High survival following bleaching highlights the resilience of a highly disturbed region of the Great Barrier Reef

Background

As climate change leads to rising ocean temperatures, coral reef ecosystems are acutely at risk. Most corals harbor photosynthetic algae, which provide the majority of the energy for the coral, while the cnidarian coral polyps harvest additional nutrients from the water. However, marine heatwaves can cause coral to “bleach” – the algae start producing excess free radicals, and the coral ejects them, usually leaving the colony eerie white in the absence of the algae’s bright colors. Without algae, bleached coral is usually destined to starve.

However, even in badly bleached reefs, coral doesn’t always die. Polyps can sometimes scavenge enough food from the water to survive until the temperature drops, then re-recruit algae from their surroundings. In the same reef, some bleached coral dies and some survives, suggesting that local environmental factors could contribute to the extent of bleaching and recovery.

Here, the authors surveyed coral bleaching over the course of a 2020 marine heatwave in the Keppel Islands of the Great Barrier Reef to determine how local factors, particularly temperature and depth, influenced bleaching and recovery.

 

Key findings

The authors surveyed eight reef sites in the Keppel Islands during and after the marine heatwave, taking photographs to capture the extent of bleaching and death. Most coral at the sites bleached initially, but almost all survived. At all eight reef sites the researchers surveyed, at least 75% of coral colonies bleached; most sites had more than 90% bleaching. However, the 2020 heatwave was followed by a cooler-than-average winter, and by the next spring, almost all colonies had recovered. The authors speculate that the widespread recovery may have been aided by the unusually high tides and strong currents of the Keppel Islands region, which can dissipate harmful free radicals, bring more food particles to algae-less polyps, and kick up debris to shield bleached coral from sun stress.

Surprisingly, the initial extent of bleaching didn’t correlate with local heat exposure, despite many previous studies that did show such a correlation. However, the high level of bleaching at all reefs suggests that temperatures were high enough to essentially “max out” the extent of bleaching everywhere, despite local variation in just how hot the water was. However, being exposed to more intense heat, or for a longer period of time – as was common at inshore reefs – did slow coral recovery after bleaching. The researchers propose that accumulated heat stress over time will likely be a better predictor of coral health than acute heat stress, especially in a warming world in which acute heat stress often surpasses bleaching thresholds.

 

Why I liked this paper

This paper showcases the importance of thinking carefully about how variables are quantified. In this case, considering the duration of heat stress, and not just its acute intensity, allowed the researchers to discover a more nuanced relationship between temperature and coral bleaching. This paper also took great advantage of existing datasets on historical and modern ocean conditions, which allowed them to infer the extent of heat stress. Finally, I liked that this paper gave me a bit of hope. While the conditions at Keppel Islands are unusual, it was heartening to read that widespread coral recovery is sometimes possible.

 

Questions for the authors:

1. Do your results change how we approach coral conservation?
2. Since this is a region that has experienced high levels of ecological disturbance in the past, is it possible that more temperature-sensitive species are already dead? How do you think the reef ecosystem had already changed by the start of monitoring?
3. Your results rely heavily on the accuracy of the seawater temperature and current velocity model you chose. Are there multiple models to choose from? If so, why did you choose this one?

 

doi: https://doi.org/10.1242/prelights.31018

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