12 July 2012 A growing threat

Extracting a coral core. Image: Eric Matson.

Coral bleaching is a serious threat to coral reefs across the globe. Scientists from the Australian Institute of Marine Science have used coral cores to track and better understand the responses of corals to documented bleaching events on the Great Barrier Reef (GBR).

Using cores extracted from massive, long-lived corals at four sites across the GBR, Dr Neal Cantin and Dr Janice Lough were able to measure changes in coral growth characteristics between annual growth bands. Growth bands are formed as the coral puts down its skeleton; a process known as calcification that is critical to the persistence of tropical reef ecosystems.

Dr Neal Cantin will be a speaker at the 12th International Coral Reef Symposium in Cairns where he will present data extracted from these coral cores. The data show that these corals exhibit a marked slowdown in growth after severe bleaching events, especially at sites that were subject to the highest levels of thermal stress.

"Coral cores provide an important window into the past," says Dr Cantin, "not only are we able to determine typical rates of growth for these corals, but by using these biological recorders, we gain insight into how and why growth slowed during times of stress.

"There are three clear growth responses to biological stress in corals. A decline in the extension rates of coral tissue, which itself causes a decline in the extension of the coral skeleton. This in turn leads to growth bands that are unusually dense, as the production of new skeleton occurs in the same physical space for a longer period of time than under normal conditions."

Through the study of these coral cores, the scientists have also been able to assess the recovery times of corals from severe bleaching events, as Dr Lough explains:

"Coral growth rates take approximately three years to recover to pre-bleaching levels. After the 1998 bleaching event, coral growth slowed by about twenty per cent. We only see a return to historical baseline growth rates by 2002."

These findings show that coral bleaching events not only have an initial impact on coral communities by killing off individual coral colonies, but also cause long-term reductions in growth even among the more resilient massive corals that were used in this study.

Dr Neal Cantin will present ‘Surviving coral bleaching events: Porites growth signatures' on Thursday 12th July, 1500, MR5 (Session 8E ‘Growth records in coral cores')

For more information:

Dr Neal Cantin, AIMS Research Scientist on an ARC Superscience Fellowship n.cantin@aims.gov.au

Wendy Ellery, AIMS media liaison, 0418 729 265, w.ellery@aims.gov.au