BROKE-West breaks ground in marine research

Four graphics showing results from the BROKE-West survey
Results from the BROKE-West survey (Photo: Ben Raymond, AADC)
Map showing the oceanic sector in which the 2006 BROKE-West survey was conducted - known as CCAMLR Division 58.4.2.Map of the BROKE-West survey region of CCAMLR Division 58.4.2, between 30 and 80E.

A comprehensive survey of the South West Indian Ocean off the East Antarctic coast has shown it to be a region rich in marine life and the site of important oceanographic processes.

The Antarctic coastline east of Mawson station has rarely been visited by Australian research vessels, and the ecosystems of this region have never been comprehensively documented. The main aim of the Baseline Research on Oceanography, Krill and the Environment-West (BROKE-West) was to characterise these ecosystems and establish an accurate estimate of krill distribution and abundance in the South West Indian Ocean (see map). This would allow the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to calculate precautionary catch limits for the krill fishery in the area. The survey was also designed to study the region's oceanography (as part of the Antarctic Climate and Ecosystems Cooperative Research Centre's climate research program), and to examine the links between biological production and ocean processes.

Using acoustic instruments (which emit 'pinging' sounds into the water and listen for the returning echo as the sound bounces off krill swarms) we found that the distribution of krill in the region is more widespread than in others areas of East Antarctica. We also found that there may be a coastal krill population (as there is elsewhere) and an oceanic population. The distribution of these populations can be related to the location of oceanic boundaries (fronts) and to the underlying current systems. There seems to be larger amounts of krill in this region compared to the area to the east (CCAMLR sector 58.4.1 on map above), which was studied in the original 'BROKE' survey in 1996. However, the density of krill is not as high as that found in the South Atlantic, where larger populations of seals and seabirds are found and where the krill fishery has concentrated in recent years.

From the results of the acoustic survey we provided an estimated krill biomass figure of 28.75 million tonnes to CCAMLR in 2007. A precautionary catch limit of 2.645 million tonnes per year was subsequently set – an increase from 450 000 tonnes set in 1992 using data collected in the early 1980s. This increase reflects the improved equipment and methodologies available nowadays, rather than an increase in the size of the krill population.

The oceanographic component of BROKE-West had a number of aims. These included describing the three-dimensional ocean circulation, estimating the exchange of water between the Australian-Antarctic Basin and the Weddell-Enderby Land basin, and examining the formation of Antarctic Bottom Water. To do this we deployed 118 casts of the 'CTD' sampling package to measure, among other things, conductivity (related to salinity), temperature, depth, current speed, oxygen and carbon dioxide.

The work confirmed the presence of a significant source of Antarctic Bottom Water in the region between 60 and 70°E. These waters are so named because they fill the abyssal ocean around Antarctica and are the densest waters (because they are cold and salty) found on earth. They play the important role of carrying oxygen rich waters to the abyss and are drivers of the global deep ocean circulation. There are very few such sources, so the confirmation of a source in this area is highly significant.

Comparison of the BROKE-West oceanographic data with historical data showed that deep waters in the eastern sector of the survey area are becoming warmer and saltier. However, the western-most part of the survey area is unchanged. This finding is intriguingly different from results found elsewhere off East Antarctica and will require further analysis and interpretation.

The vast array of BROKE-West data also allowed us to examine the relationship between oceanography and ecosystem productivity in the region, and to investigate the storage of anthropogenic carbon dioxide and pathways of carbon dioxide uptake.

We found that the highest phytoplankton production in summer occurs along the coast. Phytoplankton blooms broadly followed the surface ocean currents and polar frontal zones – where cold, northward-flowing Antarctic waters meet and mix with the warmer subantarctic waters. There is much lower productivity in the more northern waters of the survey area, despite what appear to be favourable conditions – good light and nutrients. This is possibly because of the lack of iron in the water, coupled with heavy grazing by krill and other herbivorous animals.

We also found areas of ocean where deep water is being drawn towards the ocean surface by the winds and releasing carbon dioxide to the atmosphere. In contrast, near the coast, carbon dioxide is being absorbed by the ocean. This correlates well with phytoplankton growth and other biological production near the coast, which utilises carbon dioxide.

Image A shows the mean February sea surface temperature, sea ice concentration, large scale ocean circulation (solid arrows) and the Antarctic Circumpolar Current front (lower dotted line) and its southern boundary (upper dotted line). Image B shows the chlorophyll-a concentration in mg/m2 (related to phytoplankton productivity) from 0–150 m depth. Image C shows krill density from 0–250 m depth in g/m2. D shows the number of baleen whales per sighting (red squares) and number of seabirds per three-hour observation (blue circles).

The special volume of Deep Sea Research Part II: Topical Studies in Oceanography published in May (see related story) contains the first analysis of the data we collected on the distribution and abundance of all parts of the marine ecosystem – viruses, bacteria, phytoplankton, protozoa, zooplankton, krill, fish, squid, seabirds and whales. The next step will be to create a single dataset that combines the results from the geographically adjacent BROKE and BROKE-West surveys, to produce an overview of the ecosystems off the entire coastline of East Antarctica. There are few such comprehensive datasets for the Southern Ocean so these voyages will leave a legacy that will, hopefully, be the object of study for many years to come.

STEVE NICOL

Program Leader, Southern Ocean Ecosystems, Australian Antarctic Division