Scott Reef, an Isolated Coral Reef System, Quickly Recovered from a Severe Disturbance

Isolated coral reefs are often thought to be exceedingly vulnerable to catastrophic disturbances such as cyclones and bleaching events because they do not have an external supply of larvae recruits. Gilmour et al.(2013) studied Scott Reef, an isolated reef 250 km off the coast of Western Australia that suffered mass mortality following a bleaching event in 1998. Twelve years after the bleaching event, live coral cover in Scott Reef had increased by 35 percent despite extremely low (six percent) recruitment rates for the first six years of recovery. Their results show that isolated reefs are able to recover relatively quickly from severe disturbances even though they have limited connectivity—suggesting that benefits of being away from constant human pressures such as pollution and overfishing may outweigh benefits of access to large quantities of larval recruits.—Kelsey Waite
James P. Gilmour, Luke D. Smith, Andrew J. Heyward, Andrew H. Baird, and Morgan S. Pratchett. Recovery of an Isolated Coral Reef System Following Severe Disturbance. Science 5 April 2013: 340 (6128), 69-71.

Gilmour and colleagues studied Scott Reef, a large (~650 km2) and isolated system of reefs that is more than 250 km from the mainland of Australia. There is negligible commercial and recreational fishing at Scott Reef, and the reefs isolation means it is free of many other anthropogenic pressures as well. Permanent transects were established in regions of Scott Reef in 1994, and data has been collected according to the Australian Institute of Marine Sciences Long Term Monitoring protocols every year since.
Water temperatures at Scott Reef rose quickly in February of 1998 and remained above average for another two months—the estimate of the severity of the anomaly was 13.3°C, which is the most severe anomaly ever recorded at Scott Reef. Over the next six months, a massive bleaching event occurred in which 80–90 percent of live coral on reef crests (~3m) and reef slopes (~9m) died, while live coral cover on upper reef slopes (~6m) decreased 70 percent. This extreme decrease in coral cover was followed by recruitment failure—over the next six years recruitment rates were less than six percent of what they had been prior to the bleaching event, meaning almost all initial coral growth was from local colonies that had survived the bleaching. Mean survival of recruits was between 83 and 93 percent each year during the recovery period, which is much higher than the mean survival of recruits (less than 50 percent) on reefs that are constantly experiencing anthropogenic pressures.
Gilmour and colleagues also suggest that the corals in Scott Reef would have likely recovered even faster if they hadn’t experienced a series of more moderate disturbances: two cyclones, an outbreak of disease, and a second less severe bleaching event. Their results show that coral reefs with negligible supply of larvae from outside sources (isolated reefs) can recover quickly from disturbances in when they are not under chronic anthropogenic pressures—suggesting that addressing these local pressures from human activities such as overfishing and pollution will help promote resilience against global degradation of coral reefs. 

Overabundance of Algal Symbionts Increase Susceptibility of Coral Bleaching

Increasing water temperatures due to global climate change are causing mass declines in live coral cover due to coral bleaching. Understanding the factors that make corals more susceptible to bleaching is important in managing protection efforts. However, not much information is currently known about the relationship between algal symbiont density and coral bleaching. Cunning and Baker (2012) show that symbiont cell ratio density is dependent on both environmental conditions and symbiont type. Their results support previous research suggesting that higher densities of symbionts do not buffer coral’s thermal tolerance and in turn, corals with a higher abundance of symbionts are more susceptible to bleaching events.—Kelsey Waite
Ross Cunning, Andrew C. Baker. Excess algal symbionts increase the susceptibility of reef corals to bleaching. Nature Climate Change, 2012; 3: 259–262.

                  Cunning and Baker collected 53 colonies of Pocillopora damicornis from shallow reefs (3–6 feet depth) at Taboga, Panama. Corals were maintained in an outdoor tank for a six-month period of natural seasonal warming that ended with a bleaching event. Branch tips were collected from each colony three times during the warming period and twice during the bleaching event. Symbiont DNA was then extracted and Symbiodium (algal) types were indentified. Using quantitative PCR, symbiont cell ratio densities—a ratio of the total number of symbionts to the total number of host cells—were calculated for each sample.
                  Cunning and Baker found that symbiont cell ration densities increased with warming but decreased with the onset of acute bleaching. As coral host cells are lost during warming, symbionts are expelled to keep cell ratio densities below a certain threshold. Although both symbiont and host cell densities decrease during warming, there is a greater net loss of host coral cells, resulting in an increased symbiont to host cell ratio. Since symbiont cell ratio density decreases during thermal stress (bleaching events) yet increases during natural warming, Cunning and Baker suggest the cellular and molecular underpinnings of these losses are different.
                  Coral colonies with a high density bleached more severely than colonies with low density, regardless of the symbiont type. This difference is attributed primarily to the fact that corals with more algal symbionts will produce more reactive oxygen species (ROS), which trigger bleaching—suggesting that corals with high symbiont cell ratio densities are more vulnerable to climate-change-induced bleaching. Furthermore, the severity of bleaching events may be influenced by stressors such as ocean acidification and poor water quality, which change symbiont density. Managing these stressors can significantly help corals survive climate change. 

Decline in Carbonate Production Threatens Coral Reef Growth

Corals provide complex three-dimensional habitats that sustain some of the planet’s most biologically diverse ecosystems and act as protective barriers for adjacent shorelines. However, coral reefs have been declining globally over the last few decades. Since the mid 1970s, coral cover has decreased nearly 80 percent in the Caribbean. Perry et al. (2013) show that current carbonate production rates are below historical (Holocene) values—current production rates are 50 percent lower than historical values and 37 percent of reefs surveyed were net erosional. Perry and colleagues suggest that there is an ecological threshold of about 10 percent live coral that is critical to maintain positive carbonate production rates. If a reef falls below 10 percent coral cover, it is likely that its production rate will become negative and the reef will start to degrade.—Kelsey Waite
Chris T. Perry, Gary N. Murphy, Paul S. Kench, Scott G. Smithers, Evan N. Edinger, Robert S. Steneck, Peter J. Mumby. Caribbean-wide decline in carbonate production threatens coral reef growth. Nature Communications, 2013; 4: 1402, 1–7.

                  Perry and colleagues measured current rates of reef carbonate production and bioerosion in 101 transects spanning 19 individual reefs and four countries (Bahamas, Belize, Bonaire, and Grand Cayman) in the Caribbean—Data was collected within common habitats including nearshore hardgrounds, Acropora palmatehabitats, Montastraea spur-and-groove zones, fore-reef slopes, and deep (18–20m) shelf-edge Montastraea reefs. The majority of the reefs studied had a live coral cover of around 25–30 percent, a high macroalgal abundance, and a low abundance of substrate grazing taxa (parrotfish and urchins). At each site, the ReefBudget census-based methodology was used to measure the rate of biologically driven carbonate production and erosion.
                  Perry and colleagues show that net carbonate production rates vary between different habitats studied, as well as within the same habitat. Overall, 21 percent of reefs studied had net negative budgets and 26 percent had net positive budgets, but had net rates lower than 5G (kg CaCO3m-2year-1). The highest producing reef (+3.63G at 5m and +9.53G at 10m) was located in the ‘no dive reserve’ in Bonaire. Historical (mid and late-Holocene periods) carbonate production rates are reported to be in the range of 10–17G, suggesting that current rates of production are less than half of what they used to be. Furthermore, Perry and colleagues suggest that their data may be underestimating the rate of erosion (endolithic bioerosion is challenging to quantify accurately), so the actual net production rates may be lower than calculated.
There was a significant correlation between live coral coverage and net production rates—suggesting that declines in coral cover and changes in benthic community composition are compromising coral reef accretion and carbonate production rates. Perry et al. suggest a threshold of roughly 10 percent live coral cover in order to maintain a positive reef production budget. 

2500-Year Collapse of Eastern Pacific Coral Reefs Caused by Variability in the El Niño-Southern Oscillation

Two main environmental drivers affect coral assemblages in the tropical eastern pacific (TEP): upwelling and the El Niño-Southern Oscillation (ENSO). Upwelling occurs on a seasonal scale and brings low water temperatures and reduced pH levels, causing a decrease in coral growth. Warm waters associated with multiannual El Niño events are linked to coral bleaching and sudden coral death. Together, these two drivers were predicted to be the reason for poor coral reef framework development in the tropical eastern pacific during the Holocene. Toth et al. (2012) examined the history of reef framework construction (both tempo and mode of framework) in relation to seasonal upwelling and ENSO. They found that coral reefs in TEP collapsed for 2500 years—roughly 40% of their existence—beginning about 4000 years ago. The pacific-wide phenomenon corresponds to a period of increased variability in ENSO activity, suggesting that ENSO was the main driving force behind the collapse.—Kelsey Waite
Toth, L.T., Aronson, R.B., Vollmer, S.V., Hobbs, J.W., Urrego, D.H., Cheng, H., Enochs, I.C., Combosch, D.J., van Woesik, R., Macintyre, I.G., 2012. ENSO Drove 2500-Year Collapse of Eastern Pacific Coral Reefs. Science 337, 81–84.

            Toth and colleagues removed 14 push-cores from subtidal reef-slope habitats on three reefs across Pacific Panamá. Each reef had a distinct upwelling pattern—Isla Contadora experiences intense seasonal upwelling while Isla Iguana has intermediate upwelling and Isla Canales de Tierra has no upwelling. Cores were sectioned and sorted by species and taphonomic condition to identify different modes of development. Layers were also dated by C14 analysis, accelerator mass spectrometry, and U/Th analysis by inductively coupled plasma mass spectrometry.
            Toth et al. determined reef growth had begun 6900 calibrated calendar years before the present (cal yr B.P). They found a narrow interval within the coral’s history in which the reef was dominated by Psammocora stellata, which does not branch and build framework. The interval also contained degraded Pocillopora rubble, indicating that the reef was degrading. Core dating suggests the reefs stopped growing by 4332 cal yr B.P. and recovered by 2384 cal yr B.P. The hiatus observed in coral framework cores corresponds to a period of increased climatic variability. Beginning 4500 to 4000 cal yr B.P., ENSO frequency and intensity increased. This was the same time of the onset of the hiatus demonstrated in the cores. Reconstructions show that El Niño events between 4000 and 2000 cal yr B.P. were among the strongest in the Holocene due to the coupled influence of ENSO and the Intertropical Convergence Zone.
            Toth and colleagues propose that shifts in the frequency and intensity of ENSO were the main cause of the hiatus shown in Panamá and possibly the rest of the Pacific—the intensity of seasonal upwelling acted only as a second-order process. Recently, ENSO activity has devastated coral reefs. At the current rate of warming, reefs in the tropical eastern pacific may be at risk of another collapse.  

Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds

Coral reefs, some of Earth’s most important and fascinating ecosystems, are in global decline—coral cover in has declined nearly 80 percent in the Caribbean and nearly 50 percent in Australia’s Great Barrier Reef. Branching corals such as Acroporids are essential to the growth of coral reefs because they provide topographic complexity, among which many reef species depend. Herbivorous fishes also play a key role in coral reef ecosystems by gnawing on competing algae and facilitating the colonization and growth of new corals after disturbances such as cyclones or disease. Dixson and Hay (2012) show that symbiotic gobies defend the common coral Acropora nasuta by removing allelopathic alga. The process is mediated by chemical signals and cues and may be disrupted or reversed by changes in ocean environment such as pH. Results of this study are not only interesting, but also present the first example of a species chemically cuing consumers to remove competitors.—Kelsey Waite
Dixson, D.L., Hay, M.E., 2012. Corals Chemically Cue Mutualistic Fishes to Remove Competing Seaweeds. Science 338, 804–807.

            Dixson and Hay studied how coral-dwelling fishes affected seaweed-coral interactions. They placed Chlorodesmis fastigiata (an allelopathic seaweed) on the common coral Acropora nasuta, near four different commensal fishes. Coral health was analyzed using pulse-amplitude modulated fluorometry. In field experiments, transects were run across the reef to evaluate goby occupancy. Gobies were found in 81 ± 16% of the 207 colonies assessed.
              After three days of contact with C. fastigiata, the effective quantum yield of the corals had been suppressed by roughly 80%. Most common coral colonies host at least two gobiids, which remain in that same colony for most of their adult life. In coral colonies occupied by gobies, the harmful effect of C. fastigiata on A. nasuta declined by 70–80% compared to those colonies without gobies. Dixson and Hay also determined that gobies only responded to chemical cues from Acropora nasuta, not any other—even closely related—coral species. When the coral cues gobies, the gobies begin removing alga within minutes of seaweed contact. As reefs continue to evolve due to climate change, it is important to understand interactions that enhance survival and re-growth of coral. Chemically mediated behaviors such as these can potentially be suppressed or reversed if the ocean environment changes. 

Growth of Western Australian Corals in the Anthropocene

Human interference with the global climate system can cause changes in coral growth rates by altering the coral’s physical or chemical environment. Currently, the atmospheric carbon dioxide levels are around 390 parts per million (an increase of 40% since preindustrial times). As atmospheric carbon dioxide levels rise, the oceans absorb most of it. In the process known as ocean acidification, carbon dioxide alters the seawater carbonate equilibrium and leads to a reduced pH and carbonate saturation state—both of which are currently thought to have detrimental effects on reef-building corals and other marine calcifers. Cooper et al. (2012) examined significant changes in coral calcification of southeast Indian Ocean corals over the last 110 years and related those changes to known changes in sea surface temperature (SST). Looking at a latitudinal range of 11 degrees, they determined that there was no widespread decline in calcification rates during the 20thcentury. Results of this study suggest that ocean acidification is not currently limiting the calcification of coral reefs on a global scale and that thermal changes, not ocean acidification, appear to be the largest climate change threat to reef-building corals at this time. —Kelsey Waite
Cooper, T.F., O’Leary, R.A., Lough, J.M., 2012. Growth of Western Australian corals in the Anthropocene. Science 335, 593–596.

            Cooper and colleagues collected twenty seven long cores from massive Porites sp. colonies at six locations (covering about 1000 km) off the coast of Western Australia. All of the cores were collected from colonies larger than 2 m in height and were at a depth of at least 6 m below the lowest astronomical tide. Cores were then analyzed to determine spatial and temporal variations in annual extension, skeletal density, and calcification rate (the product of skeletal density and annual extension). Calcification anomalies were calculated as the percent difference between the annual calcification rate and the long-term period 1900–2010. Linear and non-linear regression models were used to investigate the relationship between time, SST, and location in comparison to both calcification averages and calcification anomalies.
            Cooper et al. found no consistent pattern of declining calcification rates over time. In fact, the two most southern regions had the greatest recent SST warming and showed a significant increase in calcification. The two northern regions had a much lower rate of recent SST warming but showed no significant change in calcification rates over time either. Since the southern regions showed an increase in calcification despite an increasing temperature, the findings of Cooper and colleagues suggest that ocean acidification is not yet limiting the calcification of coral reefs uniformly on a global scale. 

Causes of the 27-year Decline in Coral Cover on the Great Barrier Reef

The Great Barrier Reef is the largest coral reef ecosystem in the world, consisting of over 3,000 individual reefs and spanning across an area of 345,000 square kilometers. Although these reefs are considered some of the least threatened reefs in the world, they are degrading at a concerning rate due to severe habitat disturbances such as crown-of-thorn starfish (COTS) predation, coral bleaching, declining growth rates caused by high temperatures, terrestrial runoff, tropical cyclones, and coral diseases. De’ath et al. (2012) show that mean coral cover on the Great Barrier Reef declined from 28–13.8% between 1985 and 2012. Two-thirds of that decrease occurred after 1998, suggesting that coral is dying at an increasing, non-linear rate. Furthermore, rates of coral calcification on the Great Barrier Reef have been declining due to thermal stress and ocean acidification, so reefs have been taking longer to recover from disturbances. At the current rate of disturbance and re-growth, De’ath and colleagues predict that coral cover on the Great Barrier Reef will likely decline to 5–10% by 2022. However, in the absence of cyclones, COTS, and bleaching, these reefs have the potential to increase coral cover by roughly 2.85% each year. Results of this study demonstrate the need to mitigate global warming and ocean acidification in order to maintain the biodiversity and ecological integrity of the Great Barrier Reef.—Kelsey Waite
De’ath, G., Fabricius, K.E., Sweatman, H., Puotinen, M., 2012. The 27–year decline of coral cover on the Great Barrier Reef and its causes. Proceedings of the National Academy of Sciences 109, 17995-17999.

            De’ath and colleagues used coral cover and densities of COTS surveyed around the perimeter of 214 individual reefs throughout the Great Barrier Reef. The data were obtained from the AIMS Long-Term Monitoring Program, which recorded 2,258 reef surveys between 1985 and 2012. All data analysis was done using logistic regression models. The first set of analyses modeled temporal change in coral cover, and how it differed among the northern, central, and southern regions of the Great Barrier Reef. The second set of analyses included the effects of disturbances such as cyclones, COTS, and bleaching.
            De’ath et al. show a significant decrease in hard coral cover, from 28–13.8%, over the last 27 years. This major decline means that tens of thousands of species that live in, or depend on the reefs are losing their natural habitat. Additionally, the rate at which the coral is dying has been increasing substantially since 2006. Disturbance from COTS, cyclones, and massive bleaching events have caused periodic, random fluctuations in variation of coral cover, but have not been linked to any systematic long-term effects in variation over the past 27 years. De’ath and colleagues suggest that the Great Barrier Reef is headed in the same direction as the reefs in the Caribbean, which have been declining at a rate of roughly 1.4% each year (GBR since 2006 is roughly 1.45% each year). The central and southern regions of the Great Barrier Reef are likely to decline to 5–10% coverage by 2022 if there is not a significant change in the rate of disturbances and coral growth. However, in the absence of these disturbances, the Great Barrier Reef has the potential to recover at a relatively fast rate of about 2.85% every year. 

Great Barrier Reef Corals Show Flexible Assembly Rules across a Steep Climatic Gradient

Coral Reefs are some of the most fascinating, important, and vulnerable ecosystems in the world. They hold a high environmental value due to species abundance and biodiversity, containing more species per unit area than any other marine environment. Healthy reefs also contribute to local economies through tourism and fishing. Over the past few decades, coral reefs have been degrading due to climate change and other anthropogenic causes such as overfishing and pollution. Hughes et al. (2012) evaluated the composition of assemblages in Australia’s Great Barrier Reef using multiscale sampling and analyses determined that the assembly rules, a set of ecological rules determining patterns of assemblage composition, of corals in the Great Barrier Reef are flexible, and do not change in response to latitudinal climatic drivers. Because the diverse pool of species they sampled are able to assemble in different configurations across a large range of environments, Hughes and colleagues support the hypothesis that coral reef assemblages will change extensively in the future, but not necessarily collapse due to climate change as long as greenhouse gas emissions are reduced sufficiently.—Kelsey Waite
Hughes, T.P., Baird, A.H., Dinsdale, E.A., Moltschaniwskyj, N.A., Pratchett, M.S., Tanner, J.E., Willis, B.L., 2012. Assembly rules of reef corals are flexible along a steep climatic gradient. Current Biology 22, 736-741.

Hughes and colleagues studied regional scale patterns in the composition of coral reef formations in Australia’s Great Barrier Reef. They used a multiscale sampling approach in which 33 reefs, within five different regions, were sampled over a 12-month period. The coral composition, as well as the number of coral colonies and percent of coral coverage was taken at each reef. Over 35,000 colonies were categorized into 12 ecologically relevant groups (taxa) depending on their physiology, morphology, and life history. The variation in abundance of each of these 12 groups was then analyzed across the five regions.
Coral assemblages have two main habitats: reef crests (1–2m depth) and reef slopes (6–7m depth). The characteristic faunas of reef crests and reef slopes differ greatly. Hughes et al. suggest that assemblages do not show an increasing or decreasing trend with latitude or latitude-related temperature gradients since different taxa flourish while others decrease or remain constant in abundance as the environment changes. On crests, 9 of the 12 taxa varied among the five regions, while 7 out of 12 varied on the reef slopes. Only one of the 12 taxa was uniformly abundant on both the crests and slopes, the other 11 taxa showed significant spatial variation. Hughes and colleagues suggest that this spatial variation may be due to disturbance events such as cyclones, crown-of-thorn starfish predation, episodes of bleaching, or to pulses of recruitment by more dominant species.