Benthic trawling for invertebrates in the Southern Ocean

Text: the trawling team

Photo: Noé Sardet

The trawling teams on the ACE cruise are interested in collecting marine invertebrates from the seafloor, at depths of 200 to 500 metres, for a variety of different projects and objectives. We use a specially designed Agassiz trawl to sample the animals from the sea floor, which in Antarctica is largely dominated by brittle stars, sea spiders, soft corals and a variety of worms. Some trawls are muddy while others are rocky and this has a large impact on the type of animals hauled up in the trawl.

An unsolved mystery

One objective of the trawling team is to explore the richness and distribution of diversity in Antarctica and the sub-Antarctic islands, which could lead to clues as to where regions of “refugia” existed– places where animals persisted during glacial periods. Most of the continental shelf regions of Antarctica and some of the surrounding islands were covered by grounded glacial ice during these cold periods, the glaciers grinding clean the sea floor and wiping out most of the animals from these regions. It was only after glaciation that these areas were recolonized, but where they were colonised from remains a mystery.

Collecting and sorting benthic animals.
Brittle stars are amongst the most abundant animals collected in the trawls.

We will analyse genetic data from the animals we collect in Antarctica and the subantarctic islands to determine how these populations are related to one another and find clues as to where they were recolonized from after the ice retreated. Chemical analysis of some of these organisms and their associated microbial communities will further help to determine population differences and similarities in these regions.

Many of the creatures that live on the sea floor, such as the clams, corals, sea stars and brittle stars, have hard skeletons made from calcium carbonate. The “carbon” that makes up the carbonate originates from the air, is taken in by the small plants, or algae called phytoplankton, in the sea, and are eventually eaten by these sea floor animals. It appears that in some regions of the Antarctic these animals have been growing much faster over the past few decades, probably due to climate warming reducing the amount of annual sea ice, resulting in more ice free days in each year.

A variety of sea creatures collected in the trawls and photographed in the lab.

“Blue carbon”

More ice free days means more days the phytoplankton can absorb carbon from the atmosphere and more days they can feed the animals on the seafloor. When these animals eventually die, they are buried in the mud and the carbon is captured and held for a long time (may be thousands of years), commonly termed ‘blue carbon’. The researchers on board are interested in seeing whether this carbon accumulation is happening in other regions of the Antarctic and sub-Antarctic. We will identify the different animal groups we collect and see which ones contribute to the carbon capture and sequestration in the different regions we visit.