What is it about the Keppel Islands? Even after a marine heat wave in February 2020 reaching temperatures of 32 °C and lasting weeks and affecting 90% of the branching corals in the Keppels, they recovered within a matter of months 1.
Cumulative heat stress, which scientists call ‘degree heating weeks’, refers to the length of time that corals are bathed in water considered too hot for them to cope 2.
During the 2020 summer the heat stress was highest inshore of the northern islands in the Keppel Island group (North Keppel, Conical, Corroboree, Pumpkin and Sloping Islands and Square Rocks) as well as inshore of Miall Island, a little further to the south.
There was less bleaching at the offshore sites: Barren Island, The Child, Bald and Outer Rocks.
Six months after the heat wave, the amount of coral cover on the reefs remained unchanged even though corals that were exposed to the highest temperatures even briefly, or to bleaching temperatures for longer (mainly those in the shallows), did take longer to recover.
This phenomenal resilience in the Keppels is in direct contrast to other reef systems on the Great Barrier Reef exposed to the same or even in some cases, lower heat stress 3 and is thought to be partly due to the high tidal range 4 and at times, muddy nature of Keppel Bay. The particles in the waters of the bay shield the corals from sunlight 5 which can otherwise tip them over the edge and cause them to expel their minute single cell plant partners, making them appear white – but definitely not dead. The particles also provide food for the now naked corals and allow them to survive until their plant partners return to make food for them from photosynthesis once again 6.
Another phenomenon, the ability of corals to regrow over their own dead skeleton is a factor in such remarkably fast recovery 7.
These amazingly persistent branching corals are the most dominant in the Keppel Islands and it is their resilience in the face of disturbance like the 2020 heat wave that makes the Keppel Islands group so very special on the Great Barrier Reef.
This spectacular aerial image of bleached branching corals at Big Peninsular was taken by Max Allen Jnr from Freedom Fastcats.
Read more about this extraordinary phenomenon at www.keppels.com.au/
1. Page, C.A., C. Giuliano, L.K. Bay, and C.J. Randall, High survival following bleaching highlights the resilience of a highly disturbed region of the Great Barrier Reef. bioRxiv, 2021: p. 2021.10.18.464880.
2. Berkelmans, R., Time-integrated thermal bleaching thresholds of reefs and their variation on the Great Barrier Reef. Marine Ecology Progress Series, 2002. 229: p. 73-82.
3. Skirving, W., Status of Bleaching Heat Stress on the Great Barrier Reef, Australia – 2020.
4. Kleypas, J.A. and D. Hopley. Reef development across a broad continental shelf, southern Great Barrier Reef, Australia. Proceedings of the 7th International Coral Reef Symposium in International Coral Reef Symposium. 1992. Mangilao, Guam: International Coral Reef Society.
5. Cacciapaglia, C. and R. van Woesik, Climate-change refugia: shading reef corals by turbidity. Global Change Biology, 2016. 22 (3): p. 1145-1154.
6. Grottoli, A.G., L.J. Rodrigues, and J.E. Palardy, Heterotrophic plasticity and resilience in bleached corals. Nature, 2006. 440 (27): p. 1186-1189.
7. Diaz-Pulido, G., L.J. McCook, S. Dove, R. Berkelmans, G. Roff, D.I. Kline, S. Weeks, R.D. Evans, D.H. Williamson, and O. Hoegh-Guldberg, Doom and boom on a resilient reef: Climate change, algal overgrowth and coral recovery.PLoS ONE, 2009. 4 (4): p. e5239.