Coral Reefs Protect Coastal Communities from Wave Energy Impacts

Future climate change is expected to increase the rate and severity of oceanic disturbances. Among the damages caused by disturbances such as tsunamis and storms, they create high-energy waves that damage coastal communities and ecosystems. Coral reefs and other coastal ecosystems such as sea grasses and mangroves have been recognized for their protection against the devastating effects of strong waves. Although structures such as seawalls have been built to protect coastal communities, their cost and short lifespan reduce their effectiveness as long-term solutions. In this study, Villanoy et al. (2011) seek to measure the wave energy dissipation provided by coral reefs while factoring future climate change. Using a model simulation of storm generated waves on a Philippine reef, the team of researchers measured changes in wave energy caused by varying monsoonal wind forcing and storm conditions. The results suggest that extensive reef system in the area helped dissipate wave energy, thus reducing wave run-up on land. Furthermore, a significant reduction in wave energy was observed when accounting for stronger wind and higher sea level, as well as under a non-climate change scenario. This study demonstrates that it is imperative to manage coral reef ecosystems sustainably and promote localized water quality management in order to mitigate the adverse effects on coastal communities.— Cecilia Ledesma
Villanoy, Cesar, David, Laura, Cabrera, Olivia, Atrigenio, Michael, Siringan, Fernando, Aliño, Porfirio, Villaluz, Maya. (2011) Coral reef ecosystems protect shore from high-energy waves under climate change scenarios. Climate Change doi: 10.1007/s10584-012-0399-3.

            Cesar Villanoy collaberated with colleagues at the Marine Science Institute to study the effect of coral reefs on wave propagation. The research focused on Bagacay and Rizal, two barangays located in the Municality of Sorsogon, Phillipines. The computer simulation software DELFT3D- WAVE was used to simulate wave propagation along that area and under different climate change scenarios; factoring in high resolution coastline and bathymetry from digitized maps, the model computes aspects of wind energy such as wave propagation, wave generation by wind, wind field, and water level. Bathymetry data was accessed and digitized from available navigational charts and topographic maps. Additionally, historical wind data was gathered using the QuickSCAT satellite. By measuring changes in wave energy dissipation rates and wave bottom orbital velocities, the simulations are able to determine changes in wave characteristics that are influenced by local wind and storm conditions, changes in relative seal levels, and presence of reefs. The wave orbital velocity near the bottom is used as a measurement of the energy available at the bottom for sediment transport and reworking.
            Results from the wave model simulations clearly demonstrate the importance of the coral reef areas in dissipating wave energy. While the highest dissipation takes place along the reef edges, parts of the coastline not fringed by coral reefs experienced lower energy dissipation. As a result, higher energy dissipation significant reduced the height of incoming waves across the reef edge, and lower energy dissipation areas were impacted by relatively bigger waves. Bagacay and Rizal are oriented along the southeast and northeast in the Pacific Ocean, respectively. The difference in orientation of the two areas caused different patterns in wave bottom orbital velocities. As a result, Bagacay is shown to be more sheltered than Rizal during northeast monsoon. In addition, increasing the wave height from 2 m to 4 m to simulate extreme events also increased the bottom orbital velocity. For Rizal, high bottom orbital velocity found near the town center of Gubat was consistent with observations of eroded beach and exposed tree roots. Meanwhile, significantly lower bottom orbital velocity near Rizal Beach coincided with a relatively stable beach, implying that an increase in wave height by 1-2m will have enormous consequences to wave energy reaching the coastline of Bagacay and Rizal.
             When simulating sea level rise along the edges of the reefs, the wave dissipating effects of the reefs were decreased and a higher proportion of wave energy was able to propagate across the reef and onto the coast. Assuming an increase in wave bottom orbital velocity did not mitigate the adverse effects caused by an increase in sea level. The conclusions from this study suggest that coral reefs contribute significantly to the protection of coastal communities from wave impacts. However, future increases in sea levels will likely hinder the ability of reef ecosystems to serve as adequate wave barriers. The authors stress the need to continue and enhance efforts to manage and protect these coastal ecosystems through sustainable use and preservation of biodiversity and ecosystem functions.

Local Management Options for Effective Conservation of Caribbean Coral Reefs

The degradation of coral reef ecosystems results from the combined impacts of global climate change and local activities. Although improvements to global warming trends are unlikely to occur in the immediate future, several conservation tools exist for coral reef managers. In this study, Edwards et al. (2010) examine the role that fisheries conservation tools can play in promoting the recovery  of Caribbean coral reefs from disturbance events. Using an individual-based model of ecological factors and accounting for spatial differences, they test the impact of disturbances on coral populations. Simulations for contrasting regions throughout the Caribbean demonstrate that regional differences in hurricane frequency cause different spatial patterns of reef health. While greater “patchiness” of coral cover occurred in Belize, more frequent disturbances in the Bahamas increased coral cover. Although substantial variation exists among regions, coral bleaching is shown to contribute to the decline in coral reef health over time. Additionally, the protection of herbivores fails to prevent reef degradation, but it does delay rates of coral loss over the tested time period. These results demonstrate that the impact of local conservation measures on reef ecosystem will vary spatially and temporally. Cecilia Ledesma
Edwards, H.J., Elliott, I.A., Eakin, C.M., Irikawa, A., Madin, J.S., McField, M., Morgan, J.A., van Woesik, R., Mumby, P.J. (2010) How much time can herbivore protection buy for coral reefs under realistic regimes of hurricanes and coral bleaching? Global Change Biology 17, 2033-2048.

            Helen Edwards collaborated with a number of scientists to investigate potential management interventions for preventing future degradation of coral reef ecosystems. They demonstrate that by altering the management of local factors, the vulnerability of coral reefs to changes in temperature, storms, and ocean chemistry can be reduced. By establishing a spatially realistic model of climate-change impacts on Caribbean reefs, they were able to study the extent to which local changes in grazing can influence their dynamics. An individual-based model of the Montastraea annularis zone of a Caribbean coral reef was used to model the ecological dynamics after coral populations expereienced spatially-realistic disturbances. The simulations were carried out for three contrasting regions of the Caribbean, including Belize, Benaire, and the Bahamas. Climatologies  were acquired from the US National Oceanographic and Atmospheric Administration (NOAA) and future sea surface temperatures (SSTs) were derived from the SERS A1 scenario. Although scenario A1 describes a “worst case” scenario, the authors argue that it is an appropriate prediction since current growth rates of CO2 emission exceed this trajectory.  In order to model bleaching caused by elevated temperatures, the projected temperatures and SST were then used to calculate the number of degree heating months (DHMs). Since the response of coral to bleaching events has not yet been monitored for areas in the Caribbean, Edwards et al. instead examined the response of nonacroporid Pacific species that best compared with corals in the Caribbean. Data on the frequency of past storms were used to simulate predicted environmental conditions at reef locations across the Caribbean. Changes in coral cover within reef communities were measured for a 90-year period in which bleaching and hurricanes occur.
            The results from this study show that the frequency of predicted thermal stress events increased with time. Although no severe bleaching events were predicted for the Bahamas between 2012 and 2039, sites in Belize experienced severe bleaching events between 2060 and 2099. Meanwhile, sites in Bonaire experienced few severe events before 2070. In the areas with high rates of hurricane incidence the coral cover was generally lower and more variable, increasing overall net-reef recovery. When grazing was carried out by an unexploited community of parrotfishes and reefs were only impacted by coral bleaching, coral cover maintained its initial state for approximately the first 40 years, after which it steadily declined. When hurricanes were modelled in combination with bleaching events, reefs in each region of the Caribbean exhibited net decline. Contrary to the authors’ expectations, the presence of a community of urchins did not prevent reef decline under global warming. Overall, regions across the Caribbean characterized by very different physical disturbance regimes showed a similar difference in the impacts from local disturbances, thus resulting in spatial variation in predicted coral cover.
            Of significant consequence, this study found that although the rates of decline vary substantially with geographic location, climate change impacts are detrimental throughout the Caribbean. Spatial variation in hurricane frequency and strength led to significant differences in disturbance regimes in the Bahamas and Belize. Meanwhile, analysis showed that Bonaire has the least damaging disturbance environment of the three regions. Furthermore, this study does not take into account that the frequency or severity of storms may change in the future, potentially underestimating the negative impacts of hurricanes on reef health. Even for the highest grazing scenarios with stable levels of coral cover, coral cover declined once coral bleaching was added to the disturbance simulation. This underscores the critical need to reduce global greenhouse gas and carbon dioxide emissions.

Integrated Management Actions that Safeguard Coastal Coral Communities Against Climate Change

A strong correlation is known to exist between coral bleaching and anthropogenic climate change. Expected increases in sea surface temperatures (SST) are predicted to drastically affect the biodiversity, ecology, and tourism of coral reef ecosystems. The coastal reefs in the Great Barrier Reef (GBR) are particularly susceptible to the threat from increased ocean warming and acidification; estimates suggest that coastal reefs in the central GBR area may lose a large portion of  coral structures by as early as 2030. Additionally, studies suggest that poor water quality may exacerbate effects of heat stress by lowering the thermal bleaching threshold of certain coral species. For this reason, the study by Wooldridge et al.(2011) seeks to identify appropriate management interventions that would influence thermal tolerance, improving coral reef survival rates from thermal bleaching. Using dissolved inorganic nitrogen (DIN) levels as a measure of water quality, the authors study whether elevated DIN concentrations exacerbate the damage to reef structure caused by thermal bleaching. Furthermore, they develop a modelling framework that maps coral bleaching risks when considering two factors: local land management and global warming; this modelling framework is used to support coral reef management  that aims to increase climate change resilience of reefs through localized water quality management. 
Wooldridge, S.A., Done, T.J., Thomas, C.R., Gordon, I.I., Marshall, P.A., Jones, R.N. August 2011. Safeguarding coastal coral communities on the central Great Barrier Reef (Australia) against climate change: realizable local and global actions. Climatic Change, doi: 10.1007/s10584-011-0229-z.

            Scott Wooldridge collaborated with various scientists to identify appropriate coral reef management options for the central Great Barrier Reef (GBR) in Australia. Wooldridge et al. employed two different software packages in order to model changes to reef ecosystems. “ChloroSim,” a decision support tool, was used to model the beneficial effects of reductions in dissolved inorganic nitrogen (DIN) concentrations. ChloroSim functions by providing a relationship between DIN concentrations, flood intensity, and bloom of phytoplankton biomass for different regions. Additionally, the software “ReefClim” was used to map future sea surface temperature (SST) warming patterns on the GBR. Using eight different climate models, “ReefClim” provides regional-scale SST estimates for two alternative global COemission scenarios: “no mitigation” and CO2  mitigation leading to atmospheric CO2  concentrations of 450 ppm. The SST projections were then combined with coral mortality thresholds under different DIN to simulate future coral mortality rates up to 2100.
            The results provided by this study suggest that reducing end-of-river DIN levels can considerably increase the future survival rates of locally-impacted reefs on the GBR. The simulations indicate that the potential improvement in bleaching threshold for the reef sites with the worst water quality typically require large reductions in DIN. For example, an 80% reduction in DIN results in the maintenance of the coral-dominated reef state for 50 years beyond the current coral mortality projections under the “no mitigation” warming scenario. In addition, under a CO2 mitigation warming scenario, a 50% reduction in DIN ensures the long-term survival of the impacted reefs. Thus, a combination of local and global management can help guard the long-term viability of hard coral communities within the central GBR. Regionally, the study found that the projected SST warming in the central-southern GBR is proportionately higher than the rest of the GBR.At the local level, the authors propose best methods to achieving reduced DIN concentrations; they maintain that local agricultural policy should focus on no over-application of fertilizers, reduced tillage, split fertilizer application, and removing production from the least productive soil types; doing so would eliminate over 80% of agricultural DIN exports, such as fertilizer loss. Wooldridge et al. emphasize the need for a global climate policy agreement between the largest CO2-emitting countries that will ensure atmospheric CO2 levels stabilize below 450 ppm.
            A key finding of this study points to lowered DIN levels in reef waters as crucial for enhancing reef survivability during thermal stress, and aiding its recovery afterstress. Given that DIN loading is typically lowest at mid-shelf locations, it is predicted that the mid-shelf reefs of the central GBR should display the highest resistance to heat stress; therefore, this area has important implications for the designation of a future network of marine protected area (MPA). Additionally, the results indicate that the current bleach frequency in many coastal reef communities on the central GBR inhibit the reef-building capacity of these ecosystems. This further emphasizes the significant role of good water quality in enhancing reef-building capacity between distrubance events. 

Determining Appropriate Coral Reef Management Policies Based on Multiple Interacting Stressors

Coral reef ecosystems are affected by various anthropogenic and natural disturbances, potentially crippling the resilience of the ecosystem. For this reason, identifying appropriate reef management policies is largely dependent on the interaction between multiple stressors. Notably, in Caribbean coral reefs, fishing efforts and hurricane impacts on local areas threaten the future sustainability of reefs. In this study, Blackwood et al. (2011) seek to measure the combined impacts of multiple stressors, including coral-algal interactions and grazing by herbivorous reef fish. The authors developed an analytic model that considers changes in structural complexity, the direct impacts of hurricanes, and various levels of fishing efforts. The model simulations focus on the effects of these interacting elements for specific geographic locations. However, in order to simplify the investigation, this study focuses solely on relatively short times scales, and does not account for changes in ocean temperature, chemistry, or sea level rise. The results enable the authors to determine whether the stressors behave synergistically, and to predict potential coral reef recovery patterns from interacting stressors. Additionally, they propose appropriate management policies, recommending either local reef restoration or fisheries management based on their results.
Blackwood, J. C., Hastings, A., Mumby, P. J. 2011. A model-based approach to determine the long-term effects of multiple interacting stressors on coral reefs. Ecological Applications 21, 2722-2733.

            Julie Blackwood from the University of California, Davis, collaborated with various scientists to produce this study. The research conducted is an extension of previous work  quantitatively measuring coral-algae interactions using an analytic model. Two distinct states of coral recovery were investigated: the complexity of reef structure after having suffered an acute disturbance such as a hurricane, and coral recovery from events such as bleaching or disease in which the reef structure remained intact. Rather than solely focusing on states of high coral cover and coral depletion, the authors included parrotfish dynamics and their mortality from fishing rates. The numerical responses of parrotfish to changes in habitat conditions are measured simultaneously with food limitation on parrotfish abundance. This allows the authors to determine the long-term effects of multiple coral stressors.
            Here, rugosity is defined as the structural complexity of a reef ecosystem. This analysis accounts for slow changes in rugosity that occur between hurricane impacts, direcly affected by the resilience of reefbuilding corals and by bioerosion. Hurricane impacts are measured as “jump processes,” meaning that they occur randomly and the impacts are instantaneous. Furthermore, it is assumed that recovery trajectories from hurricanes immediately resume to prehurricane rates. Three different levels of damage to coral reefs are then simulated, consideringgeographic differences. The authors assume that hurricanes reduce coral rugosity by 50% by impacting reef structure, 5% by pruning out coral colonies, and an intermediate case where a hurricane has the same proportional effect on rugosity as coral cover. Varying hurricane frequencies are considered in order to identify diverse geographic conditions, and are simulated for a 100 year time period.
            The results from this study suggest that different management guidelines are necessary for different regions in the Caribbean. Given differences in local environmental conditions and hurricane frequencies, the resilience of reef ecosystems to disturbances is highly variable. Generally, there appears to be greater resilience to fishing efforts for higher frequency hurricanes only when hurricane damage levels and the growth rate of macroalgae are sufficiently low. Otherwise, higher impact levels cause substantial damage to the strucutural complexity, reducing refugia for parrotfish and limiting the ecosystem services provided by the corals. However, when macroalgal growth rates are high, even regions with low frequency hurricanes have strong resilience to fishing effort. These results indicate that in regions with frequently occurring hurricanes, management should put greater emphasis on determining whether reef restoration methods are cost effective. Additionally, the authors maintain that managers should adapt their policy implementations according to shifts in local hurricane frequencies. For example, reefs experiencing low frequency hurricanes demonstrate less resilience to fishing effort, therefore there should be greater emphasis on fishing regulations to promote resilience. Since all levels of hurricane frequencies threaten the structural complexity of reef ecosystems, future studies need to focus on the impacts of global warming. Previous studies suggest that global warming will likely increase hurricane intensity,resulting in increased relevance of policy recommendations of high hurricane frequencies through time.

Social Vulnerability of Coastal Communities to Impacts of Climate Change on Coral Reef Ecosystems

Coral reefs play a central role in the economies of many coastal communities. Whether through fisheries or tourism, reefs contribute to the incomes and livelihoods of millions of people. For this reason, the impact of climate change on coral reef ecosystems consitutes a critical concern for governments and scientists alike. In this study, Cinner et al. examine the social vulnerability of fisheries dependent communities to changes to coral reef ecosystems. They focus on three dimensions of vulnerability, exposure, sensitivity, and adaptive capacity, in order to better gauge the source of vulnerability. They found that key sources of vulnerability differ considerably within and between the five countries studied. Additionally, this study illustrates how these differences differ from site to site, enabling the authors provide a framework of site-specific policy actions aimed at reducing different aspects of vulnerability.–Cecilia Ledesema
Cinner, J. E., T. R. McClanahan, N. A. J. Graham, T. M. Daw, J. Maina, S. M. Stead, A. Wamukota, K. Brown, and Ö. Bodin. 2012. Vulnerability of coastal communities to key impacts of climate change on coral reef fisheries. Global Environmental Change, 12−20.

            Josh Cinner from the ARC Centre of Excellence for Coral Reef Studies collaborated with various scientists to produce this study. Research was done between 2005 and 2006 at 29 sites located throughout Kenya, Tanzania, Seychelles, Mauritius, and Madagascar. Diverse sites were selected in order to provide a spectrum of social and environmental conditions. The data obtained for each site included: exposure, sensitivity, and adaptive capacity. Exposure was measured based on remote sensing data from an Indian Ocean scale stress model. Additionally, a predictive model of coral susceptibility to thermal stress and coral bleaching was used to determine exposure. The predictive model was based on past coral bleaching data and oceanographic conditions throughout the western Indian Ocean region. A systematic sampling design of 1564 households located in the 29 sites measured the level of dependence on fisheries, a metric of sensitivity. In addition, the data gathered from these household surveys were used to develop a social adaptive capacity index, specifying eight indicators of adaptive capacity. Two techniques were employed to examine vulnerability: an equation provided a quantitative vulnerability score; the three indices of vulnerability were plotted on a bubble plot, where sensitivity is plotted against adaptive capacity and the size of the points indicates exposure levels.
            The results from this study suggest that there is considerable spread of vulnerability both within and among the five countries. With a mean country level of exposure at 0.26, Mauritius had the lowest exposure. Meanwhile, Kenya and Seychelles experienced the highest exposure, at 0.58 and 0.57, respectively. Environmental conditions that contribute to high reef exposure include low temperature variability, high ultraviolet and photosynthetic radiation, high sea water temperature and low wind velocity. National-scale averages of sensitivity varied from a low of 0.10 in Seychelles to a high of 0.22 in Tanzania. Overall, the ten sites with the highest sensitivity were located in Tanzania, Kenya, and Madagascar. Seychelles and Mauritius were the countries with the highest adaptive capacity sites as well as highest overall national-level averages of adaptive capacity. At the national scale, Kenya had the highest overall vulnerability, followed by Tanzania, Madagascar, Seychelles, and Mauritius. The results for national-scale averages of local-scale vulnerability were largely consistent with national-scale studies of the vulnerability of national economies to the impacts of climate change on fishing.
            The authors used the abovementioned data to develop a policy framework at the local and national levels. Thus, they recommend specific policy tools to address different dimensions of vulnerability, accounting for varying temporal scales. Cinner et al. suggest that governments emphasize reducing the impacts on the most vulnerable in the short term, enhance adaptive capacity and reduce sensitivity in the medium term, and reduce exposure by mitigating climate change in the long term. At the local scale, efforts can include improved information about weather, evacuations from highly vulnerable areas, and diversification within the fishery. Additionally, strengthening community groups responsible for managing coastal resources would serve to decrease vulnerability. Meanwhile, national scale efforts to increase adaptive capacity include adaptation planning, improvement of coastal infrastructure, and investments in alternative energy and new industries. Cinner et al. stress that differing levels of exposure may have different implications for natural resource management. For example, high exposure areas where reefs will be damaged by climate regardless of resource management constitute poor targets for protected areas. The policy framework highlights specific measures to reduce vulnerability to the impacts of coral bleaching on fisheries.

Assessing Coral Reef Management Plans in the Western Indian Ocean

Corals reefs worldwide have suffered signficant impact as a result of rising sea temperatures. The effect on reef ecosystems located in the Indian Ocean was particularly severe in 1998 when 45% of living coral was killed. The development of management interventions well adapted to changing environmental conditions requires a comprehensive understanding of factors affecting reef thermal stress and resilience. In this study McClanahan et al. (2011) seek to identify coral reef areas of low climate stress that are resilient to climate change. The authors mapped climatic-oceanographic stress and coral reef diversity in the western Indian Ocean (WIO) in order to establish a relationship between high diversity coral reefs and areas of low climate stress. The results from an environmental stress model and susceptibility index of the WIO confirmed moderate correlation, showing that the southern and eastern parts are areas with low environmental stress. Meanwhile, regions in the north and west were identified with high fish diversity, and regions from Tanzania to northwestern Madagascar with high coral diversity. McClanahan et al. suggest that these areas are ideal locations for management efforts aimed at protecting coral reef from climate change disturbances.

McClanahan, T. R., Maina, J. M. and Muthiga, N. A. 2011, Associations between climate stress and coral reef diversity in the western Indian Ocean. Global Change Biology, 17: 2023–2032. doi: 10.1111/j.1365-2486.2011.02395.x

Tim McClanahan and colleagues at the Wildlife Conservation Society conducted their research in the western Indian Ocean (WIO), an area stretching from the coast of East Africa to the banks of the Mascarene Plateau. The authors measured the environmental stress of the WIO by using a multivariate stress model (SMI) that measured environmental exposure, sea surface temperature (SST) rate of rise, and chlorophyll concentrations, among a number of various oceanographic factors. The map generated from these variables demonstrated their relationship to coral bleaching, also measured in the model. In order to quantify biodiversity, visual observations were used to measure the richness of coral communities and belt-transect surveys projected numbers of fish species. The degree of interrelatedness of the abovementioned data was then evaluated using pairwise correlation analysis, referred to as Moran’s Index. With Moran’s Indices of 0.40, 0.13, and 0.28 for coral community susceptibility and the numbers of fish and coral species, there was less an a 1% likelihood that the clustered patterns are due to chance. These variables were then synthesized onto a final map using an algebraic sum equation.

The SMI pairwise comparisons demonstrated that several of the variables measured were statistically significant, albeit not strongly. While coral community susceptability and number of coral species were correlated with modelled stress, numbers of fish species were positively correlated with it. Although coral species exhibited the highest numbers at intermediate latitudes between 5 and 101S, the number of fish species was greatest at northern latitudes. Additionally, a map of the data identified the southern WIO region as low stress with some moderate stress regions along the Tanzanian-Mozambique border and a few very high stress areas in the northern regions. These results demonstrate the importance of the Tanzania-Mozambique border as an area of low-stress and high-diversity well suited for coral ecosystems.

This study identifies Madagascar, the Mascarene Islands, and the region from southern Kenya to northern Mozambique as areas of low environmental stress and high biodiversity. McClanahan et al. maintain that conservation strategies should focus on the protection of these coral reef ecosystems. However, the relationship between measures of environmental stress and biodiversity are often conflicting for these regions within the WIO. Although the ability to clearly identify locations possessing desired environmental conditions and species diversity is difficult, a sustainable strategy for climate change would support appropriate restrictions within these habitats.

Mapping Variations in Thermal Stress in Order to Help Manage Coral Reefs

As ocean temperatures continue to increase in the next few decades, resultant mass coral bleaching will significantly threaten reefs worldwide. Differences in thermal stress among reefs may play an important role in determining the design of marine reserves. By locating reserves in areas less prone to thermal stress, the corals inside reserves may benefit from reduced physical and biological stress, thus maximizing their overall resilience. In their study, Mumby et al.(2011) used maps of variations in thermal stress to develop hypothesis about the future response of corals to each stress scenario. Additionally, they incorporated spatially realistic predictions of larval connectivity among reefs and applied reserve design algorithms in order to create potential reserve networks for a warming environment. The results showed that reef larval dispersal is sufficient to connect reefs from desirable thermal stress conditions into a reserve network. Although such a network is viable, the reserve design is limited in its ability to account for phenotypic and genetic adaptations in corals. Mumby et al. seek to demonstrate how the design of marine reserve networks is influenced by the corals’ ability to adapt to climate change. They provide two hypotheses: that adequate larval dispersal allows marine reserve design to be stratified, and that uncertainty about coral adaptation to rising sea temperatures is a crucial component in designing marine reserve networks.
Mumby, P. J., Elliott, I. A., Eakin, C. M., Skirving, W., Paris, C. B., Edwards, H. J., Enríquez, S., Iglesias-Prieto, R., Cherubin, L. M. and Stevens, J. R. 2011, Reserve design for uncertain responses of coral reefs to climate change. Ecology Letters, 14: 132–140. doi: 10.1111/j.1461-0248.2010.01562.x

            Peter Mumby collaborated with marine ecologists from various institutions to provide an adaptive approach to reef management. All field data were collected at a depth of 710 m from 58 Bahamian sites. Several different strategies were used to test aspects of their hypotheses. A 20-year climatology of satellite-derived sea surface temperature (SST) was used to examine spatial patterns of thermal stress across reef ecosystems in the Bahamas. The information collected was then developed into four contrasting thermal stress regimes. Principles derived from previous laboratory studies were used to hypothesize the response of corals to climate change in each thermal regime. Thus, the authors predict that corals in regime A, experiencing high chronic and low acute stress, will have the most resistance to bleaching. In order to account for uncertainty over coral adaptation and acclimation, Mumby et al. considered three scenarios for testing their hypotheses. They assumed that the response of corals to thermal stress present today will continue into the future, that corals will exhibit limited local acclimation inadequate to adapt to climate change, and a “bet-hedging strategy” where larval connectivity between reefs is identified. To test the feasibility of the proposed reserve networks, the research team employed a 3D individual-based model of larval dispersal adapted for the Caribbean ecosystem. Reserve-selection algorithms were then used to design a conservation network that minimizes the cost of the reserve system and meets the conservation objectives.
            The results from this study demonstrate that a small proportion of the 58 sites, about 15 percent, are a good selection for inclusion into a reserve network. However, it is difficult to develop an optimal strategy that accounts for all the conditions present in the three scenarios. Data show that acute stress is highest in the central Bahamas, while chronic stress is highest to the west near the Gulf Stream. Generally, larval dispersal showed a west-east pattern across the Caribbean. By including thermal stratification and larval connectivity into reserve design, the authors determined that overall network performance greatly improved by as much as sixfold.

            The first hypothesis posited by Mumby et al. proved to hold true; the spatial distribution of thermal stress and larval connectivity are similar enough that networks may be stratified according to the response of corals to bleaching. Furthermore, satellite measurements of SST reveal that there is enough larval supply to generate a reserve network. Results from the second hypothesis indicate that the key difference in response scenarios is not corals ability to adapt to global warming, but the differences between the alternate scenarios. Although the authors provided potential selection sites for reef reserves based on coral adaptation to stress, they maintain that further research is needed in this area. The framework Mumby et al. develop may be adapted to future improvements in research regarding larval connection and coral response to climate change.

Coral Reef Fishes Show Decreased Antipredator Responses to Risk When Exposed to Increased CO2 Levels

Atmospheric levels of carbon dioxide are currently at 380ppm, and are expected to increase within the next few decades. Such increases in atmospheric concentrations result in increased levels of dissolved CO2 in the oceans. Elevated levels of dissolved CO2 increase ocean acidification and reduce carbonate-ion saturation. Scientists predict that these changes will have varied effects on marine species. A number of studies have demonstrated the variability in the responses of different aquatic species to increased levels of CO2. However, most studies have only compared species belonging to different orders. The study conducted by Ferrari et al. seeks to provide a comparison of physiological responses to elevated CO2 among closely related species. By doing so, they can test whether species sharing related life histories and ecology have similar tolerance levels to CO2. The ability to group species according to their CO2 tolerance could play a key role in predicting changes to marine communities in the future. Recent studies have demonstrated that CO2 exposure causes olfactory impairment in a number of reef fish. Given that reef fish rely upon their chemosensory ability to detect predators, any olfactory impairment is expected to increase mortality rates. Ferrari et al. tested the effects of CO2 exposure on the antipredator responses of four damselfish species; their research revealed that contrary to expectations, the four congeneric species varied drastically in CO2 tolerance. Additionally, using Pomacentrus chrysurus as a model species, their results suggest that reef fish larvae exposed to high levels of CO2 showed decreased antipredator responses to risk.—Cecilia Ledesma

Ferrari, M. C. O., Dixson, D.L., Munday, P. L., McCormick, M. I., Meekan, M. G., SIH, A. and Chivers, D. P. (2011), Intrageneric variation in antipredator responses of coral reef fishes affected by ocean acidification: implications for climate change projections on marine communities. Global Change Biology, 17: 2980–2986. doi: 10.1111/j.1365-2486.2011.02439.x

Maud C. O. Ferrari and scientists from various institutes collaborated on research submitted to Global Change Biology. The experiments for this study were conducted in November and December 2009 at the Lizard Island Research Station located on the Great Barrier Reef, Australia. Four damselfish species, P. chrysurus, P. moluccensis, P. amboinensis and P. nagasakiensis, were collected in light traps at the end of the planktonic larval stage. Once transferred to rearing aquariums, the fish were exposed to CO2-enriched air for 4 consecutive days. The tested levels of were approximately 700 and 850 ppm, levels of dissolved CO2 predicted under greenhouse gas emission scenarios. Immediately after the treatment period was over the fish were used in the experiment. Ferrari et al. measured the antipredator behavioral responses of the fish by quantifying their foraging, swimming activity, and microhabit use after detection of predator risk cues. These activities represent common antipredator responses in prey fishes. In order to compare behavior and mortality rates of fish exposed to CO2, P. chrysurus were placed on small patch reefs for about 11 hours and their behavior and survival were monitored.

The authors found that all four species showed varied antipredator responses when exposed to levels of CO2 higher than 390 ppm. While P. chrysurus, P. moluccensis, P. amboinensis and P. nagasakiensis all displayed significant losses of antipredator behavior, P. amboinensis remained less affected at CO2 levels of 700 ppm. The activities of CO2-treated P. chrysuru in the field were compared to those of control fish. The comparision indicates that P. chrysuru suffered much higher mortality rates, were more active, moved further and higher away from the reef, displayed higher feeding rates and were bolder than control fish. Thus, the CO2-exposed fish clearly lacked the adaptive antipredator responses to risk cues exhibited by control fish.

P. amboinensis appear as the more sensitive species; at CO2 levels of 700 ppm, it showed a 95% reduction in antipredator response. Meanwhile, at those same levels P. nagasakiensis only showed a 30% reduction in antipredator responses. Given similar history traits between the species, the authors cannot explain the variation in responses. They note that in order to better predict changes in marine communities under increasing CO2 levels future work should focus on the role of physiology in explaining these variations. Additionally, the reduced antipredator behavior observed in the lab translated into decreased survival rates under natural conditions; these fish showed as much as a fivefold to sevenfold incresae in mortality when placed in the reefs. These results indicate that interspecific variation in response to rising CO2 may result in changes to the composition of prey species; this would in turn effect the biodiversity of coral reef communities. However, the impact of increased levels of dissolved CO2 on marine ecosystems will depend on both the magnitude of variation between species and the ability of these species to adapt to changing environmental conditions.