Declining calcification on the Great Barrier Reef

As we’ve covered here before at Climate Shifts, the build up of carbon dioxide in the atmosphere has the potential to slow the growth of reef-building corals by increasing the acidity of the world’s oceans.   By burning immense amounts of fossil fuels, humans have increased the concentration of carbon dioxide (CO2) in the atmosphere by nearly 40%.  Roughly a quarter of this CO2 is being absorbed by oceans, where it reacts with seawater to form carbonic acid, acidifying the upper layers of the ocean.

Porites Cores

X-ray photographs of core sections from a massive coral (Porites lutea) showing annual density banding patterns similar to tree ring growth (Image courtesy of Bob Dunbar, Stanford University)

Several laboratory experiments suggest this could make it more difficult for corals and other organisms such as crabs and clams to secrete the calcium carbonate skeletons they depend on for survival.  The increased acidity essentially makes it more energetically costly to secrete skeletons and could eventually literally dissolve them.

A new research article published today in Science magazine (De’ath et al. 2009) takes the case for ocean acidification a step further.  The article suggests that the recent man-made increase in ocean acidity has lead to a reduced growth rate of massive reef-building corals.  A team of scientists from the Australian Institute of Marine Science measured the growth of hundreds of corals from 69 reefs on the Great Barrier Reef (GBR), Australia.  The team took core samples of the coral skeletons (up to 5m in length) and measured annual growth rings going back over 400 years. Their results strongly indicate that the vertical extension of massive, long-lived corals has slowed by roughly 13% since 1990.  This decrease in growth coincides with an increase in the acidity of tropical oceans. A previous paper from the group published earlier this year (see here) was widely criticized because it was based on corals from reefs close to shore and was limited to two geographic locations. However, the teams new paper found evidence of declines in calcification across over 300 corals growing across 2000 km of the GBR, which is indeed a cause for concern.

Whilst slower growth rates might not seem like a big problem in isolation, reef scientists are concerned that this will exacerbate the impacts of other threats to coral reefs.  Slower vertical growth of corals will make it harder for reefs to keep up with rising sea levels, whilst decreases in calcification makes corals more vulnerable to physical damage such as destructive storms and bioerosion.

Changes in (A) calcification, (B) linear extension, (C) density between 1905-2005 (328 corals from 69 different reefs across the length of the GBR. (D) shows trends in calcification between 1572-2001 (10 corals)

Changes in (A) calcification, (B) linear extension, (C) density between 1905-2005 (328 corals from 69 different reefs across the length of the GBR. (D) shows trends in calcification between 1572-2001 (10 corals). Light blue bands indicate 95% confidence intervals for comparison between years, and gray bands indicate 95% confidence intervals for the predicted value for any given year.

The fact that coral growth since the 1990’s has fallen to its lowest rate for 400 years is particularly worrying, and in stark contrast to the usual inductive reasoning portrayed in the media that “corals grow faster in warm water, therefore warm water is good for coral reefs” (see here for more discussion).

Given that global CO2 emissions are now exceeding the worse-case IPCC scenario, and the likely lag time of several decades between the addition of CO2 and the resulting increase in ocean acidity, we will almost certainly see such problems with coral calcification escalate over the coming decades. We can add this to the list of ‘surprises’ (such as deep water anoxia and the runaway collapses of iceshelves), and strongly suggests that we have seriously underestimated not only the rate of climate change, but the scale of the impacts on ecosystems across the globe. These results should compel the Rudd government to make immediate and drastic steps to decarbonise Australian economic systems as a urgent priority.

Coral springs back from tsunami

Coral transplantation in Indonesia after the impact of the boxing day 2004 tsunami.

Coral transplantation in Indonesia after the impact of the boxing day 2004 tsunami.

BBC News, 26th December

Scientists have reported a rapid recovery in some of the coral reefs that were damaged by the Indian Ocean tsunami four years ago.

It had been feared that some of the reefs off the coast of Indonesia could take a decade to recover.

The New York-based Wildlife Conservation Society (WCS) found evidence of rapid growth of young corals in badly-hit areas. A spokesman said reefs damaged before the tsunami were also recovering. Some communities were abandoning destructive fishing techniques and even transplanting corals into damaged areas, the WCS said.

“This is a great story of ecosystem resilience and recovery,” said Stuart Campbell, co-ordinator of the WCS’s Indonesia Marine Program.

“These findings provide new insights into coral recovery processes that can help us manage coral reefs in the face of climate change.”

Ove Hoegh-Guldberg, a reef expert from the University of Queensland in Australia who did not take part in the study, said the findings were not surprising since corals typically recovered if not affected by fishing and coastal development.

“We are seeing similar things around the southern Great Barrier Reef where reefs that experience major catastrophe can bounce back quite quickly,” the scientist told the Associated Press.

Countries across the Indian Ocean have been remembering the 2004 disaster, which claimed some 230,000 lives. Prayers were said in Indonesia, Thailand and India on Friday, while Sri Lanka declared a two-minute silence in memory of the dead.

Rare corals may be smarter than previously thought

Following on from a previous article at Climate Shifts, a recent article published in PLoS One shows that corals are proving to be even more non-conformist than previously thought. Zoe Richards and co-authors from the ARC Centre of Excellence for Coral Reef Studies found that ‘rare’ species of branching corals are able to cross breed with other branching corals to create hybrids, therefore avoiding probable extinction:

“Coral reefs worldwide face a variety of marine and land-based threats and hundreds of corals are now on the red list of threatened species. It is often assumed that rare coral species face higher risks of extinction than common species because they have very small effective population sizes, which implies that they may have limited genetic diversity and high levels of inbreeding and therefore be unable to adapt to changing conditions.

When we studied some particularly rare species of Acropora (staghorn corals), which you might expect to be highly vulnerable to extinction, we found some of them were actually hybrids – in other words they had cross-bred with other Acropora species.  This breaks all the traditional rules about what a species is. By hybridising with other species, these rare corals draw on genetic variation in other species, increasing their own potential to adapt to changing conditions.

When we looked at the genetic history of rare corals, we found that they exhibited unexpected patterns of genetic diversity.  This suggests that, rather than being the dying remnants of once-common species, they may actually be coral pioneers pushing into new environments and developing new traits by virtue of the interbreeding that has enabled them to survive there.

This is good news, to the extent that it suggests that corals may have evolved genetic strategies for survival in unusual niches – and may prove tougher to exterminate than many people feared. With such tricks up their sleeve, it is even possible that the rare corals of today could become the common corals of the future.”  (Link)


Corals prove to be “nonconformist”

An article published in PLoS One  has huge implications for almost everything we do in our research on corals.  In summary, using an array of genetic markers, a highly respected group of leading scientists including Fukami, Chen, Knowlton and others have shown that whilst Scleractinia (the stony corals) have a single origin in evolution, to date we have lumped many species and genera into families incorrectly, at least partly due to the traditional system of classification .  This finding has the interesting implication that morphological features (at the heart of coral taxonomy) may have been much more plastic in time than we have appreciated.  Such findings make sense given how variable the skeletal structure of corals is in response to the environmental circumstances the coral is growing in.




Fukami et al (2008) Mitochondrial and Nuclear Genes Suggest that Stony Corals Are Monophyletic but Most Families of Stony Corals Are Not (Order Scleractinia, Class Anthozoa, Phylum Cnidaria). PLoS ONE 3(9): doi:10.1371/journal.pone.0003222

Modern hard corals (Class Hexacorallia; Order Scleractinia) are widely studied because of their fundamental role in reef building and their superb fossil record extending back to the Triassic. Nevertheless, interpretations of their evolutionary relationships have been in flux for over a decade. Recent analyses undermine the legitimacy of traditional suborders, families and genera, and suggest that a non-skeletal sister clade (Order Corallimorpharia) might be imbedded within the stony corals. However, these studies either sampled a relatively limited array of taxa or assembled trees from heterogeneous data sets. Here we provide a more comprehensive analysis of Scleractinia (127 species, 75 genera, 17 families) and various outgroups, based on two mitochondrial genes (cytochrome oxidase I, cytochrome b), with analyses of nuclear genes (ß-tubulin, ribosomal DNA) of a subset of taxa to test unexpected relationships. Eleven of 16 families were found to be polyphyletic. Strikingly, over one third of all families as conventionally defined contain representatives from the highly divergent “robust” and “complex” clades. However, the recent suggestion that corallimorpharians are true corals that have lost their skeletons was not upheld. Relationships were supported not only by mitochondrial and nuclear genes, but also often by morphological characters which had been ignored or never noted previously. The concordance of molecular characters and more carefully examined morphological characters suggests a future of greater taxonomic stability, as well as the potential to trace the evolutionary history of this ecologically important group using fossils.

High coral cover and diversity at Magnetic Island, Great Barrier Reef


Here are some underwater photographs taken at a recent field trip at Magnetic Island, inshore Great Barrier Reef. These reefs are usually highly turbid, based 8km away from the Townsville shoreline.  I’m often suprised at the diversity and high cover of some of these inshore reefs, and the visibility at Magnetic this day finally lifted above it’s usual "pea-soup" consistency to get some good photographs.


Two new studies on coral symbiont specificity released in the Proceedings of the National Academy of Sciences

"Dedicated partnership may be corals weakness"

(Sampayo et al 2008, PNAS 105[30] 10444-10449)

"Great Barrier Reef coral communities may not be able to recover from bleaching as easily as previously proposed, according to new UQ research. A two-year study by a team of UQ researchers, in the Centre for Marine Studies, has found that contrary to popular theory, it is not possible for bleached corals to recover or become more resistant to bleaching by taking up more heat tolerant species of their micro-algae partners. All corals have a symbiotic (sharing relationship) with single-celled dinoflagellates, commonly referred to as zooxanthellae. The coral provides a habitat for the zooxanthellae, which in turn produce essential nutrients for the corals.  Under stressful conditions, such as high or low water temperatures, the symbiotic zooxanthellae are expelled from their host, causing a whitening of the coral tissue or bleaching.  Coral bleaching events have caused significant mortality of corals worldwide and the frequency as well as intensity of bleaching events is predicted to increase as a result of climate change. Dr Eugenia Sampayo, who performed the research as part of her PhD, said past research had suggested that bleached corals could take up new, more tolerant symbionts, which would make them less susceptible to future bleaching events" (Read more)



"New indicator uncovered that can predict coral health"

(Stat et al 2008, PNAS 105[27] 9256-9261)

A new indicator of coral health has been discovered in a community of microscopic single-celled algae called dinoflagellates. The study, released in the July 8th edition of the journal Proceedings of the National Academy of Sciences, reveals that a particular type of these algae renders corals more susceptible to disease. "Corals are fascinating organisms whose survival is dependent on dinoflagellates that live inside the coral’s tissue," says lead author Michael Stat, an assistant researcher at the Hawaii Institute for Marine Biology (HIMB) at the University of Hawaii at Manoa. "The relationship between these dinoflagellates and corals has long been considered mutually beneficial, with the dinoflagellates supplying the coral with food via photosynthesis in return for recycled nutrients and shelter. Over the last 20 years it has been made clear that there are many different types of dinoflagellates in corals and that the unions or symbiosis between a given coral and their dinoflagellates can be very specific." It had previously been considered that all dinoflagellates found in coral are equally beneficial to their coral host, but in this study Stat, along with HIMB researchers Ruth Gates and Emily Morris, present evidence that a particular type of dinoflagellate can be found in corals that are diseased or show evidence of having had a disease. (Read more)

“One third of coral species face extinction”

The Telegraph, 10th July

One third of the major reef-building coral species are vulnerable to extinction, and the pace of destruction is increasing so it is conceivable that the "rainforests of the ocean" could be wiped out this century.

The warning that coral communities are faring even worse than their terrestrial counterparts, notably tropical rainforests, is given by an international team led by Prof Kent Carpenter, Director of the Global Marine Species Assessment Of Conservation International And The International Union For Conservation Of Nature, IUCN.

Built over millions of years, coral reefs are home to more than 25 percent of marine species, making them the most biologically diverse of marine ecosystems.

The loss of reefs could have huge economic effects on food security for around 500 million people who are dependent on reef fish for food and/or their livelihoods and tourism is also likely to suffer.

"The results of this study are very disconcerting," said Prof Carpenter, lead author of the Science article.

"When corals die off, so do the other plants and animals that depend on coral reefs for food and shelter, and this can lead to the collapse of entire ecosystems."

"Whether corals actually go extinct this century will depend on the continued severity of climate change, extent of other environmental disturbances, and the ability of corals to adapt," the article concludes.

"Our results emphasize the widespread plight of coral reefs and the urgent need to enact conservation measures."

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