Coral bleaching on Ningaloo Reef

Posted on 01 March 2011

  • Ningaloo Atlas
  • Australian Institute of Marine Science

Coral bleaching has been reported on Ningaloo Reef - a reef system that has not experienced widespread bleaching to date.  Coral bleaching likelihood is largely determined by sea temperatures, and during the 2010/2011 summer, sea surface temperatures across Ningaloo were anomalously warm. 

Coral bleaching events are usually caused by long periods (usually 4 to 8 weeks) of warmer than average summer sea surface temperatures (SSTs), and SST estimates from the National Oceanographic and Atmospheric Administration (NOAA) Coral Reef Watch Program show that 2010/2011 summer SSTs around Ningaloo have been about 1°C to 3°C warmer than the long-term averages for the region.  As a result, Ningaloo Reef was on Bleaching Watch for much of the summer and reached 4 Degree Heating Weeks (DHW)1 in mid-January 2011 (Figure 1A).  In situ temperature loggers (~ 6 m depth) at Bundegi in the Exmouth Gulf and 14-Mile Beach on Ningaloo (Figure 1B) confirmed that actual water temperatures had been above seasonal averages since mid October 2010. 

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Figure 1. Temperature data for Ningaloo during 2010/2011 summer.  A. NOAA Coral Reef Watch forecasts for Ningaloo.  B. In situ temperature logger data for Bundegi and 14-Mile Beach.  The black dotted line is the NOAA seasonal averages from Fig. 1A overlaid on the temperature logger data.

Reef surveys by the Australian Institute of Marine Science (AIMS) and the Western Australia Department of Environment and Conservation (DEC) confirmed coral bleaching (Figure 2) at multiple sites during February 2011.  Initial shallow water surveys suggest low level bleaching (< 5 % of total coral cover) is wide spread, but areas such as Bundegi and Coral Bay have higher levels of bleaching of up to 80 % of total coral cover.
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Figure 2.  Coral bleaching at Coral Bay (10 February 2011).  A. Acropora and FaviaB. SeriatoporaC. Echinopora.

As of 24 February 2011, Ningaloo Reef was at 6 DHW and the Predictive Ocean Atmosphere Model for Australia (POAMA) forecast anomalous SSTs to continue through March 2011.  The fact that the temperatures are forecast to remain above seasonal averages through till the end of the 2010/2011 summer is of concern because the risk of coral mortality is increased if the thermal stress persists once the corals have bleached (Figure 3).  As such, AIMS and DEC will continue to monitor the sea surface temperatures and reef health at Ningaloo over the coming months.  Depending on the magnitude of the 2010/2011 bleaching event, the need for follow up post-bleaching surveys to estimate mortality rates will be assessed. 
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Figure 3.  Bleaching does not necessarily lead to corals dying (mortality).  Bleached corals can recover and survive if water temperatures return to normal.  Mortality normally occurs if the temperatures remain warm for an extended time period after bleaching occurs.

The bleaching reports on Ningaloo Reef follow on from multiple observations of mass bleaching from Southeast Asia, Sri Lanka and the Maldives in the central Indian Ocean between May and July 2010 - where sea temperatures were up to 4°C above summer seasonal averages. 
1 Degree Heating Weeks (DHW) is an accumulation of any temperature HotSpots greater than 1°C over a 12-week window, which shows how stressful conditions have been for corals in the last three months. DHW is therefore a cumulative measurement of the intensity and duration of thermal stress, and is expressed in the unit °C-weeks. Based on previous bleaching observations worldwide, evidence suggest that 4°C-weeks (4 DHW) results in bleaching and that 8°C-weeks (8 DHW) and above result in widespread bleaching. For example, Scott Reef in West Australia and the Maldives in the central Indian Ocean suffered significant bleaching mortality in 1998, with 13 DHW and 10 DHW respectively.  For more detailed information see Coral Reef Watch.
Acknowledgements: Thanks to Frazer McGregor (Murdoch University), Kim Friedman, Heather Taylor, Tom Holmes, Shaun Wilson (DEC), and Martial Depczynski, Ben Radford, Tyrone Ridgway, Paul Tinkler (AIMS) for data and information.