By Danielle Moloney
Edited by Sara Gagliardi
A bright spot in the midst of a lot of negative news lately: a new study shows promising signs that some corals are adapting quickly enough to keep pace with climate change. Scientists from Woods Hole Oceanographic Institution (WHOI) used a series of marine heat waves in the Phoenix Islands Protected Area (PIPA) between 2002 and 2016 as a case study by which reef resilience could be measured. PIPA is located in the Republic of Kiribati, approximately halfway between Australia and Hawaii. The group saw the impact on the reef lessen with each successive heat wave, indicating that the corals were able to adapt to some amount of heat stress over time.
Where it began As climate change continues to devastate reef communities around the globe, the pressure to understand how some corals may be able to survive this persistent threat increases. Marine heat waves have also increased in frequency in recent years, cementing the drive to solve what may make corals resilient in the face of this pressure. This is where PIPA comes in- it made a perfect study site for the WHOI researchers because it is protected from other local stressors (such as overfishing, pollution, etc; this allowed the researchers to ensure that the effects they observed were in fact due to climate change resilience rather than external factors). The site also experienced a high number of marine heat waves in the recent past (three waves in the approximately 15 years over which the study’s analysis was conducted)- one from 2002-2003, another in 2009-2010, and finally a massive heat wave in 2015-2016, which put twice the amount of stress on the reef compared to the prior two heat waves. So, what did they find?
Reefs in PIPA are keeping up- for now
The researchers used data on coral cover before and after the heat waves to determine how reefs fared after each successive heat stress event. The first heat wave in 2002-2003 wreaked havoc on the reef; the second in 2009-2010 showed comparably little devastation; the third, which was twice as strong, exhibited significantly less die-off than anticipated. Why might this be? The authors hypothesize that coral death in the first heat wave killed off less thermally tolerant corals, leaving the more resilient species to continue to grow and reproduce. By the time the second wave hit, these more tolerant corals had time to recover from the 2002-2003 wave, allowing them to withstand the heat stress with little die-off. This trend may have continued over the years until 2015, when the reef withstood an even stronger heat wave. The scientists suspect that although coral larvae can travel long distances on ocean currents, due to the remote location of PIPA reefs, it is likely that the successful new growth that followed heat waves was due to within-area repopulation. However, this success story is not the case for reefs in other locations, where often heat waves precede bleaching events and large-scale deaths from which only the hardiest corals recover.
These findings emphasize the importance of understanding how thermally tolerant corals manage to survive heat stress- some scientists believe that maintaining heat-resistant corals is the key to a successful future for reefs. With this being said, the authors of the study still highlight that even if some reefs will survive heat stress, it does not mean that climate change is any less of an urgent stressor that must be addressed. Given current climate projections, even thermally tolerant corals are at risk of death due to elevated temperatures.
This study represents an important step towards improved understanding of which corals are most likely to survive heat stress. This information can then translate to better informed conservation and management strategies. The next step is to figure out how corals are withstanding the heat stress. Overall, the findings illustrated here are a bright spot for the future of coral reefs.
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Cover Photo courtesy of National Geographic.