A comprehensive suite of scientific measurements on board the icebreaker Aurora Australis aims to improve the knowledge about food webs in the Southern Ocean. The ship has reached the Kerguelen Islands in the Southern Indian Ocean and the measurement program is under way.

The Australian ice breaker Aurora Australis is currently home for a group of international scientists studying in the Southern Ocean and its changes. The expedition’s destination: The Kerguelen Islands in the Southern Indian Ocean. This group of islands is one of the two visible parts of the mostly submerged Kerguelen Plateau. They are among the most isolated places on Earth, located 450km northwest of the uninhabited Heard Island and McDonald Islands and more than 3,300km away from the nearest populated area. The islands are part of the French Southern and Antarctic Lands.

Autonomous argo floats like this help scientists to monitor the state of the oceans. (Photo: Katja Riedel)
Autonomous argo floats like this help scientists to monitor the state of the oceans. (Photo: Katja Riedel)

The first stop and official science site for Aurora Australis was between the Shackleton and West ice shelves, a region identified as important due to its high biodiversity and biological production which makes it a popular region for whales, seals, penguins and seabirds. Here the first of five Argo floats was successfully deployed. In months to come it will provide close to real-time observations of ocean currents, temperature and salinity to scientists around the world.

Argo floats are a worldwide initiative that have revolutionised the monitoring of the ocean, with close to universal coverage over open oceans. Argos sink to around 1000m below sea level and drift with the deep ocean currents. Ten days after deployment they descend to 2000m and then begin rising to the surface, measuring temperature and salinity on the way up. Once they reach the surface they transmit the data via satellite back to scientists around the world, before returning to 1000m and starting the cycle again.

To date, Australia has deployed roughly 500 Argo floats in the past 16 years, part of global deployment of 8000 floats. This international effort has given scientists unprecedented, near real-time observations of how the current state of the ocean and how it is slowly changing.

The Kerguelen Islands are situated in the Southern Indian Ocean and were named after the French admiral De Kerguelen De Trémarec.
The Kerguelen Islands are situated in the Southern Indian Ocean and were named after the French admiral De Kerguelen De Trémarec.

The ship spent two days at the location, and along with the deployment of the Argo float, a range of scientific experiments was undertaken and compared with observations from the last survey in this area which took place in 1996. Scientists took measurements of Antarctic Bottom Water, recorded the abundance of Antarctic krill, and investigated the amount of food for krill in this area.

The team was also able to deploy a sophisticated mid-water trawl system designed to catch small fish at different depths. The system includes a series of nets and each one remains open as it is gradually retrieved from 1000m. One net closes and another opens every 200m – all while the net is slowly being dragged behind the Aurora Australis at an approximate speed of 4 km/hour.

Scientists sort through the catch from the mid-water trawl net on board the Aurora Australis (Photo: Dr Christina Schallenberg)
Scientists sort through the catch from the mid-water trawl net on board the Aurora Australis (Photo: Dr Christina Schallenberg)

The total catch consisted of 3.4 kilograms biomass including fish, krill and some salps - a small, clear gelatinous sack (the size of a squash ball) that filters phytoplankton.

This mid-water trawl net will be used more than 30 times during the duration of the voyage to estimate the abundance of small fish across the region, and to monitor how the types of fish may change as the ship moves north from Antarctica towards Heard Island. These fish are an important food source for elephant seals, toothed whales and bigger fish yet the ecology and overall importance of these fish in Southern Ocean food webs remains poorly understood.

The Division's Science Technical Support team preparing the mid-water trawl net for a trial. (Photo: Dr Christina Schallenberg)
The Division's Science Technical Support team preparing the mid-water trawl net for a trial. (Photo: Dr Christina Schallenberg)

A Continuous Plankton Recorder (CPR) is also being dragged behind the ship at a depth of 7 metres. It remains there for up to 36 hours – covering more than 450 nautical miles at a time. The CPR was designed 1931 and has been used in this region of the Southern Ocean on many occasions. The experiences on this voyage will test how a routine collection of CPR data on ships-of-opportunity, like tankers and container ships, may be used to monitor changes in foodwebs in the Southern Ocean.

The Continuous Plankton Recorder is prepared for its 450 nautical mile tow behind the Aurora Australis. (Photo: Dr Christina Schallenberg)
The Continuous Plankton Recorder is prepared for its 450 nautical mile tow behind the Aurora Australis. (Photo: Dr Christina Schallenberg)

Source: Australian Antarctic Division (AAD)