This past month brought a lot of bad news with a slight silver lining to the corals in the Great Barrier Reef. Unfortunately, the bleaching we were starting to see at the time of my last entry only continued, worsened, and even lead to mortality in some cases. National Geographic reported 75% of the northern GBR reefs being affected by the current mass global bleaching event [1].
Image of bleached corals off of Lizard Island in the GBR [6].
Australia’s National Coral Bleaching Taskforce reported in a survey of 520 individual reefs in the area between Cairns and Papua New Guinea that only four reefs showed no signs of bleaching [2]. Coral bleaching is not immediately a death sentence. If conditions improve (i.e. water temperatures cool down) then the symbionts that had been expelled from the coral tissue during the bleaching event can be replaced by new symbionts, and the coral can regain a healthy balance with these microorganisms and live. The truly devastating news from the bleaching in the northern GBR is that in a follow up survey to the one mentioned above, scientists reported approximately 50% of the surveyed bleached reefs exhibiting mortality [3].
Photos from aerial survey showing coral bleaching in the northern GBR [7].
While so much damage has already been done to the once pristine North GBR reefs, it looks like the bleaching event is finally over in the area. NOAA satellite predictions indicate that the area is currently at a watch level, whereas last month we saw many areas in alert level 1 and some in alert level 2. Hopefully these projections will ring true, and one week out from now the area will revert to a no stress level.
Maps depicting alert level for coral bleaching in the Northern GBR. The area remains at watch level during this week but is predicted to switch to no stress levels soon and remain there [8].
A recent paper published in
Science on April 15
th showed how previous bleaching events in the GBR fostered a thermal tolerance mechanism in reef-building corals [4]. The process they describe is a form of acclimatization in corals, in which “sub-lethal pre-stress events reset physiological and molecular mechanisms that underpin the innate stress response, and provide a means to survive subsequent stress events” [4]. Meaning that if a coral is exposed to high temperatures and survives, the next time it encounters high temperatures it is more likely to survive again having built up a mechanism to counteract the high temperatures during the first exposure. At least that’s the idea. The authors go on to prove that this mechanism is present in the GBR and has played a role during past bleaching events. They found that corals and their symbiodinium that had been exposed to sub-lethal, pre-stress temperatures (which have been prevalent in the GBR over the past 27 years) developed thermal tolerance. The corals that experienced single and repetitive bleaching temperatures, however, did not acquire thermal tolerance and suffered loss of symbionts and tissue.
Coral cell death (bars) and Symbiodinium density (lines) at bleaching for projective, single, and repetitive bleaching [4].
The concern of sea surface temperatures rising above the “sub-lethal, pre-stress” levels have come true during this last bleaching event, and thus the predictions made in the paper have come true, evidenced by the mass bleaching and mortality reported previously. Projections regarding total percent coral cover on corals associated with protective, single, and repetitive bleaching are also included in the paper.
Total Coral Cover projections for the next century under protective, single, and repeated bleaching trajectories [4].
These graphs clearly show a negative correlation between amount of bleaching and total coral cover. Corals subject to single and repetitive bleaching are projected to have less than 10% total coral cover by just 2030. While these projections are limited in they look at only corals from the GBR, it is likely that the trend can be extrapolated to other reefs. Additionally, the GBR is the second largest home of coral reefs in the world, so losing them would have a dramatic effect on the global coral population [5].
The silver lining to this situation is that at least the GBR has received notable amounts of global media coverage in the past few weeks. The New York Times ran a cover story on coral bleaching and discussed the GBR [3]. That being said, as you can see from the many other entries on this blog, this bleaching event has reached many other coral reefs all over the world that have not received the same amount of media attention. But at least the world seems to be starting to pay attention. In order to avoid the forecasts of minimal coral cover, however, more needs to be done, and quickly. This next decade will prove critical in determining the future of coral reef health. If we want to save this ecosystem, then we must act now.
Photo from aerial survey of northern GBR [7]. The GBR is the second largest coral reef area in the globe, and the northern section was the most pristine part of the reef.
References
1: Greshko, Michael. National Geographic. Web. 14 April 2016. http://news.nationalgeographic.com/2016/04/160414-great-barrier-reef-coral-bleaching-australia-climate-change-science/
2: Arc Centre of Excellence for Coral Reef Studies. Web. 2016. http://www.coralcoe.org.au/media-releases/coral-bleaching-taskforce-documents-most-severe-bleaching-on-record
3: Innis, Michelle.”Climate-Related Death of Coral Around World Alarms Scientists.” The New York Times. 9 April 2016. Web.
4: Ainsworth, T. D. et al. “Climate change disables coral bleaching protection on the Great Barrier Reef.” Science. 15 April 2016. Web.
5: Sheppard, C. R. C. et al. “The Biology of Coral Reefs.” Oxford University Press. 2009.
6: XL Caitlin Seaview Survey. Photograph. National Geographic. 14 April 2016. http://news.nationalgeographic.com/2016/04/160414-great-barrier-reef-coral-bleaching-australia-climate-change-science/#/01greatbarrierreefbleach.ngsversion.1460667600268.jpg
7: Terry Hughes. Photographs. ARC Centre of Excellence for Coral Reef Studies. 2016. https://www.dropbox.com/sh/yj8tt2mj5u0inmb/AAAW6D3V0eo7YVFkqAdLzhAMa/Images?dl=0
8: Far Northern GBR Satellite Coral Bleaching Alert Area and Outlook. Image. NOAA Satellite and Information Service Coral Reef Watch. 17 April 2016. Web. http://coralreefwatch.noaa.gov/vs/gauges/gbr_far_northern.php
