Reef Bleaching

Coral reefs are amongst the most vital and threatened ecosystems in the world. Reefs are critical for it provides critical ecosystem services. They are extremely important for developing countries because they provide ample food for a significant portion of the population. It operates as a coastal protection structure for most tropical islands and a primary source of income through tourism by attracting tourists from all over the world for many developing nations. It underpins essential services like fisheries and provides habitat for some of the most prominent biodiversity in the world.

For the growth, reproduction, and subsistence of the reefs, the corals are profoundly dependent on their symbiotic single-celled algae, zooxanthellae. But due to growing environmental stressors, the symbiotic relationship of the corals and zooxanthellae is being disrupted. This, in turn, prompts loss of zooxanthellae and then photosynthetic pigments. These stressors include freshwater flooding, pollution, sedimentation, disease, change the intensity of light, and primarily elevated or decreased sea surface temperatures caused due to global warming. The zooxanthellae may lose 50-80 percent of their photosynthetic pigment when serious bleaching happens. The corals are resilient and can often recover and regain their former levels of zooxanthellae. But, this depends on the intensity and duration of the stress period. Mortality of whole assemblages of reef tracts can be caused due to prolonged stress. The coral reefs, especially those that have suffered partial or complete mortality, are defenseless to algal overgrowth and diseases. This can play a toll on the resilience of the corals. These reefs start to disintegrate due to physical and biological erosion as these factors tend to outpace the rate of reef formation by aggregation of corals. Overgrowth of algae merged with the loss of structural complexity from reef degradation can lead to a tense change in coral reef species composition. It has been observed in some cases where coral bleaching has induced the local extinction of species and declined species richness.

Courtesy: Coral Reef and Tropical Fish (Shutterstock

Coral bleaching can be cast over large geographic scales or can be localized depending on the type and scope of the stressors. Local bleaching is mostly caused due to anthropogenic stressors like freshwater runoff and pollution. These causes are seen largely because of poor management practices. It can be repaired through reforestation and by other erosion and flood control stratagems to minimize threat at its origin.

On the other hand, large scale bleaching cannot be explained by local causes and is principally observed to take place because of increased sea surface temperature regionally. This acceleration in sea surface temperature has been seen to increment over the last two decades. If the global temperatures persist on increasing at this rate then the corals will suffer severe bleaching events in the near future, making climate change the single greatest threat to the reefs worldwide.

A fire coral before and after bleaching. The one on the left is a healthy fire coral, while the one on the right is completely bleached. Photo by the XL Catlin Seaview Survey.

Unlike other threats, climate change cannot be spouted in the roots to control the stress, at least not hastened enough to address the austerity of the bleaching threat. Ocean warming could stretch over to the next century even if the carbon levels in the atmosphere were to be stabilized today. This is due to the lag time in the ocean temperature response. Nevertheless, local efforts could perhaps slow down the degradation caused by climate change.

It is, however, conceivable to mitigate the negative impact of bleaching on coral reef biodiversity by utilizing the natural properties of the coral reef system. We can classify and manage specific patches of the reef where local conditions are anticipated to result in reduced temperature-related bleaching and mortality, to protect them from linear anthropogenic impacts. We can also intensify the capacity for coral reef recovery by maintaining conditions optimal for larval dispersal and recruitment to damaged sites.

Local environmental identifiers act as predictors of resistance and resilience to coral bleaching. Understanding these identifiers would help conservationists recognize, design, and operate networks of Marine Protected Areas in a way that would maximize the overall endurance of the coral reefs in the face of global climate change. Rather than focusing on expensive technological fixes, the most helpful approach would be one that utilizes natural processes. Provided that most of the coral reef countries are economically developing, it would be much more appropriate to use simple tractable strategies that any manager could begin to utilize immediately to maximize the long term survival of the coral communities.

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