Reports

Great Barrier Reef phase shift: Gardner Bank to Gardner Reef

Authors: Beaman, R. J.

Year: 2012

Publication: Voyage Summary SST02/2012, CSIRO Marine National Facility, Hobart, Australia, pp. 26.

Abstract

On a global scale, coral reefs are experiencing a period of rapid change. The world has effectively lost 19% of the original area of coral reefs since 1950, with the loss predicted of 35% of coral reefs in the next 40 years (Wilkinson, 2008). About 46% of coral reefs are regarded as healthy except for currently unpredictable global climate threats, which includes the Great Barrier Reef (GBR).

The vulnerability of coral reef habitats to climate change is high as scleractinian corals are highly sensitive to increasing sea temperature and ocean acidification (Marshall and Johnson, 2007). The increased frequency of coral bleaching due to further increases in sea surface temperature (SST) will cause a decline in coral cover, increases in algal dominance, and shifts towards species that are more thermally tolerant.

Exceptions to this pattern may occur at the southern limits of the GBR where tropical carbonates transition into temperate carbonates. The observed shift of average marine climate zones south by >200 km since 1950 (Lough, 2008) could potentially result in the GBR extending south, causing an algal to coral phase shift as coral settlement follows the changing environmental gradient.

The shelf adjacent to Fraser Island is located at the transition between tropical carbonates (GBR) to the north and cool-water carbonates to the south (Schröder- Adams et al., 2008), and represents an ideal natural laboratory to investigate this phase shift.

On the outer-shelf, Gardner Bank lies in depths of 20 to 60 m and comprises a hard substrate of red algal nodules and pebble- to cobble-sized rhodoliths (Lund et al., 2000), under the influence of the East Australian Current (Harris et al., 1996). Living platey and small massive corals grow in competition with both red algae and extensive areas of fleshy algae (Davies and Peerdeman, 1998; Marshall et al., 1998). The projected warming trend in SST would anticipate a phase shift from a red algal-dominated substrate to a coral-dominated substrate.

A hypothesis is that Gardner Bank would form an antecedent surface for future coral reef development through net coral framework accretion. In effect, Gardner Bank would become Gardner Reef, therefore it is crucial that the extent, structure, and morphology of this potentially important site be investigated.

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