Broadscale effects of bleaching
on the Great Barrier Reef  (2001-2003)

The AIMS Long-term Monitoring Program surveys sites on NE flanks of reefs annually. Reefs are surveyed at about the same time each year. Because of the area involved, the surveys span a period between October and May. Bleaching occurred in 2002 midway through the field sampling season.

Figure 2. Proportion of inshore (top) and offshore (bottom) reefs surveyed that were unaffected (<1% of reef affected) by bleaching versus reefs with moderate (1-30% affected) to high (>30% affected) bleaching in 1998 and 2002 click here to visit www.gbrmpa.gov.au and read the full report comparing 1998 and 2002 bleaching events.

As a simple measure of the broadscale effects of the 2002 bleaching event, the total living hard coral (from video transects (see Methods)) at the 2001 surveys [well before bleaching] were compared with equivalent data from the 2003 surveys [well after bleaching]. Data were available for 49 reefs spread across the GBR. The figure shows the average percent cover values on sites at each survey reef, arranged by latitude and position across the GBR lagoon [inshore to offshore]. For each reef, the darker blue bar represents the mean hard coral cover at the 2001 survey. The paler blue bar gives the same information for the 2003 surveys. Survey methods are described in a number of documents available on the AIMS web-page; it is important to note in this context that the study sites are generally at 6-9 m depth.

Figure 3. Change in hard coral cover during the period 2001-2003 on LTMP survey reefs. Click on the map to enlarge.

Taking an arbitrary figure of 5% change from pre-existing coral cover as the criterion for substantial change:

• Survey sites on 6 of the reefs lost more than 5% of the initial average coral cover over the two years. Of these 6 reefs 4 had infestations of Crown of Thorns starfish.
• Survey sites on 7 reefs gained more than 5% of the initial coral cover in that period.
• Sites on 34 reefs showed little average change (-4.9% to 4.9% of initial cover) over the two years.

LTMP surveys show that at depths of 6-9 meters there was very limited impact on hard coral cover from bleaching in 2002. Myrmidon Reef (declined from 35-24%) off Townsville and Reef 19131 (declined from 40-33%) in the Whitsundays were the only reefs where bleaching had a substantial effect on hard coral cover.  These results confirm that bleaching does not always result in coral mortality. The extent of mortality from bleaching is unpredictable in that there are many localised factors that affect recovery. In general, as LTMP sites are located on the NE flank water circulation would be expected to be high. Recovery from bleaching would be enhanced in areas of higher water flow and thus LTMP results may not be representative of other reef zones. To date it would appear that mortality from bleaching has been mostly confined to shallow water, or has been very localised spatially.

Bleaching has additional implications for reef growth apart from coral mortality. Sub lethal effects such as reduced reproductive ability, reduced calcification rate and increased susceptibility to stress have been documented by numerous studies (eg see Hoegh-Guldberg (2004) review). During the period of 1995-2003 there has been an increase in the number of LTMP survey reefs experiencing little change in percent cover over 2 year time intervals. This data reflects the combined effects of all disturbances during the survey period and represents a very short time frame in the context of reef growth.

Figure 4. Magnitude of change on LTMP survey reefs from 1995-2003.

December 18, 2008