Prinsip & Komponen Kelentingan (Resilience)
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Prinsip & Komponen Kelentingan (Resilience)
- 1. Prinsip & Komponen Daya Lenting ( Resilience ) Edy Setyawan Jakarta, Agustus 2012 Yayasan Terumbu Karang Indonesia Source: Rod Salm (The Nature Conservancy)
- 2. Pengertian daya lenting Kemampuan suatu sistem untuk menjaga fungsi-fungsi dan proses kunci dalam menghadapi tekanan dengan cara bertahan maupun beradaptasi terhadap perubahan. Komponen Daya Lenting: (a) kemampuan untuk menyerap atau menahan dampak tekanan/stres dan (b) kemampuan untuk pulih Tipe daya lenting Daya Lenting Biologis Daya Lenting Sosial
- 3. Daya lenting biologis Tutupan karang tinggi Keanekaragaman tinggi Penyakit dan gangguan rendah Rentang ukuran yang luas Photos: Rod Salm
- 4. Daya lenting biologis Pemulihan (rekrutmen) - Tersedianya substrat - Kualitas air yang baik - Banyak herbivor Pemulihan (tumbuh kembali) - Perbaikan dan pertumbuhan - Kompetitor Photos: Rod Salm
- 5. Prinsip-prinsip Daya Lenting Model Daya Lenting TNC Representasi dan Replikasi Jenis-jenis Habitat Penyebaran Resiko Wilayah Kritis Refugia Sumber larva Pemijahan yang tersedia Konektivitas Transportasi Penambahan/Pemenuhan Managemen yang efektif Pengurangan tekanan Rekrutmen yang kuat Strategi yang adaptif Pemulihan yang meningkat RESILIENCE
- 6. Representasi dan Replikasi Jenis-jenis Habitat Penyebaran Resiko shallow patch for e-r eef
- 7. Wilayah Kritis Refugia Sumber larva Pemijahan yang tersedia Melindungi refugia Photo: Rod Salm
- 8. Konektivitas Transportasi Penambahan/Pemenuhan Protect linkages Menjaga Konektivitas Photo: Paul
- 9. Efektif Managemen Pengurangan tekanan Rekrutmen yang kuat Strategi yang adaptif Pemulihan yang meningkat Mengontrol ancaman – mengurangi tekanan Photo: Rod Salm
- 10. Pengelolaan yang efektif Komunikasi Evaluasi efektivitas pengelolaan Pengelolaan yang adaptif Pendekatan kehati-hatian
- 11. Mengidentifikasi Resilience • Ecological • Biological • Physical Photo: Rod Salm
- 12. Ecological Resilience Factors Food web interactions: herbivory, trophic cascades Reproductive cycles Population connectivity Coral & fish recruitment
- 13. Ecological Resilience Factors: Herbivory • Reef Herbivores - excavators (bioeroders), grazers, browsers, scrapers • Parrotfish, surgeonfish, rabbitfish, batfish (and long-spined urchins) • Loss through overfishing – algal dominated • Phase shift reversal? Sleeper functional groups
- 14. Ecological Resilience Factors: Recruitment Recruitment = supply of new individuals to a population Photo: Rod Salm
- 15. Ecological Resilience Factors: Recruitment Determining Factors • Physical oceanographic processes (e.g., currents and eddies) transport & retention • Abundance of larvae in water column • Larval behavior (e.g., vertical migration in the water column) • Availability of settlement substrate • Ecological factors that affect survivorship after settlement (e.g., competition, predation, food supply)
- 16. Ecological Resilience Factors: Recruitment Suitable habitat for recruits? • Diversity and abundance of spp • High herbivore densities • Low corallivores, bioeroders, disease • Presence of crustose coralline Photo: Rod Salm algae (CCA)
- 17. Biological Resilience Factors • Genetic differences • Species differences
- 18. Biological Resilience Factors: Genetic Differences Coral differences: • Fluorescent tissue pigment • Colony integration • Tissue thickness Zooxanthellae differences: • Heat tolerance* *Clade D symbionts are more tolerant
- 19. Biological Resilience Factors: Species Differences Likely to bleach (e.g., Acropora, Millepora): • Quick colonizers • Fast growing • Short-lived Likely to survive (e.g., Porites, Diploastrea): • Massive growth forms • Thick or less integrated tissues • Slow growth rates
- 20. What I found in 2011 at SE Misool: • more complex • deep worse • selection for resilient corals? Photo: Rod Salm Implications for management: • reinforce resilience principles • emphasize risk spreading • larger over smaller areas Photo: Rod Salm
- 21. Biological Resilience Factors: Species Differences
- 22. Physical Resilience Factors • Cooling • Shading • Screening • Stress Tolerance
- 23. Physical Resilience Factors: • Cooling Broad scale (1000’s of km) • Regional scale (100’s of km) • Local scale (10’s of km)
- 24. Physical Resilience Factors: Shading Photo: Rod Salm
- 25. Physical Resilience Factors: Screening Photo: Rod Salm
- 26. Physical Resilience Factors: Stress Tolerance Photo: Rod Salm
- 27. Summary of Factors Herbivory Ecological Recruitment Genetics Biological Species differences Cooling Physical Shading Screening Stress Tolerance
Editor's Notes
- As managers, it is helpful to have a good sense of what resilience looks like. Resilience is more than being able to recover from a major disturbance, surviving bleaching, or resisting bleaching. For a community to be resilient, it must also be able to continue to thrive, reproduce, and compete for space and resources. For example, coral communities that have experienced bleaching but not mortality may be weakened and less able to thrive, grow, and reproduce in the competitive reef environment Multiple factors contribute to resilient coral communities, some of them known and others to be discovered. Scientists are working to identify important ecological, biological, and physical factors that managers can evaluate to determine the health or resilience of a coral community. The following sections discuss some of these factors, and how they contribute to the overall resilience of coral communities. It is important to be able to identify and better understand these factors, so management strategies can be focused on maintaining or restoring communities to these optimal conditions to maximize coral survival after stressful disturbances. Please refer to the section on monitoring resilience for help in using this information in a field monitoring context.
- The ecological processes that maintain reef function and support thriving reef communities play an important role in maintaining resilience to major disturbances such as coral bleaching. Complex food-web interactions (e.g., herbivory, trophic cascades) reproductive cycles, population connectivity, and coral and fish recruitment are among the ecological processes that scientists have recently been studying in a reef resilience context. Many questions remain about how, when and where these factors are important. But scientific evidence demonstrates the consistent importance of the presence of top predators and large herbivores as well as the importance of coral and fish recruitment rates and patterns for reef resilience.
- Reef Herbivores A variety of types of reef herbivores, including fish and invertebrates, promote coral resilience. Some commonly known reef herbivores are parrotfish (Family Scaridae), surgeonfish (Acanturidae), rabbitfish (Siganidae), batfish, and long-spined urchins ( Diadema spp.). There are four functional groups of coral reef herbivores—excavators (bioeroders), grazers,, browsers, and scrapers—and each has a particular role in maintaining healthy reef systems. Loss of herbivores, through overfishing, can cause shifts from coral dominated reefs to reefs with abundant macroalgae populations. To maintain reef resilience, management activities should focus on protecting herbivore populations. Despite this critical role, in the last few decades there have been major declines in grazer densities on many reefs around the world. Much of this decline has been regionally specific and dependent on fishing patterns. Many Caribbean reefs continue to be algal dominated, as herbivorous fish and urchin populations have not recovered. It is not yet certain if these phase shifts to algal dominated areas can be reversed, as it takes time for herbivore populations to rebuild after major declines. Even if herbivore populations recover, many prefer epilithic turf algae over macroalgae3, making it difficult to mow down mature stands of macroalgae, and thus making it even more important to prevent decline of herbivorous fish populations. Managing Herbivory Regimes A coral reef that has transformed or is in the process of transforming into an algal reef is usually fairly obvious because healthy coral reefs do not typically have substantial stands of macroalgae. Most healthy coral dominated reefs have pockets of turf algae and occasional macroalgae. Reef managers should work to maintain a balanced assemblage of coral and algal communities. Once algae have taken over, it is difficult to reverse the trend. When this occurs, management activities should focus on rebuilding and protecting herbivore populations. Following a major disturbance event, herbivores play an important role in inhibiting algal growth, providing coral larvae opportunity to recolonize dead substrate4. Recent studies have identified specific types of herbivores (large-bodied parrotfish) that seem to be more important, at least at the regional scale (see Marine Reserves Do More Than Protect Fish sidebar). Any management strategy that reduces algal cover may enhance the recovery of coral and the resilience of the community5.
- Recruitment is the measure of the number of young individuals (e.g., fish and coral larvae, algae propagules) entering the adult population, in other words, it is the supply of new individuals to a population. Recruitment can play a critical role in the resilience of coral populations through the number of individuals and different species that repopulate a reef. Its importance for community dynamics and coral populations varies by species, habitat and reef location. The rates, scales, and spatial structure of dispersal among populations drive population replenishment, and therefore have significant implications for population dynamics, reserve orientation, and resiliency of a system. For dispersing larvae, the number of new recruits entering a population is primarily related to five factors: physical oceanographic processes (e.g., current speed and direction; eddies; upwelling vs. downwelling) the abundance of larvae in the water column larval behavior (e.g., vertical migration in the water column to catch currents going in different directions) the availability of suitable substrate for settlement the ecological factors that affect survivorship after settlement (e.g., competition, predation, food supply) All of these processes affecting the magnitude of recruitment into a system can influence the spatial patterns of coral reef species communities and assemblages. For coral bleaching, larval recruitment is a particularly critical component of the recovery process. Reefs that have been severely damaged are reliant on the arrival of larvae from corals that have survived the bleaching event elsewhere and their successful settlement, survival and growth.
- Large scale physical oceanographic processes, such as ocean currents, upwelling, eddies, and El Niño events, can cause considerable mixing and long-distance transport of pelagic larvae. These large-scale processes, in turn, affect recruitment patterns at smaller, site-level scales. Currents and areas of upwelling will have a direct effect on the extent of larval transport to distant locations and the flux of larvae over particular sites, and thus overall patterns of recruitment. At the smaller, site-level scale, other physical processes can factor into larval dispersal and recruitment patterns such as micro-currents, light, areas of flow constriction, salinity, depth, surface orientation, and sedimentation. These mechanisms can facilitate larval retention near source populations. To better understand and connect these large scale processes to local areas, managers should examine the oceanographic current complex within the area. For example, areas of high localized upwelling and high flow currents, especially where these form eddies that concentrate and retain larvae, would be expected to result in high opportunities for larval recruitment because of the great flux of water over the community. Information on surface ocean currents and tides provide managers with the general movement patterns and expected larval distribution. Managers can also seek help and support from local oceanographers if information is not easily available. Furthermore, managers can also perform recruitment studies and experiments at their sites to identify the settlement and recruitment patterns. One method to quantify the density of coral recruits and to get a better idea of differential recruitment within an area is to place settlement tiles (onto which coral larvae attach and can be examined) throughout the site and compare the settlement between sites and against oceanographic patterns. This can provide general trends of the availability of larvae to the system. 2. Where are the sources of larvae for the site? The production, settlement, and survival of larvae are dependent on the availability of source areas 7. The source of larvae to an area can be an external location; or the source can be locally derived, if larval production and settlement occur within the natal site. These self-recruiting systems are not dependent on outside sources of larvae for replenishment. The pattern of larval exchange, and the degree to which larvae originate from outside populations, helps to explain connectivity. A large amount of self-seeding leads to low connectivity, while high rates of larval exchange with other populations generate high connectivity1. Historically, the consensus had been that populations of reef fishes were demographically open over large spatial scales, with recruits to a population originating from adults elsewhere. This implied that larvae settling on a reef were not derived from eggs spawned on that reef, but from eggs spawned somewhere else (external sources). However, the paradigm has shifted to consider many reef fish populations as “closed” populations that are self-recruiting2,3,4,5. Recent evidence, from a variety of fields, indicates that local retention of larvae in the natal habitat may be considerably more prevalent than previously thought, even in species with long larval durations. For example, studies in Kimbe Bay, Papua New Guinea, revealed high levels of self-recruitment (60%) in two reef fish species, clownfish ( Amphiprion percula) and butterflyfish (Chaetodon vagabundus) 6. This study, among others, suggests that the extent of larval dispersal between populations is lower than currently assumed, affecting connectivity among populations and having important implications for MPA design and resilience2,3. Most reef ecosystems are not exclusively self-recruiting or dependent on outside sources. Proportions of larvae originating from internal or external locations can vary widely within and between reef systems. In terms of recovery from bleaching, it may be optimal to have a combination of both self-recruitment and external sources of recruitment. For example, if an exclusively self-recruiting coral community suffers mass mortality from a bleaching event, there is little prospect for recovery, since all of the sources of larvae were impacted by bleaching. Likewise, recovery of a coral community that depends solely on external sources is completely dependant on the arrival of suitable coral larvae that have survived the bleaching event elsewhere. A reef that both receives and provides larvae is more likely to be resilient in a bleaching event, because it has multiple options for recovery. Monitoring of recruits should be included in the site monitoring protocol to identify new individuals arriving to the location.
- Is there suitable habitat for recruits? Preparing the Substrate Factors that may prepare the substrate for coral larval recruitment include: diversity and abundance of coral species high herbivore densities low abundance of corallivores, bioeroders and disease presence of crustose coralline algae (CCA) Finding unoccupied space and suitable habitat on coral reefs is a competitive process for coral larvae. Substrate morphology is one important factor for coral larvae settlement and a possible determinant of coral community structure. Substrates, such as live coral, sediment, and fleshy macroalgae are unsuitable for coral recruit settlement. Scientists have discovered that the suitability of a surface for coral settlement is greatly determined by chemical or biological properties of the surface. The presence of chemical stimuli in crustose coralline algae (CCA) as well as in other substrates, such as dead coral have been shown to induce coral larvae settlement8. Recent studies have found that coral larvae appear to be able to recognize and respond to chemical signatures in CCA in the selection of settlement habitat location9. When evaluating the potential for coral larvae recruitment, managers should examine the availability of suitable habitat in the site, looking specifically for areas of CCA or patches of dead coral, which may provide adequate settlement substrate for new coral recruits. New recruitment of juvenile reef fish is also dependent on the availability of suitable habitat. For example, an 8-year study in Papua New Guinea demonstrates the interdependence of the relationship between juvenile fish and coral cover10. Over 8 years, a decline in coral cover was observed, followed by a dramatic decline in fish biodiversity and an overall phase-shift in reef fish community structure. Those species with a great dependence on living coral as juvenile recruitment sites revealed the greatest decline in abundance. This suggests habitat-limited recruitment and that obligate coral-dwelling species’ populations decrease with a decrease in healthy coral cover, compromising the long term resilience of the community. 4. What is the herbivory regime at the site? Abundance and community structure patterns of herbivorous fish and invertebrate grazers can influence coral recruitment. The presence of grazing reef fish, such as parrotfish, reduce macroalgal cover and can facilitate enhanced coral recruitment11. In areas where there has been overfishing of herbivores, recovery of coral communities following a bleaching event has been compromised. Managers need to evaluate the types and extent of grazing occurring within the area. Management that is designed to enhance coral recruitment and resilience should include strategies that reduce algae cover through the maintenance of high levels of grazing reef fish and invertebrates.
- This section of the toolkit is designed to describe some of the biological characteristics of both corals and zooxanthellae that support resilience to bleaching. Bleaching is a dynamic process and there are few data with which to predict the capacity of corals to withstand climate change. However, several known biological factors of both coral and zooxanthellae influence the degree of resistance or resilience to coral bleaching. Resilience or resistance to bleaching is highly variable, with differences observed among coral colonies of the same species, between colonies of different species, and within individual coral colonies. Different responses of species and individuals to thermal heat stress can be partially attributed to biological factors of individual coral and symbiotic zooxanthellae. Knowledge of biological factors of individual corals enhances the ability to understand factors that confer resilience and guide management actions in response to threat of elevated sea temperatures and bleaching.
- Genetic connectivity between and within coral reefs is an important component of resilience. Larval exchange between reefs promotes genetic diversity, which is critical in terms of resilience against any disturbance, particularly mass bleaching events. The spread of selectively advantageous genetic traits, such as bleaching resistance, is a potential consequence of larval coral exchange and migration1. Within species, susceptibility to bleaching and mortality can differ, even under the same environmental conditions. These differences between individuals suggest that genetic variation within coral populations can create resilience to increased thermal stress. Three biological characteristics of corals may contribute to their resilience2: 1. Fluorescent tissue pigment Fluorescent pigments are common in many corals, providing a system for regulating light. These pigments protect the coral from broad-spectrum solar radiation by filtering out damaging UVA rays (blue light portion of spectrum), as well as by reflecting visible and infrared light, thereby reducing light stress on the corals. Concentrations of the pigments vary among species (pocilloporids and acroporids have relatively low densities of pigments, while poritids, faviids and other slow-growing massive corals have high densities). The protective capacity of these pigments provides a kind of internal defense mechanism that may have important implications for long-term survival of corals exposed to thermal stress. Corals containing fluorescent capacity have been found to bleach significantly less than non-fluorescent colonies of the same species. 2. Colony integration The extent of colony integration influences the degree to which the whole colony responds to thermal stress. Characteristics of colony integration include polyp dimorphism , intra-tentacular budding and complex colony morphology3. Species with a high colony integration (e.g., milleporids, pocilloporids and acroporids) are predicted to have a greater whole-colony response to increased temperatures than species with a low colony integration (e.g., poritids, faviids, and other massive corals). This pattern of mortality has been observed between Acropora and Porites . Acropora , with high colony integration,displayed high rates of whole-colony mortality and little partial mortality, while Porites , with low colony integration, had patches of bleached areas with little whole-colony mortality. 3. Tissue thickness The thickness of coral tissues may contribute to the level of susceptibility to bleaching. Thin tissue is found in coral species that are more susceptible to bleaching. Thicker tissue may help shade zooxanthellae from intense light, thereby increasing the resilience of the coral. Zooxanthellae Genetics Heat Tolerance in Zooxanthellae In Panama, corals of the Pocillopora genus were surveyed, and those that contained zooxanthellae clade D were unaffected by bleaching, while colonies containing zooxanthellae clade C bleached severely8. Adaptation to higher temperature in Symbiodinium D can explain why Pocillopora spp. hosting them tolerate warmer temperatures, whereas corals hosting Symbiodinium C do not6. Symbiodinium is a genetically diverse group of dinoflagellates, including eight phylogenetic types, distinguished as clades A-H4,5. The genetically distinct clades possess unique environmental, ecological and geographic variations, which influence the resilience of corals to elevated temperatures and bleaching. Studies have revealed that the different clades of zooxanthellae have different susceptibilities to thermal stress. Evidence indicates that corals with clade D symbionts are more stress-resistant, opportunistic and thermally tolerant than other zooxanthellae types. Additionally, some corals which have experienced (and survived) repeated bleaching events have been observed to host a greater proportion of D-type zooxanthellae6,7 than other clades. This indicates that coral species that host the more heat tolerant, clade D zooxanthellae may be more resilient, or resistant, to coral bleaching Many coral colonies appear to associate with a single zooxanthellae clade, while some coral species are relatively flexible in the types of algal symbiont they contain, and can associate with several types simultaneously. Corals that host mixed zooxanthellae types may have an ecological advantage over those that do not, in terms of their ability to cope and respond rapidly to thermal stress and climate change. Additionally, several studies have revealed that some corals can vary the ratio of zooxanthellae clades, resulting in an improvement of their thermal tolerance. The selective exchange of zooxanthellae (from suboptimal to more heat tolerant clades) represents a mechanism for rapid acclimatization by corals to thermal stress. Thermal acclimatization/adaptation, catalyzed by zooxanthellae change is a natural operation of coral reefs, and shifts toward more heat tolerant populations of corals and zooxanthellae may arise naturally, but may be accelerated due to rising temperatures.9 Understanding the conditions under which corals can change their symbionant type, the biology of the zooxanthellae, and the factors that shape the coral-zooxanthellae relationship, remains the focus of active research which will have a role in helping determine the bleaching response patterns of coral reefs.
- Not all coral species are equally susceptible to bleaching. In response to elevated sea temperatures, some corals may immediately show apparent signs of bleaching, while other coral species at the same location may not bleach at all. Furthermore, some corals have revealed the capacity to acclimatize to local temperature increases. Corals that have experienced (and survived) higher temperatures and light levels may be more resistant to bleaching stresses in the future. From a colony perspective, species that are characterized by fine-structured, branching or tabular growth forms, and thin or well-connected tissue, tend to be less resistant to bleaching. Coral species that are more susceptible to heat stress are characterized by branching or tabular growth forms, such as Seriatopora and Acropora . Photos left to right: © J. McManus; NOAA Corals that are less resistant to bleaching tend to be those corals that are quick to colonize free space, are fast growing, and often short-lived. Those less resistant genera include: Acropora Millepora Stylophora Coral species that are more resistant to bleaching can be characterized by massive growth forms, thick or less-integrated tissues and slow growth rates. Coral genera recognized as more resistant to thermal stress include: Coral species that are more tolerant of thermal stress have massive growth forms, thick tissues and slow growth rates. Photos © S. Kilarski/TNC Acanthastrea Porites Hydnophora Merulina Favia Goniastrea Leptoria Montastrea Platygyra daedalea Cyphastrea Galaxae
- I found resilience indicators in abundance at most places I went, though to be honest the reefs of the Andaman islands, Aceh in Sumatra, Indonesia, Thailand and Malaysia were very hard hit, suffering high levels of mortality. Wakatobi NP where we work in Indonesia suffered 60-65% bleaching but only 2.9% mortality – we’ll put this to good use to revise the zoning of the area when it comes up for review in about one year. In the Raja Ampat islands, e.g., I found little coral bleaching in the shallows while deeper ones were more affected – and dying. I also found very different patterns in the way corals of the same species responded to hot water in areas smaller than a football field. This was exactly what I found in Mozambique just last year where I was helping our Africa program with their first marine project there. My GBR colleagues have confirmed similar observations there. What I think this means is that the shallow water corals are becoming stress hardened – the more susceptible individuals were culled out in 1998 leaving hardier ones that tolerate heat stress better. If this is true, it’s good news because it illustrates emergence of differences in corals’ ability to tolerate heat stress, with some doing better than others and surviving to breed and populate the community with more resilient strains. This complexity is really reinforcing of two things: the need to include examples of all habitat types into MPA networks or zones, to replicate them, and to spread out the replicates – it’s equivalent to diversifying an investment portfolio to spread risk; and also the need to go for larger over smaller areas because in choosing an area that is too small we are likely, despite best intentions, to capture just one piece of a pattern and miss other essential ones. By investing time and effort in large areas, we are more likely to capture all pieces of these mosaics of different patterns that make up the picture of how coral reefs will respond to heat stress and bleaching and other similar large scale stress events.
- A general hierarchy of resistance to bleaching provides a reasonable indication of susceptibility to heat stress1,2,3. ResistanceGrowth typeCoral FamilyExamples Fine branching; thin or well-connected tissuePocilloporidaeSeriatoporaStylophoraPocilloporaBranching, tabulate, encrusting/folioseAcroporidaeAcroporaMontiporaMassive, brainFaviidaeFaviaFavitiesLeptoriaGoniastreaPlatgyraMassive, boulder; thick or less-integrated tissuePoritidaePoritesGonioporaGalaxea Pavona A broad spatial resilience approach is recommended for MPA network site selections. Sites within the network should include corals that exhibit a range of resistant properties. Sites that contain corals exhibiting resistance properties serve as refuges and souces of seed, while sites with more vulnerable species may be vital to connectivty and other ecological dynamics at larger scales. The MPA network approach highights the management of sites with varied resistant properties as critical element within a connected network.
- Cooling: Oceanographic conditions that cause mixing of heated surface waters with cooler deeper water can reduce temperature stress. Shading: High island shadow or overhanging vegetation may reduce the harmful effects of sunlight. Screening: Naturally occurring suspended or dissolved matter reduces sunlight penetration and may reduce bleaching. Stress Tolerance: Coral communities that are exposed to extreme conditions regularly are often populated by species with a high tolerance for stress. Others do not survive. Conditions only become stressful outside of normal ranges tolerated by the species at its location change. A coral at higher latitudes, for example, may be acclimatized to much lower water temperatures than the same coral species at the equator. A rise above its normal temperature threshold would cause bleaching at temperatures easily enough to cause bleaching when they deviate significantly from those tolerated by the same species at the equator.
- Local meteorological conditions, bathymetry, and tidal and oceanic currents affect local current patterns. These conditions can result in upwelling of deep, cooler water to the surface. Complementary factors such as longshore or offshore winds, tropical storms, eddies behind reefs, or shelving seafloors also may act as mixing agents. Below are some resources and suggestions to assist in identifying areas of potential cooling: At the broadest scale (1,000s of kilometers) , consult NOAA’s global temperature maps that provide the histories of sea surface temperature (SST), including global hotspots of high temperature. Regions that now regularly receive SSTs approaching average hot season maximums are particularly vulnerable to coral bleaching. Note: Use the highest resolution available. Currently, 50km resolution SST maps are freely available, and 4km resolution maps are now available through the National Oceanographic Data Center’s Pathfinder SST Program. At the regional scale (100s of kilometers) , look for reefs that are close to local currents, and deep water and whose corals may benefit from cooler water temperatures. Identify major obstructions in the path of currents (e.g., sheer reefs, islands and seamounts rising from great depths, ridges across currents, and promontories that extend underwater) where cool waters are unlikely to extend, as opposed to high flow areas that are more likely to be cooled by deeper waters. Also identify constrictions, such as channels and narrow passages between landmasses, that funnel and intensify the flow and mixing of oceanic and major regional and tidal currents. At the local scale (10s of kilometers) , the same features of bathymetry, current flows and eddies, constrictions, and obstructions to current flows apply, but at a much finer scale. Look for areas that have not bleached because they might be ones in which cooling occurs. FROM ROD: Cooling is caused by mixing of deep cool water with the heated surface water. Managers can find cooled areas by asking those who know, consulting SST maps or using hydrodynamic models. They can verify bleaching resistance, and can protect and manage resistant sites.
- Shading reduces bleaching risk, but to be effective it must be reliable. Heavy cloud cover may offer protection to corals from increased ultraviolet light and ultimately from, bleaching, but clouds are not a reliable source of shade. On the other hand, topographical and bathymetric features are reliable year-round. Shading is important in certain locations such as Palau, the Philippines, and anywhere there are steep-sided limestone or volcanic islands that offer protection. Islands of high relief that are oriented along a north-south axis provide reefs with shade for half of each day. Steep-sided reefs with a north-south axis may also provide shade for half of each day. Aspect relative to the sun for steep-sided reefs in high latitudes may provide greater shading of slopes facing away from the equator. Limestone islands, or coastlines undercut by erosion at the waterline, may provide shade for reefs growing on shelves around their perimeter. Trees on steep islands further enhance their shading effect. FROM ROD: Shading is a second factor. When corals bleach, they essentially lose their sunscreen and become vulnerable to the harmful effects of the intense sunlight. Intense sunlight will kill corals if they remain stressed and bleached for too long. Among some island clusters, there is no shortage of deep shade especially beside high north-south orientated islands (POINT TO SHADOWS), which protects reefs fringing their bases from morning or afternoon sun and helps the corals there survive a bleaching event. (Mushroom island slide) Many limestone coastlines are deeply undercut at the waterline. If we were to dive in and have a closer look, (shaded coral slide) we would find deeply shaded corals growing right up to the surface under the dark, overhanging rock. These corals survive bleaching better than more exposed ones nearby. The most diverse coral site currently being monitored in Palau is indeed in just such a place.
- Sediments from urban and agricultural run-off can cause stress and kill corals. However, specific kinds of corals that live in sheltered locations may tolerate the naturally higher levels of dissolved and suspended matter that color the water. Corals on these sheltered reefs may benefit from the screening of sunlight by suspended particles that scatter light, and by colloids and colored dissolved organic matter (CDOMs) that absorb light. Turbid water caused by excessive nutrient run-off or other land-based sources of pollution do not provide screening protections to corals. These conditions are harmful and weaken the resilience of corals exposed to them for even short exposure periods. FROM ROD: Another factor is screening by particles in the seawater. These particles act as shade netting that screens out harmful radiation to help corals resist bleaching. There are places near estuaries or in sheltered bays where coral communities have adapted to these conditions and thrive there. Have you found coral reefs in similar conditions that have fared better than those in clearer waters offshore? These cloudy water areas, often overlooked for conservation, provide important refuges for resistant corals and would be valuable places to invest our conservation effort. The screening works where the coral communities have adapted to cloudier water, but it doesn’t work if the cloudiness caused by erosion (communities not dev under those conditions). Some bays naturally cloudy (organic material). Colored dissolved organic matter (eg tea trees). Not recommending cutting down forests to cause erosion and protect seas – won’t work. These cloudy areas are not where we normally go and look for reefs, but they are important. One thing you can do is protect these areas (will be covered by represented). Eg if required: Ngeremeduu Bay in Palau
- Although corals are very sensitive organisms that generally require narrow ranges of certain conditions to survive (e.g., temperature, salinity, light), some corals have adapted to exist in highly stressful conditions at the outer limits of these ranges. A history of exposure to high temperatures can influence the thermal tolerance of corals and their resistance to bleaching. Parts of reefs that regularly experience heat stress conditions, such as reef flats and crests, may be populated by corals that are more tolerant and resistant to bleaching. For example, corals on reef flats are exposed to air during the lowest tides. These corals may be accustomed to extreme stress from heat, desiccation, and great fluctuations in salinity. This may help to explain why corals in some inner reefs appear less susceptible to bleaching than the same species growing in deeper waters. Guidelines for identifying stress tolerant corals include the following: Check for living corals on reef flats and reef crests. Important areas to protect are those with multiple species and a wide range of sizes. Size often relates directly to the age of the coral, and thus a wide range of sizes may indicate a wide range of ages and serve as a proxy for survival prospects through bleaching events. Coral species that tend to be more resistant can still bleach, but surveying species composition of a reef may indicate past history of exposure to bleaching, possible resistance, and provide indications of future bleaching risk. FROM ROD: A fourth factor that helps corals to survive a bleaching event, and the greatest surprise of all, is stress tolerance developed through exposure to harsh conditions. The corals on reef-flats or shallows, where you might expect the sun to have the greatest effect, can and often do survive; while those deeper down the reef slope will bleach and die. These shallow corals are routinely exposed to harsh conditions at low tides. They can become exposed, dry out, get scorched by the sun, and flooded by rainwater – their very survival requires tolerance of widely ranging conditions. These four factors (cooling, shading, screening, and stress tolerance) help corals either to resist bleaching altogether or to recover quickly from it. These four factors will be important determinants for the selection of conservation areas in Palau – a forward looking strategy indeed. We are learning a great deal about the physiology of coral bleaching and how and why corals die, or don’t, as a consequence. The story of bleaching resistance and resilience is complex. But, we are also learning that we cannot wait for perfect science; we need to arm ourselves with the best available knowledge and take action now to protect corals.