Chronicles of the deep

Aimée Komugabe. Photo by James Giggacher.

With the help of one of the planet’s oldest marine organisms, an ANU scientist is revealing the natural environment’s true history. By LUCY WEDLOCK.

She may have grown up in Africa’s land-locked Republic of Uganda, but Aimée Komugabe has always felt the inescapable pull of the ocean tide.

But it wasn’t until after finishing school, when Komugabe was living in another land-locked country, Austria, that the siren call of the deep blue sea became irresistible.

Komugabe applied for the Summer Scholar program at ANU and began a project with the Research School of Earth Sciences looking at black corals of the deep ocean.

"One week into the program, my supervisor realised that I had no idea if coral was a plant or an animal. He sent me away with a pile of books - and I was hooked. I fell in love with these carnivorous creatures that are popularly known as the ’rainforests of the sea’."

This new romance inspired her to move to Canberra to continue studying these tree-like corals. Three years on, Komugabe has reached the mid-point of her PhD on the deep sea corals of south Tasmania. These black corals are revealing how ocean currents have changed over the past 5,000 years.

"Climate change is inextricably linked with regional ocean circulation. For example, studies have shown that the East Australian Current (EAC) has intensified in the past half century due to a multitude of things, one of them being increased atmospheric carbon dioxide levels. That’s why we’re so interested in the changes in ocean currents and water masses."

The EAC has been extending further southwards. As it penetrates into east Tasmanian waters, it brings warm, low-nutrient waters into a region where cold, nutrient-rich Antarctic waters are typically present. This is causing a shift in the ecosystems along the east coast of Tasmania. Fewer local fish species can survive in the nutrient-poor waters, whilst tropical fish larvae are now being found further south.
 
"At present, all the records of ocean movements and temperatures are from instrumental records dating back only 60 years, not long enough to indicate whether these events have happened before, and how the natural system reacted.

"Corals are recording changes in ocean currents as they grow. So they can give us an insight into movements over the last few millennia.

"Shallow water corals, such as those in the Great Barrier Reef, have skeletons which are sourced from the water they grow in. Deep sea black corals, however, form a skeleton of chitin and protein, similar to the exoskeleton of an insect. The traces seen in these corals reflect what it has been eating, instead of being a reflection of the ambient water that surrounds it.”

Black corals feed on marine ’snow’ - organic matter that rains down from the ocean surface, such as dead plankton and faecal pellets. Their diet is a direct reflection of surface processes, even though the corals are up to 3,000 metres deep.

"What makes these corals even more fascinating is that their skeleton is made of rings quite similar to the trunks of a tree. Reading the corals is like reading the rings on a tree. Their slow growth rate and longevity makes it possible to determine variations in the local environmental conditions - for example, water mass movements - over thousands of years."

Water mass movements tell us about changes in ocean circulation in a particular region and can be measured using radiocarbon dating techniques, Komugabe says. The radiocarbon age of the coral gives the true age of the coral plus the age of the water above it.

"By finding the age of the overlaying surface water, we can map the ocean currents in that location over centuries. If an increase in age is detected, there must have been movement of older, deeper waters."

It’s a case of looking into the past in order to predict the future. Reading the corals will reveal whether present changes are a case of history repeating itself.