The resilience of coral reefs is the capacity of coral reefs to resist natural and anthropogenic disturbances such as storms and bleaching episodes, and recover after damage. Resilience refers to the ability of biological or social systems to maintain key functions or adapting to change in order to overcome pressures and stresses in the environment. It measures how well coral reefs tolerate changes in ocean chemistry, sea level, and sea surface temperature.Reef resilience toolkit model: Introduction The_Nature_Conservancy Retrieved 7 December 2011. Together, reef resistance and resilience influence coral reef recovery following disturbances such as ocean acidification. Natural reef resilience processes can be used as a recovery model for coral reefs and an opportunity for management in marine protected areas (MPAs).__TOC__
Mechanisms of resilience
Thermal tolerance Many corals rely on a symbiotic algae called zooxanthellae for nutrient uptake through photosynthesis. Corals obtain about 60-85% of their total nutrition from symbiotic zooxanthellae.Fujise, L., Yamashita, H., Suzuki, G., Sasaki, K., Liao, L.M., Koike, K. (2014) Moderate thermal stress causes active and immediate expulsion of photosynthetically damaged zooxanthellae (Symbiodinium) from corals PLoS ONE, 9(12): 1-18. Slight increases in sea surface temperature can cause zooxanthellae to die. Coral hosts become bleached when they lose their zooxanthellae. Differences in symbionts, determined by genetic groupings (clades A-H), may explain thermal tolerance in corals.Sampayo, E.M., Ridgway, T., Bongaerts, P., Hoegh-Guldberg, O. (2008) "Bleaching susceptibility and mortality of corals are determined by fine-scale differences in symbiont type" PNAS Environmental Sciences, 105 (30): 10444–10449. Research has shown that some corals contain thermally-resistant clades of zooxanthellae. Corals housing primarily clade D symbionts, and certain types of thermally resistant clade C symbionts, allow corals to avoid bleaching as severely as others experiencing the same stressor. Scientists remain in debate if thermal resistance in corals is due to mixing or shifting of symbionts, or thermally resistant vs. thermally-sensitive types of zooxanthellae. Species of coral housing multiple types of zooxanthellae can withstand a 1–1.5 °C change in temperature. However, few species of coral are known to house multiple types of zooxanthellae. Corals are more likely to contain clade D symbionts after multiple coral bleaching events.
Biodiversity and functional diversity Biodiversity and functional diversity both influence coral reef ecosystem functioning and resilience. Higher coral species richness has been associated with improved ecosystem performance, including contributions to reef growth and structural development, although the strength of this relationship is non-linear, with benefits increasing rapidly at lower levels of diversity and then leveling off at higher levels. This pattern indicates that reefs with low diversity may experience larger declines in ecosystem functioning with additional species loss, while more diverse reefs may show smaller functional changes when species are lost. In addition to species richness, functional diversity, the range of ecological roles or traits represented by species, varies across coral reef regions and is not always directly correlated with the number of species present. A global analysis of reef-building corals found that some regions with high species richness exhibit relatively low functional diversity, indicating a degree of functional similarity among species.This variation in functional trait diversity suggests that reefs differ in the distribution of ecological functions they support, which may influence their responses to environmental change. Together, these findings indicate that both species diversity and the diversity of functional traits contribute to patterns of ecosystem functioning on coral reefs.
Ecological impacts and recovery
Research studies of the Mediterranean species of coral Oculina patagonica reveal that the presence of endolithic algae in coral skeletons may provide additional energy which can result in post-bleaching recovery. During bleaching, the loss of zooxanthellae decreases the amount of light absorbed by coral tissue, which allows increased amounts of photosynthetically active radiation to penetrate the coral skeleton. Greater amounts of photosynthetically active radiation in coral skeletons cause an increase in endolithic algae biomass and production of photoassimilates. During bleaching, the energy input to the coral tissue of phototrophic endoliths expand as the energy input of the zooxanthellae dwindles. This additional energy could explain the survival and rapid recovery of O. patagonica after bleaching events. A study by the Australian Research Council proposed that the loss of fast-growing coral could lead to less resilience of the remaining coral. The study was undertaken in both the Caribbean and the Indo-Pacific and reached the conclusion that the latter may be more resilient than the former based on several factors; the process of herbivory and the rates of algal blooms forming. Coral bleaching is a major consequence of stress on coral reefs. Bleaching events due to distinct temperature changes, pollution, and other shifts of environmental conditions are detrimental to coral health, but corals can restore from bleaching events if the stress is not chronic. When corals are exposed to a long period of severe stress, death may occur due to the loss of zooxanthellae, which are vital to the coral's survival because of the nutrients they supply. Coral bleaching, degradation, and death have a great effect on the surrounding ecosystem and biodiversity. Coral reefs are important, diverse ecosystems that host a plethora of organisms that contribute different services to maintain reef health. For example, herbivorous reef fish, like the parrotfish, maintain levels of macro algae. The upkeep of seaweed contributes to decreasing space competition for substrate-seeking organisms, like corals, to establish and propagate, creating a stronger, more resilient reef. However, when corals become bleached, organisms often leave the coral reef habitat which in turn takes away the services that they were previously supplying. Reefs also administer many ecosystem services such as food provision for many people around the world who are dependent on fishing reefs to sustain themselves. There is evidence that some species of coral are resilient to elevated sea surface temperatures for a short period of time.
Disturbances and Stressors
Physiological Mechanisms of Coral Bleaching Coral bleaching results when an environmental stress negatively affects the symbiotic relationship that exists between a coral's photosynthetic algae (Symbiodiniaceae) and itself. This causes either expulsion of the algae or loss of those same algae. In addition, warmer-than-normal sea-surface temperature levels reduce the photosynthetic efficiency of the symbiont's cells. As a result, damaged algae produce reactive oxygen species, which further damage the host cells.
To mitigate further damage to their cells, corals expel their symbionts. The expelled symbionts cause the colourless appearance of the white coral. The expulsion of symbionts greatly decreases the amount of energy available to the coral since 80-90% of all of a coral's energy needs come directly from photosynthesis. If stressful conditions continue for an extended period of time, then long-term energy deficiencies can lead to decreased growth rates, diminished reproductive capabilities, and eventually death. However, under certain circumstances, corals that have experienced severe bleaching due to environmental stressors can regrow and regain their symbionts, provided that the environmental factors causing the stress have been removed, and the coral's species has sufficient ability to withstand stress.
Natural disturbances Natural forces such as disease, storms and tidal emersion degrade corals. The frequency of coral disease caused by microbial pathogens has increased over the years, contributing to coral reef mortality. Bacterial, fungal, viral, and parasitic infections can result in physiological and morphological effects. Some of the most common coral diseases include black band disease, white pox disease, white plague, and white band disease, all of which involve tissue degradation and exposure of the coral skeleton. Diseases such as these can quickly spread among healthy coral reefs, potentially making them more susceptible to injury from disturbances like storms. Storms, including cyclones and hurricanes, can cause mechanical destruction to reefs and a change in sedimentation. The strong waves that result from these disturbances can strike corals, causing them to dislodge, and can also cause the reef to come into contact with released sediments and freshwater. Tidal emersion, or low tides, can leave shallow reefs unprotected from UV radiation and heat, leading to stressed and dry corals that may expel its zooxanthellae if exposed for too long.
Anthropogenic disturbances Anthropogenic forces contribute to coral reef degradation, reducing their resiliency. Some anthropogenic forces that degrade corals include pollution, sedimentation from coastal development, and ocean acidification due to increased fossil fuel emissions. Carbon emissions cause ocean surface waters to warm and acidify. The combustion of fossil fuels results in the emission of greenhouse gases, such as carbon dioxide into the atmosphere. The ocean uptakes some of the emitted carbon dioxide, injurious to the natural processes that occur in the ocean. Ocean acidification results in a lower ocean water pH, negatively affecting the formation of calcium carbonate structures which are imperative to coral development. Developing coastal areas has the potential for chemical and nutrient pollution to run off into surrounding waters. Nutrient pollution causes the overgrowth of aquatic vegetation which has the ability to out-compete corals for space, nutrients, and other resources. Occasionally, water runoff containing pathogens from human and livestock waste can also infect coral in large outbreaks and cause damage to reefs. Plastic pollution is another anthropogenic disturbance that affects coral reef resilience. Corals may recognize microplastics as food and ingest them, possibly making the corals feel full while their digestive system is actually congested. Microplastics can also land on the outside of corals, causing irritation to the coral and blocking them from accessing proper food. Larger plastic debris is also dangerous for corals, getting caught in the coral's structure and increasing the risk of disease due to pathogens that the plastic may carry.