Review: Walker PJ, Mohan CV (2009). Viral disease emergence in shrimp aquaculture: origins, impact and the effectiveness of health management strategies. Reviews in Aquaculture, 1: 125-154.

Feature Paper: Walker PJ, Mohan CV (2009). Viral disease emergence in shrimp aquaculture: origins, impact and the effectiveness of health management strategies. Reviews in Aquaculture, 1: 125-154.

Author Abstract: Shrimp aquaculture has grown rapidly over several decades to become a major global industry that serves the increasing consumer demand for seafood and has contributed significantly to socio-economic development in many poor coastal communities. However, the ecological disturbances and changes in patterns of trade associated with the development of shrimp farming have presented many of the pre-conditions for the emergence and spread of disease. Shrimp are displaced from their natural environments, provided artificial or alternative feeds, stocked in high density, exposed to stress through changes in water quality and are transported nationally and internationally, either live or as frozen product. These practices have provided opportunities for increased pathogenicity of existing infections, exposure to new pathogens, and the rapid transmission and transboundary spread of disease. Not surprisingly, a succession of new viral diseases has devastated the production and livelihoods of farmers and their sustaining communities. This review examines the major viral pathogens of farmed shrimp, the likely reasons for their emergence and spread, and the consequences for the structure and operation of the shrimp farming industry. In addition, this review discusses the health management strategies that have been introduced to combat the major pathogens and the reasons that disease continues to have an impact, particularly on poor, small-holder farmers in Asia.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: For a couple weeks we've been looking at coral reefs and I thought that we would shift gears this week and look at fisheries resources (specifically, shrimp aquaculture) and the challenges that face removing stress from overfishing by aquaculturing resources.

Still, shrimp aquaculture has a lot more to do with coral reef management than many people realize. To collect most shrimp in the wild, nets are dragged across the bottom, destroying benthic habitat and catching many other organisms, usually disgarded as "by-catch." For every one pound of wild caught shrimp, about 8-9 pounds of other animals get caught and die in the process (ones caught by the nets dragging across the bottom to catch shrimp), with most disgarded rather than being kept for other food sources. Additionally, the benthic cover is destroyed, which usually means corals are destroyed. And while most commercially harvested shrimp do not exist directly on coral reefs, they still exist in mixed sandy bottom and coral (and other invertebrate and algae) habitats, so animals are still killed in the process of catching wild shrimp.

Therefore, shrimp aquaculture, in general, is good. There are exceptions, where mangrove swarms are cut down to make room for shrimp aquaculture facilities, or where fish pond densities are so close that disease is rampant and overflow of waste water is discharged into the natural environment, thereby degrading surrounding coastal reefs. But on the whole, aquacultured shrimp is better for the environment than wild caught shrimp. Please note that this is not true for all aquacultured products, so for those who enjoy seafood, do your research if you care about the impact of your source.

This paper addresses the situation of shrimp aquaculture and some of the diseases that have emerged in the attempt to culture shrimp in captive environments.

The main stressors that are cited as causes of shrimp disease in aquaculture settings are: stress from "displacement from natural environments, artificial or alternative feeds, being stocked in high densities, exposed to stress through changes in water quality, and being transported nationally and internationally."

This paper then reviews the recently emerging viral diseases that have become rampant in shrimp aquaculture and led to the production of special strains of disease-free shrimp, increasing the cost and complexity of shrimp aquaculture. As of 2006, shrimp aquaculture in Asia alone is an $8 billion industry so it is vital to maintain the future production of shrimp.

The major shrimp viral diseases in aquaculture identified by the paper are: "GAV, gill-associated virus; IHHNV, infectious hypodermal and haematopoietic necrosis virus; IMNV, infectious myonecrosis virus; MBV, monodon baculovirus; MSGS, monodon slow growth syndrome; TSV, Taura syndrome virus; WSSV, white spot syndrome virus; YHV, yellow head virus." Their origin is show on the map below from the paper, but they are now global due to the international trade of aquacultured shrimp products, and some diseases have been shown to survive freezing.

Besides the 8 viral diseases noted above and in the figure, there have been more than 20 viruses found in shrimp aquacutlure. However, the eight in the figure above are the most devistating to production, with WSSV being the worst of the worst, and is responsible for "much of the economic impact of disease on production globally." It is a devastating disease because not only does it kill shrimp and lower fecundity (reproductive outcome) of survivors, it affects multiple species in production (whereas some viruses are unique to certain species and carriers in others). From being discovered in China in 1992, it had spread to the United States by 1995 from imported frozen shrimp. By 1997 it had spread into the wild shrimp stocks of the southeastern US.

The paper continues to describe all the other major shrimp diseases and details the history of their spread from their sources of original identification (on the map above) so I won't reiterate the great review that can be read freely by following the full paper link at the beginning of this review.

However, a couple other noteworthy points in the paper involve their discussion of why viruses have emerged in shrimp aquaculture and the future of shrimp production.

1. Viruses can emerge when not causing major disease outbreaks in nature through shifts in the natural environment, thereby creating an imbalance in the artificial aquaculture environment that benefits diseases and lessens the immune protection of captive shrimp. This can be addressed in part through multiple generations of shrimp in captivity, since captive-raised shrimp are better able to withstand disease than wild-caught shrimp, which often host multiple diseases.

2. High shrimp densities in aquaculture allows for diseases to spread like wildfire in a population, but a balance must be struck between aquaculture facility size and the ability to effectively feed large populations at low enough costs to make production worthwhile.

The authors also point out two major kinds of shrimp pathogens:

1. "Those for which shrimp are the natural host in which they have existed in for, perhaps, millions of years"

2. "Those that have been introduced to shrimp as a result of a recent cross-species transmission"

WSSV, TSW, and IMNV have all emerged through the latter category, in some cases through complex life histories among aquatic snails and terrestrial insects before spreading to shrimp in aquaculture environments.

In 1996 shrimp diseased cased a 40% loss in production capacity worldwide, but some new diseases have nearly bankrupted production of certain shrimp species. While losses have decreased since the mid-1990s, some diseases continue to affect production worldwide.

The industry has adapted in part by shifting production species of shrimp, with the three major species today being Penaeus vannamei, P. chinensis, and P. monodon.

In Asia, the major barrier to disease eradication continues to be the multitude of "low-income, small-holder farmers (<1 ha holding)," who represent a significant sector of the Asian industry but their lack of education and resources to "implement comprehensive disease management practices are major barriers to achieving reliable production."

Many international organizations (including the FAO, NACA, UNEP, WB, and WWF) have worked towards creating "better management practices" to ensure adequate production in order to maintain food security, expecially among the poor.

Industrial nations have mostly switched to high-security facilities with expensive, disease-free and disease-resistent cultures of shrimp, with workers maintaining strict health standards (such as not being allowed to eat crustaceans outside of the workplace) to ensure production capacity.

The take-home message is that disease emergence is to be expected in any new intensive agricultural or aquaculture facility, but today most "aspects of the biology and epidemiology of infection are now known for most of the major pathogens and technological advances have enabled the production of SPF [specific-pathogen free] broodstock and provided sensitive detection tools that allow screening of seed [stock] to eliminate pathogens."

What is also interesting about the paper is that the authors note that the industry is facing "challenges presented by global warming," which threatens not only coral reefs but a host of natural and production environments.

Next we'll look at a paper about public speaking, which is one of the greatest fears of people around the world. Besides the paper, I'll also share some of my perspectives as I've given over 500 hours of public speaking engagements.

Review: Hughes TP, Baird AH, Bellwood DR, Card M, Connolly SR, Folke C, Grosberg R, Hoegh-Guldberg O, Jackson JBC, Kleypas J, Lough JM, Marshall P, Nyström M, Palumbi SR, Pandolfi JM, Rosen B, Roughgarden J (2003). Climate change, human impacts, and the resilience of coral reefs. Science, 301: 929-933.

Feature Paper: Hughes TP, Baird AH, Bellwood DR, Card M, Connolly SR, Folke C, Grosberg R, Hoegh-Guldberg O, Jackson JBC, Kleypas J, Lough JM, Marshall P, Nyström M, Palumbi SR, Pandolfi JM, Rosen B, Roughgarden J (2003). Climate change, human impacts, and the resilience of coral reefs. Science, 301: 929-933.

Author Abstract: The diversity, frequency, and scale of human impacts on coral reefs are increasing to the extent that reefs are threatened globally. Projected increases in carbon dioxide and temperature over the next 50 years exceed the conditions under which coral reefs have flourished over the past half-million years. However, reefs will change rather than disappear entirely, with some species already showing far greater tolerance to climate change and coral bleaching than others. International integration of management strategies that support reef resilience need to be vigorously implemented, and complemented by strong policy decisions to reduce the rate of global warming.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: Now that we've looked at reasons for coral reef decline and looked at what happens to coral reefs once they decline, as well as the role of Marine Protected Areas (MPAs) in protecting coral and fish stocks, we will turn our attention to evaluating how coral reefs can fare given climate change effects that are outside of the control of most coral reef managers, and therefore, can still impact MPAs.


Today's paper looks at growing global threats to coral reefs, while our past papers dealt more with specific impacts that could more easily be addressed and policed.

As the authors state in their abstract, "the diversity, frequency, and scale of human impacts on coral reefs are increasing" globally. The authors also point out sadly that "there are no pristine reefs left" and from my own experience in the field, even remote uninhabited and unfished reefs suffer from floating rubbish, nets, and regional fisheries declines. Furthermore, "local successes at protecting coral reefs over the past 30 years [for example, through MPAs and grassroots efforts] have failed to reverse regional-scale declines."

And with global carbon dioxide increasing steadily (levels are already higher than anything in recorded recent history since about 600,000 years ago), the oceans will absorb the carbon dioxide and lead to decalcification of oganisms (as discussed in two past Science Corner reviews, or in the author's words, "future changes in ocean chemistry due to higher atmospheric carbon dioxide may cause weakening of coral skeletons and reduce the accretion of reefs, especially at higher latitudes."

The authors include some good figures in their paper to show the global extent of impacts on coral reefs as organisms and fisheries resources are traded globally. The authors also point out that as atmospheric carbon dioxide has increased steadily, coral bleaching has also increased in severity as atmospheric changes have lead to elevated temperatures and increasingly frequent El Niño-Southern Oscillation (ENSO) events.

The authors point out that corals start to bleach at about 1ºC above their mean summer maximum temperature exposure, and that "this threshold will be chronically exceeded as temperatures rise over the next 50 years, leading to predictions of massive losses of all corals."

While some corals may adapt, when I was working in the Maldives I was exposed to reefs that still had not recovered beyond 5-25% live coral cover 10 years after a particularly intense 1998 ENSO killed over 90% of corals on reefs throughout the central and northern Maldives (and 30-70% of corals in the southern Maldives died). Therefore, as bleaching events increase in frequency, the adaptation of a few coral species may be outweighed by the loss of most corals. 

If reefs continue to be impacted before they can fully recover, they may never reach a pristine state again and rather end up slowly (or rapidly in the case of the Maldives and Caribbean) dying off.

So, to get back to the title of this paper, we've discussed climate change and impacts, but what about the natural resilience or coral reefs? As shown in past papers, healthy coral reefs are more likely to ward off disease and to recover faster from impacts. But can corals adapt to climate change?

Yes, corals can adapt. They have symbiotic algae within their tissues that they rely on for photosynthesis, and some researchers (such as Dr. Rob Rowan of the University of Guam Marine Laboratory) have shown that there are multiple clades (or genotypes) of zooxanthellae (the algae living within coral tissues and providing food for corals through photosynthesis) and that some clades prefer warmer water and others colder water. So when corals bleach, the corals that survive have been shown to often switch their zooxanthellae clades to warmer-adapted symbionts so that they have fewer bleaching events in the future. 

However, as the authors point out, "a major concern is that the accelerating rate of environmental change could exceed the evolutionary capacity of coral and zooxanthellae species to adapt." And during the 1998 ENSO event, scientists showed through core samples drilled in large corals that many specimens of 500 - 1000 year old corals died in 1998 from bleaching stress. ENSOs aren't new... they've been around at least 13,000 years from the shift out of the last glacial ice age. But the main point is that in the last 1000 years, corals did not face the kind of stress that they did in 1998, and so many corals were stressed that year from high sea-surface water temperatures that 13% of corals died worldwide and nearly 70% of corals on many reefs in the Indian Ocean.

You can think of corals as very flexible to environmental changes when young but very specific to those conditions once they are a couple years old. So, corals in the Persian Gulf, for instance, often get exposed to temperatures in excess of 36ºC, while those same species might have only 25ºC maximums near the equator. Yet, even though two corals might be the same species, once a coral adapts to such conditions, if you were to take a specimen from more stable temperatures and move it to conditions like the Persian Gulf, it would almost certainly perish it's first year.

The authors show that MPAs do help corals after bleaching events because healthy herbivorous fish communities keep algae from overgrowing newly bleached (and dead) coral skeletons, thereby giving corals time to grow back quicker. However, there are still many research and management challenges, since MPAs do not stop the cause of global climate change, which effects corals worldwide.

The authors point out that more large-scale experiments and sampling of coral reefs needs to occur to model the scale of impacts and key biological processes to "go beyond the current emphasis on routine monitoring and mapping." Critically, the authors state that "most coral reef research is parochial and short-term, and provides little insight into global or longer-term changes." More emphasis on "temporal, regional, and global patterns of relative abundance or functional attricutes of species that could be exploited for management of resilience" needs to occur. The authors also call out for "studies of intergenerational (genetic) responses to climate change" for coral reef organisms, "particularly corals and zooxanthellae."

The take-home message is that conservation efforts and MPAs need "to be complemented by heightened protection of adjacent areas and by strong international policy decisions to reduce the rate of global warming." In the meantime as governments work for such changes in greenhouse gas emissions (the main cause of global climate change), the authors suggest that "30% of the world's coral reefs should be no-take areas to ensure long-term protection and maximum sustainable yield of exploited stocks." 

Since coral reefs are mostly tropical, and many tropical countries are poor, incentives need to be given to developing nations to protect their coral reef resources. Unfortunately, industrialized nations don't bear the onus of responsibility though they have the majority of coral reef researchers. As the authors point out, "even in affluent countries, such as the United States and Australia, less than 5% of reefs today are no-take areas. Wealthy countries have an obligation to take the lead in increasing the proportion of reefs that are no-take areas, while simultaneously controlling greenhouse-gas emissions."

Next we'll shift gears away from coral reefs, but stick with an important component of the marine environment: fisheries resources. One way to protect coral reefs and fisheries resources is to aquaculture sea life. Yet, it is not without its own challenges, often centered around on reproduction in confined spaces, nutrition, and diseases.

Review: Selig ER, Bruno JF (2010). A global analysis of the effectiveness of marine protected areas in preventing coral loss. PLoS ONE, 5(2):e9278. doi:10.1371/journal.pone.0009278

Feature Paper: Selig ER, Bruno JF (2010). A global analysis of the effectiveness of marine protected areas in preventing coral loss. PLoS ONE, 5(2):e9278. doi:10.1371/journal.pone.0009278

Author Abstract: A variety of human activities have led to the recent global decline of reef-building corals [1], [2]. The ecological, social, and economic value of coral reefs has made them an international conservation priority [2], [3]. The success of Marine Protected Areas (MPAs) in restoring fish populations [4] has led to optimism that they could also benefit corals by indirectly reducing threats like overfishing, which cause coral degradation and mortality [2], [5]. However, the general efficacy of MPAs in increasing coral reef resilience has never been tested. We compiled a global database of 8534 live coral cover surveys from 1969–2006 to compare annual changes in coral cover inside 310 MPAs to unprotected areas. We found that on average, coral cover within MPAs remained constant, while coral cover on unprotected reefs declined. Although the short-term differences between unprotected and protected reefs are modest, they could be significant over the long-term if the effects are temporally consistent. Our results also suggest that older MPAs were generally more effective in preventing coral loss. Initially, coral cover continued to decrease after MPA establishment. Several years later, however, rates of coral cover decline slowed and then stabilized so that further losses stopped. These findings suggest that MPAs can be a useful tool not only for fisheries management, but also for maintaining coral cover. Furthermore, the benefits of MPAs appear to increase with the number of years since MPA establishment. Given the time needed to maximize MPA benefits, there should be increased emphasis on implementing new MPAs and strengthening the enforcement of existing MPAs.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: Now that we've established how many reefs worldwide have been declining (in the last Science Corner) we will look at seeing whether marine protected areas (MPAs) are effective in helping corals recover and preventing further losses of live coral cover when neighboring reefs are being damaged.


As the authors point out, MPAs have been long established to be effective in maintaining and growing fish populations, but what about their effectiveness on longer-lived animals, such as colonial reef-building corals?

This paper is important because it is one of the first papers to address health of corals in MPAs, and as pointed out by Raymundo et al. in our third review, healthy reef fish communities help to prevent coral diseases, thereby helping the entire ecosystem to function in a "give and take" whereby the health of one component ensures and reinforces the health of other components. This is important because people rely on reefs for multiple resources, so as long as all components are functioning properly, everyone can be happy since all resources will remain at sustainable levels.


To address their question (do MPAs help prevent loss of live coral cover?) the authors looked at over 8000 surveys over nearly 40 years where live coral cover was recorded by scientists. The authors then filtered sites by MPA and non-protected area and determined trends in percent live coral cover over those four decades.


Their results were a bit startling in some regards and common sense in others. For instance, in the Philippines and Indonesia, many ornamental marine fishes (for aquariums) are collected using bleach, cyanide, and other chemicals that stun the fish (often with them dying inside aquariums from organ failure months later linked to the chemical exposures) and damage or kill the live corals in which the fish hide. Food fish is often collected using dynamite, which destroys live corals below it. Additionally, in many Indo-Pacific nations, corals are harvested for building materials, with cements and lye produced from coral skeletons.


The authors are also careful to note that "protection within MPAs may not necessarily result in positive effects on coral cover. Coral loss that is driven by regional or global stressors like climate change and coral disease outbreaks seems unlikely to be mitigated by MPAs or other local management actions [14], [15]. Indeed, several studies of individual reefs or small groups of reefs have found that MPAs do not prevent coral loss and other forms of reef degradation [7], [15], [16], [17]."


However, overall, the authors found that MPAs provided a stabilizing effect on the ecology of the reef ecosystem and that all functional levels of the reef remained constant. This effect seemed to help ensure that corals remained healthy alongside fish and invertebrate communities within MPAs, whereas live coral cover declined steadily outside of MPAs.


The authors also provide a nice map of MPAs worldwide, which is certainly better than in past decades. Hopefully more MPAs are established since the take-home message is that when you protect an entire ecosystem (as with an MPA) you also protect all the individual organisms within that ecosystem. With complex ecosystems, it is heartening to know that a hands off approach (meaning, keep your hands off the resources and out of the cookie jar) helps, though regional and global effects can play a negative effect on the progress of MPAs globally, so we must still worry about global climate change.

Review: Bruno JF, Selig ER (2007). Regional decline of coral cover in the Indo-Pacific: Timing, extent, and subregional comparisons. PLoS ONE, 2(8):e711. doi:10.1371/journal.pone.0000711

Feature Paper: Bruno JF, Selig ER (2007). Regional decline of coral cover in the Indo-Pacific: Timing, extent, and subregional comparisons. PLoS ONE, 2(8):e711. doi:10.1371/journal.pone.0000711

Author Abstract: A number of factors have recently caused mass coral mortality events in all of the world’s tropical oceans. However, little is known about the timing, rate or spatial variability of the loss of reef-building corals, especially in the Indo- Pacific, which contains 75% of the world’s coral reefs. We compiled and analyzed a coral cover database of 6001 quantitative surveys of 2667 Indo-Pacific coral reefs performed between 1968 and 2004. Surveys conducted during 2003 indicated that coral cover averaged only 22.1% (95% CI: 20.7, 23.4) and just 7 of 390 reefs surveyed that year had coral cover .60%. Estimated yearly coral cover loss based on annually pooled survey data was approximately 1% over the last twenty years and 2% between 1997 and 2003 (or 3,168 km2 per year). The annual loss based on repeated measures regression analysis of a subset of reefs that were monitored for multiple years from 1997 to 2004 was 0.72 % (n = 476 reefs, 95% CI: 0.36, 1.08). The rate and extent of coral loss in the Indo-Pacific are greater than expected. Coral cover was also surprisingly uniform among subregions and declined decades earlier than previously assumed, even on some of the Pacific’s most intensely managed reefs. These results have significant implications for policy makers and resource managers as they search for successful models to reverse coral loss.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: For the last few weeks we've been discussing various causes of coral decline worldwide. This week we'll look at what happens after a coral reef is already degraded and address issues that managers use to help recover damaged coral reefs.

This paper is important because of its scope: 6001 surveys of 2667 Indo-Pacific coral reefs performed between 1968 to 2004 were analyzed. In Indo-Pacific contains nearly 75% of reef-building coral cover globally. Their analysis showed that coral cover over the last 20 years decreased 1% annually (a larger 2% loss was seen between 1997 - 2003). Coral cover on surveyed reefs decreased from greater than 75% live coral cover to less than 25% live coral cover throughout the study period. 

This paper represents a database analysis, whereby surveys from many different people conducted along similar guidelines (so that comparisons are applicable) are compared over a large geographical area to determine trends... in this case, trends in % live coral cover over time.

The authors "describe a comprehensive analysis of the timing, rate, and geographic extent of the loss of coral cover across the Indo-Pacific," which the authors bound by Sumatra in the west and French Polynesia in the east. 

Throughout their study range, more reefs were found over time with lower % live cover of coral towards the end of the study than at the beginning of the study. Live coral cover is an important measure of reef health because corals are the chief architects of reef structure, thereby providing a home for thousands of organisms. When corals die, many of the organisms associated with corals die as well. Corals also act as breakwaters to waves, protecting coastlines from erosion. Healthy reefs also produce more fish than unhealthy reefs, providing food for millions of coastal peoples worldwide.

Most of the reefs in the database were surveyed only once, though the authors included 651 reefs surveyed two or more times. Some survey regions were studied more intensively than others, with the Great Barrier Reef having the most number of surveys followed by the Philippines and Mainland Asia. 

By 2003 all 10 subregions studied had a mean percent live coral cover below 30% (the region-wide average was only 22.1% live coral cover), compared to the early 1980s when most surveyed subregions had a mean percent live coral cover of nearly 50% (region-wide estimates are 42.5% mean live coral cover between 1980-1983 for the Indo-Pacific). As the authors note, "our results indicate that coral cover on Indo-Pacific reefs is currently lower and far more uniform than expected."

The authors note that percent live coral cover is just one measurement (or proxy) of reef health, but use it because they did not have abundant data on other important measures of reef degradation, "in particular the abundance and diversity of reef inhabitants." Also, while the authors note that between 1984 and 1996 coral cover was slightly lower in eastern Indonesia than on the Great Barrier Reef, it is known today that eastern Indonesia and Papua New Guinea have the highest total species diversity of corals. Therefore it is important to understand that high percent coral cover does not necessarily mean high diversity of species. There are over 800 species of reef-building (hermatypic) corals in Papua New Guinea compared to less than 50 species of reef-building corals in the Hawaiian Islands, yet some areas of the Hawaiian Islands have over 90% live coral cover. Sometimes, certain species overgrow and outcompete other species.

Therefore, keep in mind that percent live coral cover is one proxy of reef health, but not necessarily a proxy of reef diversity, which depends on a variety of regional factors. 

Nevertheless, the take-home message is well-known... most reefs worldwide are declining in live coral cover. This means that while corals may still be around in 100 years, coral reef communities are likely to look a lot different. Even a pristine and well-managed reef like the Great Barrier Reef in Australia showed a shift towards more study sites with lower % cover of live coral than sites with high live cover. The authors found that only 7 of 390 surveyed reefs in 2003 with greater than 60% live coral cover. Something in management will have to change to protect coral reefs for the coming century.


Now that we know that coral loss has been occurring steadily since the 1970s, we'll address the major approach aimed at halting reef degredation: Marine Protected Areas. Next we'll look at another Bruno and Selig paper that discusses whether marine protected areas (MPAs) have been effective globally in preventing coral loss.

Review: Diaz-Pulido G, McCook LJ, Dove S, Berkelmans R, Roff G, Kline DI, Weeks S, Evans RD, Williamson DH, Hoegh-Guldberg O (2009). Doom and boom on a resilient reef: Climate change, algal overgrowth and coral recovery. PLoS ONE, 4(4):e5239 1-9.

Feature Paper: Diaz-Pulido G, McCook LJ, Dove S, Berkelmans R, Roff G, Kline DI, Weeks S, Evans RD, Williamson DH, Hoegh-Guldberg O (2009). Doom and boom on a resilient reef: Climate change, algal overgrowth and coral recovery. PLoS ONE, 4(4):e5239 1-9.

Author Abstract: Background: Coral reefs around the world are experiencing large-scale degradation, largely due to global climate change, overfishing, diseases and eutrophication. Climate change models suggest increasing frequency and severity of warming-induced coral bleaching events, with consequent increases in coral mortality and algal overgrowth. Critically, the recovery of damaged reefs will depend on the reversibility of seaweed blooms, generally considered to depend on grazing of the seaweed, and replenishment of corals by larvae that successfully recruit to damaged reefs. These processes usually take years to decades to bring a reef back to coral dominance.

Methodology / Principal Findings: In 2006, mass bleaching of corals on inshore reefs of the Great Barrier Reef caused high coral mortality. Here we show that this coral mortality was followed by an unprecedented bloom of a single species of unpalatable seaweed (Lobophora variegata), colonizing dead coral skeletons, but that corals on these reefs recovered dramatically, in less than a year. Unexpectedly, this rapid reversal did not involve reestablishment of corals by recruitment of coral larvae, as often assumed, but depended on several ecological mechanisms previously underestimated.

Conclusions / Significance: These mechanisms of ecological recovery included rapid regeneration rates of remnant coral tissue, very high competitive ability of the corals allowing them to out-compete the seaweed, a natural seasonal decline in the particular species of dominant seaweed, and an effective marine protected area system. Our study provides a key example of the doom and boom of a highly resilient reef, and new insights into the variability and mechanisms of reef resilience under rapid climate change.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This paper is important because it deals with a subject that is becoming all too common on coral reefs worldwide: high temperatures cause coral bleaching (the expulsion and loss of symbiotic microalgae living within reef-building corals' tissue and without which, corals die); The symbiotic algae (zooxanthellae) don't return in time; The bleached corals die; Algae overgrow coral skeletons; Coralline algae cover goes down; Less recruitment area is available for coral larvae (which can't settle on filamentous algae); If coupled with overfishing of herbivores, the algae grows out of control (especially when also coupled with increased nutrients, as from agricultural and sewarage runoff); A phase shift occurs and the "coral" reef that once was is no longer and won't be again until environmental conditions return to normal.

This paper deals with the situation when just such a scenario occurs (corals bleaching and dying and their skeletons being overgrown by a seaweed; in this case, herbivores were present but the seaweed overgrowth was of a noxious seaweed that prevented herbivore grazing) but later, the corals still recover quicker than expected.

An important point to keep in mind is that this study occurred on the Great Barrier Reef, which is one of the best managed reef ecosystems in the world. Unlike the US Government, which has something like 40+ government agencies with a say in the fate of coral reefs under US jurisdictional waters, Australia is unique in having a single government agency (The Great Barrier Marine Park Authority) overseeing all aspects of recreation, fishing, use, and take on Australia's major reef.

As Raymundo and her team showed in our third review, healthy coral communities are better able to recover from stress. The same was seen in this study, where fragments of coral tissue that still remained were able to regrow and chemically kill microalgae in their path as they regrew on the skeletons where most of their tissue died back from. When coupled with a natural seasonal decline in the particular algae in question and healthy waters without overfishing, the coral reef was able to completely recover in less than three years.

As El Niño Southern Oscillation (ENSO) events increase in frequency due to anthropogenic climate change, coral reefs are bleaching more frequently. With major fisheries around the world near collapse stressors on reefs continue mostly unabated. Yet this paper shows that if there is proper management of reefs, their ability to recover from catastrophic events is relatively great. However, if catastrophic events keep occurring more frequently than the ability of corals to recover fully, ecosystem collapse may occur. And as seen in the Caribbean, once that happens, it can take decades for reefs to recover, if at all.

The take-home message is that coral reefs still have some surprises up their sleeves, with previously undocumented ways to recover still being found. However, for these protective mechanisms (likely acquired over millions of years) to "kick in" reefs have to be resilient in the first place, which they can only do when they are not under undue stress. Coral reefs seem relatively able to recover from brief, catastrophic events (such as a single hurricane or ENSO event) but are less able to cope when continuously bombarded with stressors, which can compound decline by leading to more disease and algal overgrowth.

Now that we've looked at enough examples of how coral reefs can decline, we'll look at the extent of coral decline throughout the Indo-Pacific region of highest coral diversity.

Review: Kuffner IB, Andersson AJ, Jokiel PL, Rodgers KS, Mackenzie FT (2008). Decreased abundance of crustose coralline algae due to ocean acidification. Nature Geoscience, 1(2):114-117.

Feature Paper: Kuffner IB, Andersson AJ, Jokiel PL, Rodgers KS, Mackenzie FT (2008). Decreased abundance of crustose coralline algae due to ocean acidification. Nature Geoscience, 1(2):114-117.

Author Abstract: Owing to anthropogenic emissions, atmospheric concentrations of carbon dioxide could almost double between 2006 and 2100 according to business-as-usual carbon dioxide emission scenarios. Because the ocean absorbs carbon dioxide from the atmosphere, increasing atmospheric carbon dioxide concentrations will lead to increasing dissolved inorganic carbon and carbon dioxide in surface ocean waters, and hence acidification and lower carbonate saturation states. As a consequence, it has been suggested that marine calcifying organisms, for example corals, coralline algae, molluscs and foraminifera, will have difficulties producing their skeletons and shells at current rates, with potentially severe implications for marine ecosystems, including coral reefs. Here we report a seven-week experiment exploring the effects of ocean acidification on crustose coralline algae, a cosmopolitan group of calcifying algae that is ecologically important in most shallow-water habitats. Six outdoor mesocosms were continuously supplied with sea water from the adjacent reef and manipulated to simulate conditions of either ambient or elevated seawater carbon dioxide concentrations. The recruitment rate and growth of crustose coralline algae were severely inhibited in the elevated carbon dioxide mesocosms. Our findings suggest that ocean acidification due to human activities could cause significant change to benthic community structure in shallow-warm-water carbonate ecosystems.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: This paper is important because it deals with the topic of what will happen to coral reefs as carbon dioxide increases from human greenhouse gas emissions. Unlike the previous science corner dealing directly with corals, this paper looks at calcarious crustose coralline algae (CCA). CCA are important on reefs because they:
1) Bind perhaps as much atmospheric carbon dioxide as corals do on reefs;
2) They cement reefs together by growing over adjacent rocks;
3) Coral larvae use CCA as settlement cues and when CCA aren't present, settlement can be negatively affected (as discussed in last week's other Kuffner paper).


As mentioned in last week's Science Corner, Ilsa Kuffner's lab produces high-quality marine ecology experiments based on laboratory manipulations of environmental conditions. In this experiment, they had open-system cylindrical aquaria with established populations of CCA that they exposed to elevated carbon dioxide concentrations. Following the experimental manipulation, they monitored recruitment rates, growth, and eventual decalcification of CCA populations in the flow-through systems.


Like the group studying Mediterranean hard corals (in week 7's Science Corner), Kuffner et al. found that increased carbon dioxide does decalcify their experimental organism.


This paper is important because it shows another group that will be affected as greenhouse gas concentrations continue to increase over the current century. And while the general public might not know CCA as well as corals, it is also important to note that since coral larvae use CCA as settlement cues (meaning the larvae chemically sense the CCA and preferentially choose to settle on areas of the reef with dominant CCA growth, among other ideal conditions), the lower the CCA presence (which will happen with increasing greenhouse gases, as this paper shows), the more area is left for other algae and marine organisms, which will all combine to lessen coral recruitment.


The take-home message is that coral can be affected by more than just direct impediments to their survival; they can be affected by changes in their immediate environment. The purpose of ecological studies is to show that organisms are often interdependent upon each other and certain environmental conditions in order to properly thrive. Once one "leg" is affected, the whole house of cards can fall down so to speak.


Now that we've discussed several aspects of coral reef decline, next we'll examine the case of what happens after a coral reef is already degraded. How can managers help recover damaged coral reefs to their once-former glory?

Review: Kuffner IB, Walters LJ, Becerro MA, Paul VJ, Ritson-Williams R, Beach KS (2006). Inhibition of coral recruitment by macroalgae and cyanobacteria. Marine Ecology Progress Series, 323:107-117.

Feature Paper: Kuffner IB, Walters LJ, Becerro MA, Paul VJ, Ritson-Williams R, Beach KS (2006). Inhibition of coral recruitment by macroalgae and cyanobacteria. Marine Ecology Progress Series, 323:107-117.

Author Abstract: Coral recruitment is a key process in the maintenance and recovery of coral reef ecosystems. While intense competition between coral and algae is often assumed on reefs that have undergone phase shifts from coral to algal dominance, data examining the competitive interactions involved, particularly during the larval and immediate post-settlement stage, are scarce. Using a series of field and outdoor seawater table experiments, we tested the hypothesis that common species of macroalgae and cyanobacteria inhibit coral recruitment. We examined the effects of Lyngbya spp., Dictyota spp., Lobophora variegata (J. V. Lamouroux) Womersley, and Chondrophycus poiteaui (J. V. Lamouroux) Nam (formerly Laurencia poiteaui) on the recruitment success of Porites astreoides larvae. All species but C. poiteaui caused either recruitment inhibition or avoidance behavior in P. astreoides larvae, while L. confervoides and D. menstrualis significantly increased mortality rates of P. astreoides recruits. We also tested the effect of some of these macrophytes on larvae of the gorgonian octocoral Briareum asbestinum. Exposure to Lyngbya majuscula reduced survival and recruitment in the octocoral larvae. Our results provide evidence that algae and cyanobacteria use tactics beyond space occupation to inhibit coral recruitment. On reefs experiencing phase shifts or temporary algal blooms, the restocking of adult coral populations may be slowed due to recruitment inhibition, thereby perpetuating reduced coral cover and limiting coral community recovery.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: There are basically three areas of main research: 1) Laboratory work (either of a chemical / biochemical nature; computer modeling; mapping and other analyses-intensive research); 2) Field work; and 3) Experimentation and observation. This paper falls into the last classic methodology of science: experimentation. 

Last week we discussed the ability of reef-building hard corals to survive decalcification of their skeletons, which is a threat that may occur as greenhouse gas concentrations continue to rise in the atmosphere through human activities and burning of fossil fuels. That would be a catastrophic destruction of reefs as we know them, though some coral species would likely survive (as last week's paper showed).


A quicker destruction of reefs occurs when various elements are out of sync with optimal conditions for healthy reefs, and through this imbalance algae often gain the advantage (through growing faster than corals) and can overgrow and kill corals, leading to a phase shift (mentioned a few weeks ago)
This paper is important because it set out to show experimentally how algae and cyanobacteria can impede the successful recruitment of coral larvae (the hard coral Porites asteroides and the gorgonian Briareum asbestinum). This paper provides solid proof that "macrophytes affect coral recruitment," at least for the species of corals tested. This paper also showed that without a suitable substrate, coral recruits would die when in contact with algae and cyanobacteria.

The take-home message is that algae and cyanobacteria can produce chemicals and otherwise irritate the recruitment success of tested coral species.


It has been known for some time that coral larvae prefer to settle on crustose coralline algae (CCA) and it may be for the reason that other algae are deterrents. As CCA is an important cement on coral reefs and absorbs perhaps as much carbon dioxide as reef building corals, coral larvae may have evolved to seek out noncompetitive substrates where they can grow relatively unimpeded until they are large enough to overshadow small filamentous algae like those researched in this paper.


In the scientists' words, "We have presented evidence that locally common species of algae and cyanobacteria deter coral larvae from recruiting nearby despite the availability of suitable settlement substrate; most of the species we tested affected the location of coral recruitment."


Ilsa Kuffner and her team and collaborators are very good at putting out clear, concise experimental marine science papers aimed at provided proof of general concessions in the international marine science community. I highly recommend all her papers, and we'll continue with another study of hers, also about increased carbon dioxide concentrations in the ocean, like the last paper.

Review: Fine M & Tchernov D (2007). Scleractinian coral species survive and recover from decalcification. Science, 315:1811.

Feature Paper: Fine M & Tchernov D (2007). Scleractinian coral species survive and recover from decalcification. Science, 315:1811.

Author Abstract: Anthropogenic-driven accumulation of carbon dioxide in the atmosphere and projected ocean acidification have raised concerns regarding the eventual impact on coral reefs. This study demonstrates that skeleton-producing corals grown in acidified experimental conditions are able to sustain basic life functions, including reproductive ability, in a sea anemone-like form and will resume skeleton building when reintroduced to normal modern marine conditions. These results support the existence of physiological refugia, allowing corals to alternate between nonfossilizing soft-body ecophenotypes and fossilizing skeletal forms in response to changes in ocean chemistry. This refugia, however, does not undermine the threats to reef ecosystems in a high carbon dioxide world.


Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.


Review: This paper is one of the Science journal's "Brevia" reports. Generally, these reports are 1 page in length and provide a timely and non-jargon-filled scientific breakthroughs or accounts of wide value. Basically, they are short, important papers, often with brief and well thought out figures / images. They are very difficult to write because, as Mark Twain once said, "I'm sorry I wrote you a long letter. I didn't have the time to write a short one." Editing is the hardest job with any writing and when you must condense sometimes years of complex research into a 1-page summary, you'd be surprised how hard it can be.

This paper is important because it deals with the topic of increased ocean acidification, which is predicted over the current century as atmospheric carbon dioxide (and other greenhouse gases) increase. As a quick note though for naysayers: humans have already affected sea surface pH since the 1700s, from which we have global measurements.

[Figure showing "change in sea surface pH caused by anthropogenic carbon dioxide between the 1700s and the 1990s." Lower numbers are more acidic pH conditions from base measurements in 1700.]

Most people know that trees absorb carbon dioxide, but the oceans absorb just as much (or more) than all the trees in the world. Some of the carbon dioxide absorbed is used by phytoplankton -- microscopic plants that form the base of the food chain in the open ocean. But with excess carbon dioxide, the ocean acts as a big chemical sponge and responds to the increasing carbon dioxide by changing its pH balance. When the pH of seawater is too acidic, calcium gets dissolved. Calcium is found in a lot of living things in the ocean (basically everything with a shell or a skeleton) and if more calcium gets dissolved every day than can be accreted by the host body, then the potential for the organism to die is great (and certainly, growth is impeded or stopped).

The authors took the hypothetical situation of a high-greenhouse gas world and replicated it in the laboratory. Basically, they pumped carbon dioxide into flow-through seawater systems under otherwise natural Mediterranean Sea environmental conditions (temperature, salinity, photoperiod) and recorded changes in coral physiology in 2 species of Mediterranean corals (Oculina patagonica and Madracis pharencis) over 12 months in pH ranging from 7.3 - 7.6 (normal Mediterranean Sea water is pH 8.0 - 8.3). 

What is very surprising about the results of their study is that their corals survived but their skeletons completely dissolved. The corals went from being colonial organisms to individual polyps that looked functionally similar to sea anemones. Bleaching (or loss of symbiotic algae) occured in 20% of corals in the study but they recovered.

The take-home message is that "this has far-reaching impolications for better understanding the natural history of corals but mainly implies that corals might survive large-scale environmental change, such as that expected for the following century." 

I want to note however that just because 2 coral species (out of more than 800 total species) survived the experiment, they were functionally very different. No calcium or calcification means no reef formation. No reef formation means no ability to "grow up" towards the light as islands subside (sink) naturally or as sea levels rise (which will increase with increasing atmospheric carbon dioxide). I should also note that coral "reefs" are geologically recent even though coral "non-reefal communities" have been around since the dinosaurs. 

So yes, at least some corals will survive even under major environmental changes but in no way should people assume that the diverse coral reefs we can dive or snorkel on today will necessarily be there. 

Life goes on and nature adapts, but wouldn't it be a shame if something so beautiful were gone?

[Photo by Dave Burdick of Guam Reef Life: Gun Beach, Tumon, Guam low tide, exposed Acropora spp. corals]

Review: Hochberg EJ, Apprill AM, Atkinson MJ, Bidigare RR (2006) Bio-optical modeling of photosynthetic pigments in corals. Coral Reefs, 25:99-109.

Feature Paper: Hochberg EJ, Apprill AM, Atkinson MJ, Bidigare RR (2006) Bio-optical modeling of photosynthetic pigments in corals. Coral Reefs, 25:99-109.

Author Abstract: The spectral reflectance of coral is inherently related to the amounts of photosynthetic pigments present in the zooxanthellae. There are no studies, however, showing that the suite of major photosynthetic pigments can be predicted from optical reflectance spectra. In this study, we measured cm-scale in vivo and in situ spectral reflectance for several colonies of the massive corals Porites lobata and Porites lutea, two colonies of the branching coral Porites compressa, and one colony of the encrusting coral Montipora flabellata in Kaneohe Bay, Oahu, Hawaii. For each reflectance spectrum, we collected a tissue sample and utilized high-performance liquid chromatography to quantify six major photosynthetic pigments, located in the zooxanthellae. We used multivariate multiple regression analysis with cross-validation to build and test an empirical linear model for predicting pigment concentrations from optical reflectance spectra. The model accurately predicted concentrations of chlorophyll a, chlorophyll c2, peridinin, diadinoxanthin, diatoxanthin and beta-carotene, with correlation coefficients of 0.997, 0.941, 0.995, 0.996, 0.980 and 0.984, respectively. The relationship between predicted and actual concentrations was 1:1 for each pigment, except chlorophyll c2. This simple empirical model demonstrates the potential for routine, rapid, non-invasive monitoring of coral-zooxanthellae status, and ultimately for remote sensing of reef biogeochemical processes.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This paper is important because as reefs are more threatened every year through direct human impacts (e.g., dynamite fishing, dredging of coral for building materials and lime, overfishing, etc.) and global climate change (e.g., increased El Niño Southern Oscillation events that contribute heavily to bleaching of coral reefs), scientists struggle to map coral reefs now. Eric Hochberg, Marlin Atkinson, Serge Andréfouët, and a handful of other coral reef scientists are using satellite imagery of the oceans to map coral reef communities. This greatly speeds up the process of mapping in situ (on location) but isn't nearly as accurate. Therefore, "ground truthing" is always required but remote sensing scientists hope that in the future, once all the obstacles are overcome, that all coral reef communities can be mapped accurately. This would allow frequent monitoring of all reefs around the world (theoretically though cloud cover would be an issue), almost in real time, which would be a critical tool to coral reef managers.

So, before I get into the review of this week's paper I'll give a very brief overview of remote sensing (mapping from remote locations, as opposed to mapping while observing an ecosystem directly).

How is it possible for a satellite to see a coral reef? Just check out Google Earth and use the satellite function, such as this image of the upper Caribbean. Coral reefs are high-contrast light blue (because many reefs are shallow and often because of sand within the reef environment) and easy to see below.

The challenge is not seeing where shallow-water tropical potential reef boundaries are. The challenge is being able to accurately map coral reef communities. To do this you have to first obtain cloud-free and calm seas pictures of the area you want to survey (because without radio waves, you can't map corals through clouds). NASA has been building a "coral library" for some time to build the best pictures of reefs around the world, which they've mapped (in red below).

Once you have the kind of images you'd like, there are still A LOT of problems. Eric Hochberg has done a lot of work in correcting for distortions in wavelength due to seawater, but the short story is that it is quite difficult to map coral reefs. However, once the obstacles are overcome, you get an image like that below (from Eric Hochberg's research website).

I've left Eric's explanatory figure notes above, but because of his image resolution, it may be difficult to read for some. The main point is to look at the left image (satellite) of a coral reef in an area of Hawaii and see that through his computer processing of the image using various algorithms to correct for water column effects, he generates a false-color map (right) with different reef communities discriminated (corals are in red).

In Eric's image, corals are able to be distinguished from algae, bare rock, and sand because living corals reflect light differently than those other categories. This is because corals have color, which is a reflection of pigments inside their tissue. Some pigments are sunscreens that shallow-water corals develop to protect them from harmful UV radiation (just like humans can tan) and other pigments come from symbiotic algae that live in their tissues (called zooxanthellae), which are responsible for photosynthesis inside corals (after all, corals are animals and only plants can photosynthesize, so it is a parnership or symbiosis between some corals and algae). Pigments within corals are there to absorb certain wavelengths of light (some that are harmful, thereby protecting coral tissues, and some that are needed in the photosynthetic process). Because corals have a unique combination of pigments compared to algae and other substrates, they can be discriminated once pigment peaks are determined.

With that background in mind, we'll get on to the review of this week's paper, which deals with attempting to discriminate photosynthetic pigments within living corals in the field (i.e., underwater) instead of having to collect coral samples, kill them, and process them in a laboratory for pigment analysis.

The authors point out a lot of the above in their paper's introduction, but when dealing with coral pigments, the important point to keep in mind is that there are three sources of pigments for corals: 1) pigments produced by corals; 2) pigments produced by zooxanthellae living inside coral tissues; 3) pigments by invasive organisms (some algae and sponges bore into living coral skeletons) and diseases. The authors concentrate on zooxanthellar pigments and state that the "major photosynthetic pigments" for this group are "chlorophyll a, chlorophyll c2, peridinin, diadinoxanthin, diatoxanthin, and beta-carotene." From past work in the pigment community, peridinin was shown to be unique to dinoflagelates (the group of algae that include zooxanthellae). For a more technical review of the photo system responsible for photosynthesis in zooxanthellae, look to the full paper.

This paper is unique in that the authors look to the complete suite of coral pigments, which has promise for determining how ratios vary according to environmental conditions (thus potentially helping determine coral reef health, turbidity, or nutrient stress through pigment ratios) or how ratios of pigments vary according to coral species (though no one has yet been able to, or may be able to, determine different species of corals through remote sensing technology). Pigment concentrations are also critical for determining bleaching status of corals and may allow researchers fearing a bleaching event (through tracking satellite images of elevated sea surface temperatures) to track the response of corals in situ to temperature stressors. Not only is this of practical management value, but this would also allow tracking of variable coral species response to heat stress in the field or laboratory.

The researchers studied corals in Hawaii that belonged to 3 growth morphologies: massive, branching, and encrusting. The researchers measured coral reflectance (in the visible wavelengths of 400-700nm) in the field using a spectrometer. They then took micro-samples of coral tissue (6mm diameter using a cork borer) from the area of spectral measurement and processed the samples in the laboratory through techniques shown to release more pigments over time from sample tissues (freezing within 3 hours of sampling at -50ºC for 2 weeks, followed by liquid nitrogen immersion for 2 months). Samples were then prepared for high-performance liquid chromatography (HPLC) analysis of pigment concentrations. Again, for full methods, see the paper.

The authors then used multivariate multiple regression analyses "to determine the ability of spectral data to predict pigment concentration." Basically, the authors were taking their observed measurements of various pigment concentrations and trying to predict coral reflectance at different wavelengths. This means that given a certain combination of pigments (therefore, a pigment-concentration matrix), what will the color reflectance be? If corals have a distinct ratio between their pigments it will be possible (if the above analyses proved significant) to determine that a given sample does or does not come from coral. This is necessary to make the maps shown earlier where corals are shown as distinct from other benthic categories.

In short, the authors demonstrated that all corals (tested) showed 3 statistically-significant pigment reflectance peaks at 575, 605, and 650nm, though strength of peaks varied with coral species, form, and whether bleaching or normal. In other words, corals can be predicted by their pigment ratios (which affect reflectance peaks). The authors also showed that an entire system could be built for pigment analyses of corals for less than US$5,000 total, thereby opening the door for "simple and low-cost spectral measurements to provide for routine in situ monitoring of coral-zooxanthellae status, and eventually for remote sensing of coral reef color." This is great news for beginning researchers and small university laboratories or private field stations looking to contribute to ground-breaking science without breaking the bank.

I hope everyone enjoyed this review and that it provides a motivation to learn more about coral pigments, coral reflectance, and remote sensing of coral reefs. Stay tuned for next week when I'll look at a good paper showing that sometimes a healthy reef has a healthy, diverse, and dominant algal community. This is important because many coral reef managers equate algae with bad nutrient stress, which isn't always the case.

Review: Locker SD, Armstrong RA, Battista TA, Rooney JJ, Sherman C, Zawada DG (2010). Geomorphology of mesophotic coral ecosystems: current perspectives on morphology, distribution, and mapping strategies. Coral Reefs, 29:329-345.

Feature Paper: Locker SD, Armstrong RA, Battista TA, Rooney JJ, Sherman C, Zawada DG (2010). Geomorphology of mesophotic coral ecosystems: current perspectives on morphology, distribution, and mapping strategies. Coral Reefs, 29:329-345.

Author Abstract: This paper presents a general review of the distribution of mesophotic coral ecosystems (MCEs) in relationship to geomorphology in US waters. It was specifically concerned with the depth range of 30–100 m, where more than 186,000 km2 of potential seafloor area was identified within the US Gulf of Mexico/Florida, Caribbean, and main Hawaiian Islands. The geomorphology of MCEs was largely inherited from a variety of pre-existing structures of highly diverse origins, which, in combination with environmental stress and physical controls, restrict the distribution of MCEs. Sea-level history, along with depositional and erosional processes, played an integral role in formation of MCE settings. However, mapping the distribution of both potential MCE topography/substrate and existing MCE habitat is only beginning. Mapping techniques pertinent to understanding morphology and MCE distributions are discussed throughout this paper. Future investigations need to consider more cost-effective and remote methods (such as autonomous underwater vehicles (AUVs) and acoustics) in order to assess the distribution and extent of MCE habitat. Some understanding of the history of known MCEs through coring studies would help understand their initiation and response to environmental change over time, essential for assessing how they may be impacted by future environmental change.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This is a review paper, meaning that it collates current knowledge about a certain topic. These kinds of papers are very useful to scientists and students wishing to know a base of information on a given topic that they are unfamiliar with, as well as providing a good article on a discipline of interest that can be "bookmarked" for following up on specific theories posed.

This paper is important because it deals with a kind of coral reef environment that has generally gone unexplored: the mesophotic coral reef. Colloquially this region is known as the "twilight zone" (a term coined by Dr. Richard Pyle, one of a small group of regular explorers of this zone) because it lies below the normal photic zone (region of highest photosynthetic activity, generally between 0 - 30 meters depth) of reefs, but still has enough light (generally defined as at least 1% of equivalent solar radiation at the surface) to allow some photosynthesis. But because the amount of light is so small, there are only a few zooxanthellate corals (those containing symbiotic photosynthetic algae within their tissues) and algae that can survive. Most of the reef looks quite different, dominated by whip corals and sponges. Fish life, however, is abundant.

As a side note, there are other deep reef communities that survive in cold waters absent from light (generally below 300 meters depth) but we won't be reviewing those communities this week. However, it should be noted that deep water coral communities likely have as much diversity as shallow water coral reefs (with over 2000 species of "hard" corals, only about 850 species are known from tropical coral reefs) and are extremely important for fish communities (and subsequently suffer damage from trawling by deepwater fishermen). For scientists working in the mesophotic deep reef realm, there is now a Mesophotic Research Organization that collects and posts information about that research community.

Okay, so now that the background is out of the way, on to the key points of the paper:

1. Mesophotic reefs are poorly studied. This is because they are too deep for conventional scuba diving (the main research tool of coral reef scientists) but too shallow to be cost-effective for research submersibles. A good overview of this point was made by Dr. Richard Pyle at the TED 2004 conference:

2. The paper gives an overview of the current geographical distribution of known mesophotic reefs, concentrating heavily on the United States (northern Gulf of Mexico, Florida, US Caribbean, the main Hawaiian Islands, American Samoa, and the Mariana Islands). It is clear from the paper that the potential habitat for mesophotic reefs is large, but known mesophotic reefs are few and far between (because of point #1 above).

3. The paper discusses the various habitats where mesophotic reefs can be found, including: continental shelves, isolated banks, terraces along sloping drop offs, deep buttresses and continental shelf-edge spurs and grooves, and relic reefs (often from old sea stands during glaciation periods).

4. "Until recently, little effort has been made to systematically identify, map, and study scleractinian [reef-building] coral reefs found at depths below ~ 30 m."

5. The paper overviews various methodologies used for sampling and mapping mesophotic reefs, noting that they are below the limit of conventional scuba diving and "beyond the scope of airborne and satellite remote-sensing instrumentation." Successful research methods used are acoustic technologies (to map reefs based on distance traveled from surface ship based sounds, such as with swath-mapping systems, multibeam and side-scan sonar, and electromechanical "boomers" such as seismic-reflection profiles), underwater still and video cameras (either with technical scuba divers or remotely operated vehicles, or ROVs), pulsed-laser line imaging to construct a "3D digital representation of the seafloor," other submerged vehicles (ROVs as noted as well as autonomous underwater vehicles, or AUVs, and towed vehicles or sleds with attached instrumentation), and technical scuba diving techniques (discussed by Dr. Richard Pyle in the video above).

6. Geomorphology can limit the "occurrence and distribution of [mesophotic reefs] by providing favorable hard substrates for colonization and by directing the down-slope transport of sediment" that would smother reef corals. Corals and many benthic organisms need hard substrates to survive and grow.

7. And in a sobering note, the authors state that "the single most important issue from a mapping perspective is the lack of knowledge on the potential extent" of mesophotic reefs. Dr. Pyle has also noted that more is known about the surface of the moon than mesophotic reefs and more scientists have surveyed the abyssal deep ocean bottom than "deep reefs."

8. Continuing with mapping and surveying mesophotic reefs will require the use of emerging technologies. Mesophotic reef scientists who use scuba-based technologies either require multiple staged bottles with different gas mixtures (to lessen the effects of nitrogen narcosis or "getting the bends") and technical diving skills or they have switched over to closed-circuit rebreather technology adapted for deep diving.

9. The thickness of mesophotic reef benthic communities is unknown, meaning that the rate of calcification and accretion of corals and other calcifying organisms is also unknown at depths below 30m. This is important because as ocean acidification due to increasing carbon dioxide concentration continues, the effect on mesophotic reefs may be more acute than on shallow-water reefs since calcification is often tied (in corals) to photosynthesis and metabolic rates. In layman terms, too much carbon dioxide will dissolve coral skeletons faster than they can be laid down, destroying reefs even without conventional pollutants like increased nitrogen or phosphorous.

And on that note, next week we'll look at labratory experiments meant to determine the harm that decalcification of living coral skeletons has on coral organisms.