Coral reefs offer natural coastal protection, especially in areas frequently impacted by hurricanes and tropical storms. The great biodiversity of coral reefs serves as an important source for new medicinal remedies.
Coral reefs are among the most complex ecosystems and are revealing the degraded status of coastal environments. Their alarming status represents the poor health of our oceans and if coral reefs disappear other marine realms will follow. Corals have existed for more than million years; yet stresses and changes from human activities are happening faster than their ability to adapt.
Corals may not survive the intensity and swiftness of these ongoing changes. A matter of vital importance is sexual reproduction of corals ; as it is for most species. Sexual reproduction maintains genetic diversity and, in turn, enables species to adapt to a naturally dynamic environment in the long-term. Corals under stress are likely to stop sexual reproduction, which puts their survival at risk. More heat stress led to more bleaching which led to more long-term coral loss.
There was a threshold beyond which corals died off very quickly. Just like any other animal, corals span a wide range of species. These species have different characteristics, shapes, and grow at different rates.
Some grow like platelets; others like branching trees. Some grow rapidly; others much more slowly. This means that reefs can have very different compositions depending on the types of corals that are present.
Coral taxa respond differently to heat exposure. Some are highly intolerant to heat stress and die off immediately, while others can withstand high levels of stress and recover quickly. For example, the tabular staghorn Acropora, Seriatopora hystrix, and Stylophora pistillata are fast-growing branching species that dominate many reefs in the Indo-Pacific region.
It becomes more dominant in the resilient species and loses all of the vulnerable ones. Researchers can measure changes in reef composition using a metric called the nMDS score.
A higher score means a larger change in the make-up of a reef ecosystem. In the chart we see the shift in composition across the sampled Great Barrier reefs in This is measured relative to the amount of heat stress that each reef experienced.
Again, we see that the relationship is not linear. At lower levels of heat stress, bleaching levels are lower and there is not much differentiation between winners and losers. This leaves only the species that can withstand high levels of heat stress. They become the dominant species, which can completely transform reef ecosystems. With satellites, surveys and climate modelling we can now track changes in the atmosphere and the oceans at high-resolution. This means scientists can understand when and where we would expect coral bleaching to be a problem in any given season.
To make these forecasts we need to make assumptions about what levels of heat stress lead to seriously bleaching, and mortality. But, these recent studies on the Great Barrier Reef suggest the current guidelines on tolerable levels of heat stress might be too low.
The experience of the heatwave on the Great Barrier Reef suggests that these guidelines are too high. Corals can bleach and die at lower levels of stress.
The loss of many of its corals is almost an inevitability. The biggest thing we can do to protect them is to slow global climate change by reducing our greenhouse gas emissions as quickly as possible. Globally we have seen an increase in the frequency and intensity of coral bleaching events. This has been driven by increasing temperatures, with warmer waters putting corals under stress.
The average time between mass bleaching events has fallen five-fold, from once every 27 years in the s to once every 5.
This is a truly global change. Very few corals have managed to escape these impacts completely. Most reefs in the world have experienced bleaching events in the last few decades. All have been affected, but some more than others. In this article, I look at how coral bleaching varies from ocean to ocean, and whether this has changed with warming waters in recent decades.
Before we look at the bleaching trends for specific oceans, there is a larger meta-point about the disproportionate impact on some reefs versus others. But there appears to be a marked distinction in the vulnerability of corals across these zones. Reefs that lie closest to the equator have often fared better; those across mid-latitudes experienced worse bleaching.
This is despite the fact that corals in both zones have been exposed to similar levels of thermal stress. Purple markers highlight reefs where very little only one to a few percent of the corals were bleached. Yellow markers show reefs where most of the corals were affected. Reefs closer to the equator might be less vulnerable to bleaching because they have adapted to larger swings in temperature over time.
Equatorial reefs often experience much more variability in temperature over short timeframes: they see large swings of heating and cooling on a daily basis. This exposure to constant fluctuations might have made them more resilient and adaptable over time. Reefs at higher latitudes will have less experience of this. When larger changes in sea surface temperature occur, they are less prepared.
In a study published in Science , Terry Hughes and colleagues tracked the frequency of coral bleaching events across pantropical locations from to In the charts here we see the number of moderate and severe bleaching events that occurred in each ocean from onwards. There are large geographical differences in the timing, intensity, and frequency of these events. The Western Atlantic warmed earlier than elsewhere, and therefore also saw an earlier uptick in the frequency of bleaching.
Its reefs were experiencing regular bleaching much sooner — as early as the s. By its was seeing an average of four bleaching events per location, compared to 0. Over the course of the entire period — from to — the Western Atlantic has seen the highest number of bleaching events, averaging ten events per location.
This is two to three times higher than other regions. It has also seen the greatest number of severe mass bleaching events. The Western Atlantic might have been the most exposed to bleaching in the s, but this has also changed a lot over time.
In the s bleaching risk was highest in the Western Atlantic, followed by the Pacific, with relatively low risk in the Indian Ocean and Australasia. But this has since flipped. We see in the charts that the Indian Ocean and Australasia have seen the strongest rise in bleaching over time.
In the s, bleaching events were rare. This is even true of severe mass bleaching events. The return times of severe bleaching events — how long following an event that we would expect another — have declined in all regions apart from the Western Atlantic.
In contrast to the earlier period, the Western Atlantic managed to escape a major bleaching event between and For most, this is 10 to 15 years. Getting hit by one bleaching episode after another means corals will struggle to recover.
But they differ in how much pressure and rate that this has changed over time. What might explain these differences? The corals that had seen the largest rise in bleaching were not necessarily the ones that had encountered the biggest increase in average water temperature. This might seem counter-intuitive since bleaching is driven by warming waters.
This point matters for our understanding of how reefs will be affected by bleaching in the future. This is relatively simple to do. But the relationship to bleaching is a bit more complex. Instead, we need to understand how warming oceans will affect the frequency and intensity of short-term episodes of extreme warming. An important question is whether reefs will be able to adapt to these extreme temperatures.
And if some, but not all of them can, whether it will mean very different coral ecosystems from what we have today. In our follow-up article we look at how adaptable our corals might be to a warming ocean. This is when they expel their algal symbionts — which are their primary source — due to environmental stress. Some corals die immediately when exposed to extreme temperatures.
Others become bleached then either recover or slowly die over the coming months. As the time between successive bleaching events get shorter and shorter, corals do not have the time they need to recover. Some have suggested that there might be a silver lining. Perhaps coral reefs are more resilient than we give them credit for, and can adapt to a warming ocean. The broader question of whether corals can adapt touches on two related but distinct questions.
First, the question of whether an individual coral becomes more resilient to bleaching over time. If a coral experiences coral bleaching, are they more resilient to future events? Effectively, is there a protective effect of past bleaching? Second, the question of whether coral reef systems can adapt. If a coral experiences and recovers from an intense bleaching episode, will they be protected from another event years later? There are a couple of ways in which organisms can acclimatize or adapt to different conditions.
Organisms can often adjust to new environments — such as a change in temperature, pH, moisture level or altitude — to make sure they can survive across a range of conditions. We see many examples of this in nature. Sheep, for example, grow thicker coats in colder climates then shed them in the Spring or Summer.
We even see examples in humans. When we climb a mountain our bodies produce more haemoglobin — the protein in red blood cells — which transport oxygen around the body. This helps us to adapt to higher altitudes. When we move to a hot climate our bodies adjust by sweating at a lower core body temperature while also reducing the amount of salt in our sweat.
In corals, there are a couple of ways that this might work. First, the coral hosts can release higher levels of specific genes involved in stress-resistant traits. By releasing heat shock proteins and antioxidants, they can respond efficiently to pulses in warming and cooling. One study, published in Science, took the species Acropora hyacinthus — an important coral in the Pacific that tends to bleach easily — and transplanted it to an environment with very frequent swings in temperature.
This resistance that comes with being in a rapidly shifting environment mirrors the heat tolerance we see in corals elsewhere: corals close to the equator tend to experience much less bleaching than those at mid-latitudes; this is because they experience much larger swings in temperature on a day-to-day basis.
Second, acclimatization traits could be passed onto offspring. When corals reproduce, most of the inheritance of key traits is based on genetic factors. But experiments on species such as Acropora , Goniastrea , Platygyra and Porites , suggest that some acclimatization from generation-to-generation could influence the tolerance of offspring.
The third involves the selection of particularly heat-resistant symbionts. There are large differences in how well different types of Symbiodinium can repair photosynthetic damage — the cause of heat stress and bleaching.
These are often much more resistant to bleaching. There is mixed evidence on how well these three processes work to protect reefs in practice. Some results suggest that the threshold for coral bleaching has increased over the course of decades, or even years. In contrast, more recent studies on the Great Barrier Reef have found no evidence of a protective effect of past bleaching. They bleached just as much. This would suggest that previous exposure did not improve their resilience to subsequent bleaching events.
It was the largest mass bleaching event on record in Australasia. This might point towards an explanation for why we have conflicting results — that some studies show that previous bleaching provides some adaptive protection while others do not.
Perhaps reefs can and do adapt to previous heat stress, but only up to a certain point. There may be a limit to what they can adapt to. Their adaptive success might show in more moderate bleaching episodes but when extreme events — like the Great Barrier Reef bleaching in — arrive, they are pushed beyond their limits.
How well corals can acclimatize or adapt to climate change will therefore depend on the frequency and intensity of these very extreme bleaching episodes. Coral B is still going strong and soon takes the place of the disappearing Coral A. In the end our reef system looks very different to how it started. How does this play out in the real world? Do we really see such large differences in the tolerance of different coral species? We already know that different species can tolerate very different environmental conditions.
Rather than comparing corals in different environments we should compare the bleaching or mortality thresholds of species within their current environments. Even there we see large differences in response from species to species. In a study in Moorea in the French Polynesia, researchers looked at how different species of coral were affected in a mass bleaching event in The results are shown in the chart. Montipora experienced moderate bleaching. The susceptibility of Pocillopora was very different: small colonies of these species were very tolerant to heat stress, but the largest ones experienced a lot of bleaching.
Finally, at the bottom we have Porites, a stony coral that tends to form small finger-like structures. It was very resistant to heat stress — almost none of its corals experienced bleaching.
This variability in response to heat stress is not unique to these particular coral taxa. A number of studies have shown the same across a range of coral taxa and reefs across the world. This is true of all coral taxa. But there are large differences in the sensitivity of each. As we saw from our study in the French Polynesia, Acropora is highly susceptible to bleaching and comes out on top. Other taxa — such as Siderastrea siderea and Stephanocoenia intersepta — experience small amounts of bleaching even at very low levels of heat stress.
The difference is that these corals are much less responsive to even more heat stress. This means that not only do different corals respond differently to warmer temperatures, the magnitude of these differences really depends on how extreme the warming event is and the amount of stress they are put under. Changes in reefs will also reflect differences in how quickly corals recover and grow. If some corals bleach — and possibly die off — much more easily than other species, it seems likely that reefs will begin to be dominated by the most resilient ones.
This seems likely. But sensitivity to bleaching is just one part of the story. We also need to consider how quickly corals can recover and grow. If corals that bleach easily then bounce back quickly, they might be able to maintain their spot on the reef.
Conversely, if more resilient corals that experience only moderate bleaching take 10 to 20 years to recover and grow back slowly, they could lose theirs. Researchers have looked at this dynamic between some specific corals in detail. The two corals which dominate many reefs in the Indo-Pacific region — which includes corals off the coasts of Southeast Asia and the Great Barrier Reef — are Acropora palmata and Porites. These corals could not be more different.
Porites is much more resilient. But their growth rates and recovery times are also very different: Acropora grows quickly; Porites grows slowly. This means infrequent but severe disturbances tend to favor Acropora because it can grow back quickly. But moderate, frequent events tend to favor Porites which is much less-affected by moderate warming.
Models that look at the dynamics of these two corals on a reef system suggest that Acropora continues to dominate as long as the interval between bleaching events is more than two years. If the interval between events is less than two years then Porites starts to dominate because even the fast-growing Acropora cannot recover quickly enough. Researchers therefore expect susceptible corals like Acropora to decline in abundance as a result of increased warming.
But depending on the frequency of bleaching events they may not decline by as much as their response to heat stress would suggest. The corals themselves can adapt and acclimatize to changing temperatures through genetic changes in the host, or changes in the selection of heat-resistant symbionts.
But reefs as a whole can also change and adapt, with more resilient species becoming more dominant while others die away. Reef assemblages will be different.
Just how different, and how much coral cover we will lose completely will depend on the intensity and frequency of extreme bleaching events.
The research suggests that corals themselves can adapt to changes — but only up to a certain point. More and more extreme events will push beyond these limits. Human emissions are driving climate change.
The intensity of coral bleaching depends on how much greenhouse gases we emit. When we think of the impacts of carbon dioxide CO 2 emissions we tend to focus on its impact on climate change. But for marine organisms, these emissions pose a double threat. CO 2 emissions could risk the future of marine life, including coral reefs through ocean acidification. If we emit more than can be absorbed by forests, other vegetation and the ocean, then the atmospheric concentration increases.
The Global Carbon Project shows this nicely in the graphic. What this means is that the ocean absorbs a lot of CO 2. As we can see from the chart — the amount of absorbed CO 2 has been increasing over time as a result of increased emissions from fossil fuels. Now, on to chemistry. When CO 2 is absorbed in water, it reacts to form other substances. But now, in the lead-up to World Oceans Day on June 8, scientists caution that these and other strategies may only buy reefs time until world leaders implement aggressive climate change action.
Scientists often compare coral reefs to underwater rainforests, yet unlike the leafy plant base of a forest, corals are animals. The soft polyps inside the hard parts of corals are naturally translucent and get their famously vibrant color from algae living inside them.
When corals experience stress from hot temperatures or pollution, they end their symbiotic relationship with this algae, typically expelling them and turning white, though one recent study indicates some coral turn a bright neon color when stressed.
People first noticed coral bleaching events in the s. Scientists around the world are looking for all kinds of ways to protect and maybe even revive corals. One option is to create more marine protected areas —essentially national parks in the ocean. Scientists say creating marine refuges, where fishing, mining, and recreating are off limits, make the reefs healthier, and so more resilient.
An estimated 4, fish species, and some 25 percent of marine life, depend on coral reefs at some point in their existence. Fish keep the algae that grow on corals in check, allowing corals to breathe and access sunlight. At a talk hosted by the Woods Hole Oceanographic Institution on Wednesday, renowned marine biologist Sylvia Earle promoted the idea of using marine parks to protect coral, which she does through her organization Mission Blue.
A recently published assessment of 1, reefs in 41 countries found that only 5 percent of reefs were able to provide all of their lucrative byproducts, such as healthy fish stocks and biodiversity. To increase that percentage, new marine reserves will need to be strategically placed in areas well away from humans, say experts.
Beyond such nature preserves, some conservationists are looking to more hands-on methods. One research center in the Florida Keys is exploring a form of natural selection to keep corals afloat. To keep the wild ecosystem alive, Muller and her team are harvesting samples of the corals that have survived the environmental stresses naturally, breeding them by hand, and reattaching them to the reef.
At any given time, the center has 46, corals growing on underwater plastic lattices in its nursery. So far, the center has regrown over 70, corals from five different species on damaged reefs. In Massachusetts, Cohen's research has found two key elements that seem to protect corals. On average, these lagoons submerge coral in water that is two degrees Celsius warmer than the water outside the lagoons.
All the scientists interviewed for this article noted that mitigating climate change is the only long-term, sustainable solution to conserve and restore coral reefs.
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