Reef surveys at Arlington Reef showing areas of bleached coral; image Neal Cantin, Copyright AIMS 2017.


IMOS data charts the course of the 2015-16 coral bleaching event on the Great Barrier Reef

The 2015-16 summer saw widespread coral bleaching along the Great Barrier Reef. Data from the IMOS Wireless Sensor Networks provided daily real-time measures of temperature, bleaching stress and risk which helped guide survey and monitoring work. The analysed data, recently published, documents the local and regional environmental factors that led to the bleaching event.

The Wireless Sensor Network Facility of IMOS is the world’s largest coral reef sensor network, with stations along the Great Barrier Reef (GBR) providing real-time information on reef conditions.

Following warmer-than-usual ocean temperatures in the northern hemisphere and predictions of an increased coral bleaching risk over the 2015-16 austral summer, data and web systems were developed from historical and real-time sensor network data to provide measures of daily water temperatures and associated bleaching risk.

The IMOS historical data was processed to give daily average temperature values for each site along with the statistical variability; these ‘climatologies’ provided a context to understand the daily temperatures as the summer progressed. The real-time temperatures were also compared to known empirical bleaching thresholds, temperatures above which field observations show bleaching can occur, allowing thermal stress and risk to be measured over the summer.

The on-reef temperature data for the summer indicated a number of unusual warm water events, with a prominent north-south gradient in warm water events and bleaching stress. On the southern reefs, water temperatures were within normal limits and were below the empirical bleaching threshold. Reefs in the central GBR closely followed historical bleaching thresholds, therefore predicting moderate bleaching. However, reefs to the north, such as Lizard Island, had four days over the bleaching threshold. Further north, in the Torres Straits, the Thursday Island station logged a new temperature record. Here, temperatures were above the empirical bleaching threshold for ten consecutive days.

The IMOS data successfully predicted bleaching severity along the GBR with field surveys confirming the north (extensive bleaching) to south (moderate to no bleaching) patterns. The IMOS data also revealed that warmer temperatures continued well past the summer - temperatures remained 1⁰C above normal from March through to June 2016. This late summer warming may have implications for the fate of corals that were exposed not only to record high temperatures, but to subsequent months of above-normal temperatures.

The IMOS data provided real-time indicators of thermal stress and bleaching risk to the research community, including the AIMS Bleaching Response Group. Atmospheric and ocean data are currently being analysed to understand the factors that caused localised heating and cooling, such as occurred at Thursday Island. Additionally, data from the IMOS sensor networks, moorings and ocean gliders are being compiled to deliver the most comprehensive record of the ocean conditions during the 2015-16 bleaching event. This information is critical in understanding the relationship between environmental forcing factors and the observed biological response. With a subsequent 2016-17 bleaching event currently underway the work demonstrates the value of IMOS in generating the long term data required to record, contextualise and understand events such as coral bleaching.

The full paper is available at:

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