High coral cover doesn’t tell the whole story.
New paper:
Dangerous demographics in post-bleach corals reveal boom-bust versus protracted declines.
by Juliano Morais, Renato Morais, Sterling Tebbett, Morgan Pratchett and David Bellwood,
published now in Scientific Reports
This reef looks pretty with its high coral cover! But is this what a “recovered”, healthy reef looks like?
photo: RP Streit
Coral reefs are beautiful.
But especially in recent years many people have mostly
heard about one thing that coral reefs seem to be doing: dying.
But what’s really going on with corals on reefs?
In particular since 2016 “coral bleaching” has become a quite well-known term outside of science. It’s even been the protagonist of an award-winning Netflix documentary. Coral bleaching happens when corals are exposed to water temperatures that are too high for too long. If corals remain bleached for too long, they die.
Coral bleaching itself is a natural process. But because of human-made climate change, ocean temperatures are reaching these critical highs more often and across larger areas of the world.
The early months of 2016 were particularly bad on the Australian Great Barrier Reef, where we work. Within a few weeks our main study site, Lizard Island, looked very very different. The reefs around the island had already been stripped of some of their coral because tropical cyclones had hit the reefs in 2014 and 2015. But in 2016 things really started to change. All types of coral were turning white, some soft coral even looked like they were literally melting:
Massive (back), soft (middle) and branching corals (front) bleached because of high water temperatures at Lizard Island in March 2016. photo: RP Streit
Many corals really were not a very pretty sight after the bleaching event. Many had become overgrown with brown fuzzy algae (like on the right image below). A lot of corals died, especially branching corals, like from the genus Acropora.
Not feeling the Love: especially branching corals, like this Acropora, had a tough time in early 2016 on the Great Barrier Reef.
photo: RP Streit
This bleaching event and coral mortality did not only happen on Lizard Island but throughout Indo-Pacific coral reefs. And the bleaching did not only happen in 2016, but also in 2017, 2020 and early this year in 2021.
Since corals are at risk from high water temperatures and bleaching,
one has to ask the question:
Do they have a chance to survive into the future,
if bleaching events now happen so regularly?
One way to track how the corals are doing, is to measure “coral cover” through time. That simply means counting how much area of reef is covered by living corals. Coral cover measurements can be used to measure whether corals can “recover” in-between bleaching events.
In February this year, 2021, we went back to Lizard Island and got quite a surprise! What we saw at one of our well-known dive sites was this:
We clearly got quite excited! Some of us had only ever known Lizard Island reefs after the 2016 bleaching event, having very little coral, but being dominated by algae.
This really was the best coral many of us had ever seen on Lizard Island reefs!
But is it really good news, or just pretty?
While it is nice to see such coral cover, we did not want to get carried away in our excitement. Does this mean the reefs have “recovered”? Do we need to worry less about coral bleaching, if corals can grow so well, even after 4 bleaching events in 5 years?
Now we have some first answers, thanks to the brand-new paper lead by our PhD student Juliano Morais that was just published in Scientific Reports.
Together with Renato Morais, Sterling Tebbett, Morgan Pratchett and David Bellwood, Juliano used a dataset of repeated photos of 362 one-squaremeter “quadrats” across Lizard Island. We took the first set of photos just before the bleaching event in 2016. Since then, we have returned to Lizard 5 times to re-take these photos of the exact same locations on the reef.
With these pictures, Juliano was able to track the fate of individual corals, through three bleaching events and across 5 years. By focusing on individuals, it is possible to collect “demographic data” (growth, mortality, recruitment).
This makes it possible to tease apart which corals may be more vulnerable, better at growing, or better at reproducing - compared to measuring coral cover.
Coral cover gives a current snap shot, but does not give information on how we got here. Who died? Who survived? Is it mostly large old survivors? Or a new generation of many tiny new recruits?
Figure 1 from the study:
“Timeline of the study with data collection instances (camera icons) and bleaching events (temperature gauges) with examples of quadrats (1 m^2) of the same reef section across repeated sampling periods showing the growth of new Acropora colonies. January 2018, 24 months after first sampling, January 2020, 48 months after first sampling, and February 2021, 60 months after first sampling. All photographs taken at Lizard Island by SB Tebbett.”
Using this technique of analyzing repeated photographs, Juliano and his co-authors followed 1,069 individual coral colonies for up to 5 years. In their study they focused on two dominant types of coral: branching Acropora corals and massive Porites corals.
Acropora coral (red in the figure below) disappeared completely from the study sites after 2016. Already they only covered about 1% of reef and following the bleaching event, all these remaining corals died. However, they did come back!
A new generation of Acropora corals settled on the reefs and within a few years Juliano observed a 1000% increase in the number of corals. These 897 new colonies grew very quickly, on average by 201% in size per year!
Figure 2 from the study:
(a) Coral cover of Acropora and massive Porites based on 362 quadrats over the 60 month time period. (b) Total number of Acropora and massive Porites coral colonies tracked over the 60 month time period spanning three bleaching events at Lizard Island, northern Great Barrier Reef. Photographs: Victor Huertas.
Massive Porites corals (blue in the above figure) on the other hand, showed very different patterns. Almost none (only two!) of the existing colonies died. And most of the remaining colonies were growing!
However, while Porites were seemingly better at surviving the bleaching, Juliano found not a single new Porites colony across 5 years. A new generation did not appear!
So, while Porites seem to be more stable in surviving coral bleaching events, they are not very good at being replenished. And in fact, if they do bleach, they tend to shrink by losing live tissue and may ultimately die.
Juliano found that the proportion of colony area that was bleached on Porites corals was very good at predicting, whether a Porites coral would subsequently grow, shrink or die (see the colour of the lines in the figure below, the yellower a line, the more bleached the coral had been).
Figure 4 from the study:
(a) Relative area of live tissue on massive Porites colonies over 60 months. Each line represents a single colony, with line colors representing the proportion of bleaching in each colony (during the 2016 bleaching event). The red dotted line represents the average increase of 21% in colony area of massive Porites. (b) Effect of the proportional bleached area (in April-2016) on the subsequent relative change in live tissue area of massive Porites. Line and band show the prediction and 95% confidence intervals of a Gamma GLMM, while dots show raw data points. Modelling was performed in the software R34, using the glmmTMB package35. The solid horizontal line and arrows indicate where colonies effectively increased or decreased live tissue area. The dotted vertical line represents the minimum bleached area required, on average, to trigger tissue loss. mR2 = marginal R^2 and cR^2 = conditional R2.
So what does this mean? Is this good news for coral reefs that have been through multiple bleaching events already and are likely to face many more?
Well, it depends.
On the one side, it is reason to be carefully optimistic about Acropora corals. These branching corals are very susceptible to dying from bleaching events, but they have proven themselves to be very good at re-establishing quickly. But, once the next major bleaching event comes along, all these new, quickly growing colonies will likely vanish again. As Juliano and his co-authors say in the paper:
“It may represent, therefore, just a short-term boom in a new Anthropocene configuration, where fast-growing corals persist but are unlikely to attain their former abundance due to successive disturbances and suppression of recovery dynamics.”
On the other hand, the patterns of Porites are a little more concerning. Although most of them survived the bleaching events, some did suffer and die. And, critically, no new corals appeared.
“Porites colonies exhibit a precarious degree of resilience, increasing in area but with an underlying recruitment deficit and a strong negative response in tissue area to bleaching severity.”
In the end, it is nice to see so much live coral returning to Lizard Island (and many other) reefs! But most of this new live coral are just Acropora colonies (as in the first photo on the top of this page). It is hopeful to see that these corals are able to re-colonize and grow so quickly, even after being locally lost because of coral bleaching. However, this may just be a short-lived, weedy response, which may happen again and again. Loss > Regrowth > Loss. In the end, it will depend how much “recovery time” remains for corals in between bleaching events.
Further, Porites corals may slowly disappear in the long term, if bleaching events increase in frequency. Some will die or shrink each time, with only few or no new colonies replacing those losses.
These details of “demographic patterns” of two major types of corals could not have been found by only recording coral cover. The vulnerability of Porites to slow chronic losses would not be picked up, if high coral cover, driven by quick growth of Acropora, seems like cause for celebration.
“Without replacement, increasing repetitive bleaching events may drive a slow, protracted decline of massive Porites that could be easily overlooked. These markedly different demographic patterns offer grounds for both optimism and concern. Massive Porites are resistant, but potentially compromised in the long-term, while Acropora are vulnerable, but have greater capacity to recover in the aftermath of major disturbances. In both cases their dangerous demographics require caution when interpreting the susceptibility and perceived resistance of corals to disturbances.”
To read this new research paper in full, find it here. To find more about Juliano’s future work (also on coral dynamics!) keep an eye on his profile here.
