Research Voyages

Sifting through the sediments

Dr Tom Okey has an impressive collection of netting and bucketware prepared for sifting the sediments.

The box corer.

Tom Okey enters the realm of the mini maintainers

Dr Tom Okey has a roll of fine netting next to his desk. He'll be using it onboard the Southern Surveyor to enter the miniature world of benthic macrofauna: tiny creatures that live in the top 1–10 centimetres of sediment on soft-bottomed seabeds.

"Benthic macrofauna inhabit the oxygenated layer, the living part of the sediments where it all happens," Dr Okey says.

"At each sampling station we'll lower the box corer and scoop up a perfect cube of sediment. This will be sieved through a screen with 0.5 mm openings, and my studies will focus on the all the creatures left on that screen: small invertebrate animals such as polycheate worms, amphipods crustaceans, and small bivalves and snails.

"By identifying, counting, and weighing the different species, we can estimate the abundance, biomass, and diversity in the sediments across broader areas of the seafloor.

The sampling will be repeated in a range of habitats subject to different trawling intensity as part of the search for clues to the effects of trawling on seabed habitats and communities.

"People often picture aquatic food webs in terms of fish eating copepods (planktonic, shrimp-like crustaceans), but in shallow systems such as the Gulf of Carpentaria, much of the food web is supported by primary and secondary production that occurs on the seafloor," Dr Okey says.

"Many fish and invertebrates such as prawns feed on bottom in this shallow system, so much of their diet consists of these smaller animals, which are produced in high rates on the seafloor. In a pristine state, such systems have spatially complex habitats with diverse architectures of attached invertebrates and algae, which in turn maintains a diverse community of mobile fish and invertebrates."

Dr Okey says little scientific information has been gathered on the benthic macrofauna (these smaller, soft-bottom invertebrates) that inhabit Australia's continental shelves.

"There is no public concern for polycheates and amphipods but they are incredibly useful tools for studying ecosystem function and the effects of disturbance, competition, and predation," he says.

"For example, seafloor trawling might cause a shift in the relative abundance of different species groups, with a few disturbance-adapted worms, for example, dominating in a similar way to a wooded landscape shifting to grassland under an intensified fire regime. If we do see such differences, it will indicate that trawling has an ecological effect. It will be more difficult to find areas that have pristine assemblages of habitat-forming invertebrates."

Patterns in the distribution of seabed macrofauna will be studied together with information on the geology of the seabed to identify any relationships between geological and biological features in the region.

"If you can ground-truth enough of an area you can try to use geological features to predict the distributions of biotic assemblages," Dr Okey says. "This kind of extrapolation might be useful in marine planning and conservation through identifying protected areas, but the relationships are never simple because the abundances and distributions of marine organisms are shaped by a variety of factors, not just geomorphology."

Dr Okey came to CSIRO last year from the University of British Columbia. His research has taken him from studying seabeds of the Monterey Submarine Canyon, to Alaskan fjords, Indonesian coral reefs, and Galapagos rocky reefs.

He has been on many scientific research expeditions throughout the Pacific and has worked as a commercial fisherman on a salmon seiner out of Kodiak, Alaska, and as a seaman "chasing rust around the boat" on the RV Melville, the largest vessel maintained by SCRIPPS Institution of Oceanography (University of California).

No doubt it won't take him long to regain his sea legs, netting in hand, on the RV Southern Surveyor.