Map of the wireless sensor network at Heron Island. Click to enlarge.


2014-15: A warm summer with more to come.

Under conditions of high temperatures and light corals become stressed to the point where the symbiotic relationship with single cell algae (zooxanthellae) that live within the corals tissues breaks down and the algae are expelled.

This makes the corals look white as much of the colour of the coral comes from the algae, it also means that the coral is no longer getting food from the algae and if this relationship is not re-established the coral will die.

Coral bleaching and subsequent death due to exposure to warm water is a major concern as previous warm events have resulted in some areas losing much of their coral cover. As the climate swings from La-Nina to El-Nino conditions (the Bureau of Meteorology confirms that the Pacific is in the early stages of an El-Nino pattern) we can expect more warm summers to come.

Wireless Sensor Networks located on five reefs along the Great Barrier Reef give real time information about environmental conditions at the level of coral bommies and so can give information about the current conditions as they relate to empirically established coral bleaching temperature thresholds. This allows managers to track temperatures against known thresholds and to get day by day reports of the health of the corals at the sensor network sites.

The data from Davies Reef shows that in early 2015 temperatures were just at the known thermal thresholds for that reef, although only for a few days. What was just as interesting was that temperatures later into the year were abnormally high being above the known historical limits as measured from the seven years of IMOS data from this site. This showed not only a hot summer but a late one with a series of above normal temperatures going into April. This shows a summer that is not only hotter at its peak but one that has more warm days later in the summer with temperatures in April above the known historical values.

The data from Heron Island reef shows a similar pattern with temperatures nudging the empirical coral bleaching temperature threshold in late January but then a period of late warm conditions in late February and early March that again pushed temperatures close to the bleaching threshold.

The data for the 2014-15 summer from the Wireless Sensor Network Facility of IMOS shows that the summer was warm to the point where a number of reefs experienced short term temperatures close to the empirical bleaching temperature thresholds. What the data also showed was a period of warm temperatures late into the summer (through to the end of April) and so there was a delay or lengthening of the summer warm period that may also have implications to the summers that lie ahead if warm conditions persist under an El Nino ocean pattern.

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Graph showing daily water temperatures (red line) for Davies Reef against the long term mean (dashed green line), historical upper and lower limits (purple and blue lines) and the known bleaching limits (dashed black line). Click to enlarge.