Australian waters.
Named after the Bonney Coast west of Portland, the upwelling phenomenon of spring winds shifting to the south-east driving an ocean current to the north-west along the coast where the surface waters drift offshore. This shifting water is replaced with cold nutrient-rich water from Antarctica drawn up from the deep onto the continental shelf.
Each season, from November to May, the upwelling will pump more nutrients onto the shelf. The strength of the upwelling and the amount of nutrients it brings vary from season to season depending on large-scale climatic processes that aren’t well understood.
It’s impossible to predict in advance what sort of upwelling season we’re about to have, or exactly when it will start. Often it is during the last week of October or the first week of November.
The cold (as low as 9°C) nutrient-rich water is upwelled into the sunlit zone, allowing biological processes to begin – Nutrients + sunlight = photosynthesis.
Phytoplankton (single-celled green plants) bloom in countless billions, turning the ocean from clear blue in winter to murky green or even khaki brown during the upwelling season.
Phytoplankton is the basis of the food web, being food for zooplankton (animal plankton) including copepods and krill, which are in turn eaten by a range of larger animals.
The krill Nyctiphanes australis is the most significant zooplankton in the upwelling, forming swarms of millions of individuals that may occur anywhere between the surface and the bottom of the ocean.
Krill live in the system year-round, but probably time their growth and breeding cycle to coincide with upwelling, and become very noticeable after the upwelling season starts. They grow to about 2cm, but swarms may be 1km or more long.
Blue whales, Balaenoptera musculus, arrive in the Bonney Upwelling to feed on the krill near the very start of the upwelling season. The earliest blue whale sighting recorded so far on 8 November 2004 and in 2008, a colleague in South Australia sighted the first blue whale on 10 November, on the west coast of Eyre Peninsula in the Great Australian Bight.
At this stage we don’t know if they arrive from the west or from the east, or both. We know that blue whales migrate up both east and west coasts, to and from unknown tropical breeding grounds – perhaps in the Indonesian archipelago, or around the Solomon Islands.
About 3% of all our sightings involve mothers with calves, which are born elsewhere but are weaned from their mothers at about 7 months of age while in the Upwelling.
Blue whales’ feeding season lasts from around November to May. The Bonney Upwelling is one of a handful of feeding areas in the Southern Hemisphere, with others off Perth, off southern Chile, across the southern Indian Ocean, and in the Southern Ocean to our south.
Blue whales are classified as Endangered under Australian law; their numbers are still low as a result of uncontrolled commercial whaling during the 20th century.
Information provided by the Blue Whale Study Inc. For more information go to www.bluewhalestudy.org