Spotlight on the K-Axis

Draw a line from the South Pole, through the Amery Ice Shelf on the East Antarctic coast, and out into the Southern Ocean, between Heard Island and Kerguelen Island, and you have an ‘axis’ that intersects three key habitat areas. The Kerguelen Axis or ‘K-Axis’ is one of only three lines of longitude where the Antarctic Circumpolar Current flows across the Antarctic continental shelf, the deep ocean and subantarctic islands, resulting in one of the most highly productive regions for polar plants and animals, and valuable toothfish, icefish and krill fisheries.

Early next year, the K-Axis will be the focus of a major Australian marine science voyage, led by Dr Andrew Constable of the Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC). The voyage will study the physical, biological and chemical conditions that drive the krill-based food web in the southern part of the axis, and the fish and copepod (small crustaceans) dominated food web in the north. It will also establish methods for the long-term observation of the region in the face of climate change.

‘The K-Axis is an important foraging area for seals, whales, penguins and flying seabirds, and is the site of valuable toothfish and icefish fisheries on the northern Kerguelen Plateau and potential krill fisheries in the south,’ Dr Constable said.

‘Because of its high levels of primary production [phytoplankton growth] the region also contributes significantly to the drawdown of atmospheric carbon dioxide into the deep ocean.

‘All these characteristics make the K-Axis an excellent place to identify physical, chemical and biological drivers of the different food webs found in the Southern Ocean, and to measure ecosystem change as a result of climate change.’

Despite the region’s importance, the voyage will be the first to study the K-Axis as a whole. To ensure maximum return on their research investment, the Aurora Australis-based voyage will also coordinate with three other research vessels conducting additional or complementary research in the region — the French ship Marion Dufresne, the Japanese Umitaka Maru and CSIRO’s new national research facility, Investigator. There will also be some oceanographic input from the US vessel Roger Revelle. The data collected will help address a number of research questions.

The first area of research will investigate the factors that affect the distribution of Antarctic krill and determine the species’ northern limits. These factors could include temperature, the distribution of their food source (phytoplankton), sea ice extent, and the southern boundary of the Antarctic Circumpolar Current. This information will be used in ecosystem models of the krill-based food web to understand how the species may be affected by climate change and ocean acidification. This in turn will feed into management measures and catch limits for krill fisheries.

The second area of research will examine the relationships between planktonic species, including phytoplankton, zooplankton and krill, with different habitat characteristics. These characteristics include temperature, salinity, depth, iron supply (for phytoplankton growth) and carbonate concentration (used to form shells and other hard structures in planktonic and other species).

‘As the Southern Ocean warms and winter sea ice extent decreases, we could see temperate species migrating south towards the poles,’ Dr Constable said.

‘So knowledge of the key habitat variables that limit the ranges of species is crucial for determining whether such temperate marine food webs will migrate towards the poles, resulting in a contraction in polar food webs,’ Dr Constable said.

A third research area will assess phytoplankton productivity and food web structure in three habitat areas of the K-Axis — close to the Antarctic continent (continental shelf habitat), the BANZARE Bank and adjacent open ocean, and the northern Kerguelen Plateau, near to subantarctic islands.

The productivity component of the work will focus primarily on iron sources in each habitat area.

‘Each of these areas has different potential sources of iron, which is critical for blooms of phytoplankton at different times,’ Dr Constable said.

‘However, the sources of iron in the Southern Ocean are a big question at present. It could come from the adjacent continents via the wind, or be concentrated in sea ice through a variety of processes. Or it may be released from the sediments or hydrothermal vents.’

If the biogeochemical teams onboard the Aurora Australis and Investigator can identify which iron sources are the most important in the K-Axis, they can help improve the way ecosystems models represent primary production in the region.

For the food web structure analysis, the research team will examine the distribution of marine mammals and seabirds across the K-Axis and look for any ‘hotspots’ of activity. To do this they will draw on existing predator monitoring work by France and Australia at Kerguelen Island and Davis station respectively.

‘Both France and Australia monitor the movement of penguins, seals and flying seabirds in the region each year,’ Dr Constable said.

‘The Davis-based predators migrate to the north of the region we’ll be traversing by ship and the Kerguelen predators migrate into the south of the region.

‘We’ll also use acoustic technology and observers to locate any whales in the region.’

The team will also look at the distribution of small ‘mesopelagic’ fish. These fish live in the top 200 to 1000 metres of ocean and are an important food source for seals, penguins and other predators. DNA analysis of the stomach contents of the fish will provide information about their diet — different phytoplankton and zooplankton species (see DNA barcoding plankton) – which can in turn be related back to the effects of different habitats and ocean chemistry on plankton productivity.

‘The best way to think about all these data is as layers,’ Dr Constable said.

‘So we will have a layer showing where the predators forage and another layer showing the densities of fish and krill in areas where they are and are not foraging. Below that we will see where the zooplankton are found — are they in the same place as the fish or is there a flux over time, in different places, which the fish exploit? Then we can look at the data layer showing the distribution of phytoplankton and its productivity, so that we can see where it is and how much is available for krill, fish and zooplankton to eat. Below that, we’ll have a layer of information about ocean chemistry and dynamics.

‘We’ll be able to see the relationships between all these layers, which will give us a better understanding of what drives the movement of predators and fish.’

These drivers could include trace metals such as iron, ocean currents, sea ice, or a combination of physical and chemical factors.

The research team aims to place all the ecological observations in the context of physical and chemical drivers of possible change in the region. This will feed into ecosystem models being developed at the Australian Antarctic Division and the ACE CRC.

‘The main outcome of this work will be to enhance the realism of these models, to identify methods and technologies that will allow long-term monitoring of the effects of climate change and ocean acidification on Southern Ocean ecosystems in the region, and to provide information for conservation and fisheries management,’ Dr Constable said.

Wendy Pyper
Australian Antarctic Division

This project is funded by the Australian Antarctic Division, ACE CRC and Australian Research Council Special Research Initiative.