Coral Bleaching: Causes and Consequences

Coral bleaching is a global environmental issue. It has been observed in coral reefs around the world, from the Great Barrier Reef in Australia to the Caribbean.

coral bleaching

Coral reefs stand as the epitome of the Earth’s unparalleled heterogeneous marine ecosystems. Encompassing a mere 2% of the expansive oceanic realm, these intricate coral reef systems span across more than a hundred nations and serve as the abode for a staggering 25% of the complete spectrum of marine species.

Beyond their opulent biological multifariousness, coral reefs assume a role of paramount significance in affording protection to coastlines against the erosive forces of nature and in tempering the ferocity of tempestuous storm surges. However, in the wake of escalating global temperatures and the concurrent acidification of oceans, as delineated by the term “ocean acidification,” the well-being of coral reefs undergoes a state of deterioration, precipitating a wide-scale phenomenon of coral bleaching across the expanse of the planet’s oceans.

Thus, an exploration into the essence of coral bleaching, its underlying causative agents, and its far-reaching ramifications becomes a matter warranting profound consideration.

Key Takeaways: Coral Bleaching

  • Coral reefs consist of tiny coral creatures called polyps that live together with special algae called zooxanthellae in a mutually beneficial partnership. These algae perform photosynthesis, providing energy to the corals and adding vivid colors to the reefs.
  • When corals encounter stressors like high sea temperatures, pollution, or changes in their surroundings, they expel the algae they depend on. This causes the corals to become pale, a phenomenon known as “bleaching.”
  • Coral bleaching leads to the disappearance of the vibrant algae, impacting not only the stunning look of the coral reefs but also depriving the corals of essential nutrients and energy.
  • Coral reefs have a critical role as ecosystems, giving homes and nourishment to many sea creatures. Widespread coral bleaching disrupts these ecosystems and can result in a decrease in the variety of life.
  • While mild coral bleaching can be temporary, with corals regaining their health as conditions improve, severe or prolonged bleaching can result in coral mortality if the corals fail to recover their algae partners.

What Is Coral Bleaching?

Bleached branching coral (Acropora sp.) at Heron Island, Great Barrier Reef.
Bleached branching coral (Acropora sp.) at Heron Island, Great Barrier Reef. Image: J. Roff.

Coral reefs encounter stress and respond by expelling symbiotic microalgae known as zooxanthellae from their coral polyps, resulting in a complete paling of their appearance when subjected to alterations in environmental conditions such as temperature shifts, light variations, or nutrient levels. Zooxanthellae microalgae constitute the principal nutritional source for corals, contributing to their vibrant chromatic manifestation. These microalgae engage in photosynthesis, yielding glucose, glycerol, and amino acids, which the coral employs for synthesizing protein, lipids, and carbohydrates, while concurrently generating calcium carbonate to fortify its rigid exoskeleton. In reciprocity, the coral affords continuous sunlight exposure and refuge to the zooxanthellae microalgae. Environmental pollution and escalating water temperatures culminate in coral stress, triggering the expulsion of zooxanthellae from the coral’s organism.

Coral bleaching is the physiological process by which coral reefs get rid of tiny algae called zooxanthellae from their coral polyps. This happens when the coral reef is under stress from things like high temperatures, acidification of the ocean, or human actions. This process leads to the translucency of coral tissues, uncovering the ivory-hued underlying coral skeleton. The manifestation of coral bleaching does not invariably signify the demise of the coral colony, although numerous coral entities face potential demise due to malnutrition or heightened susceptibility to pathogenic agents following the bleaching occurrence. Prolonged periods of coral bleaching amplify the complexities entailed in the reestablishment of algae symbiosis and the overall recovery of the coral ecosystem.

The two images of the Great Barrier Reef, with the warmest water (upper picture) matching the coral reefs (lower picture), establish the conditions that could contribute to coral bleaching.
The two images of the Great Barrier Reef, with the warmest water (upper picture) matching the coral reefs (lower picture), establish the conditions that could contribute to coral bleaching. Image: Wikimedia.

Coral bleaching represents an inherent phenomenon typically observed during the warmer seasons when oceanic temperatures surge. Nonetheless, recent extensive oscillations in sea temperatures have instigated widespread coral bleaching episodes, inflicting severe impacts on an array of coral species. These pervasive bleaching occurrences are demonstrating increased recurrence and heightened intensity, posing formidable challenges to the intrinsic recuperative mechanisms of coral reefs.

An exemplar of considerable prominence in the context of coral bleaching pertains to the Great Barrier Reef, a colossal and extensive coral reef system situated along the coastline of Queensland, Australia, encompassing an expanse of approximately 350,000 square kilometers. This awe-inspiring coral reef assemblage encountered five notable instances of mass bleaching in the years 1998, 2002, 2016, 2017, and 2020. The bleaching incidents witnessed in the years 2016 and 2017 were of such grievous magnitude that nearly half of the reef ecosystem perished.

In accordance with comprehensive evaluations, an estimated 14% of the global coral population has succumbed to coral bleaching events since 2009. The fatality of coral colonies has assumed a pervasive character, particularly in geographical regions encompassing South Asia, the Pacific, East Asia, the Western Indian Ocean, the Gulf region, and the Arabian Sea, apart from the Australian domain.

Cause of Coral Bleaching

Healthy coral
Healthy coral, Port Ghalib, Egypt. Image: J. Hutsch.

Coral bleaching manifests as a natural phenomenon, prone to manifest during the summer season or in response to climatic events like El Niño, which usher warm water currents. Nevertheless, the escalation of profoundly intensified episodes of coral bleaching is intrinsically linked to anthropogenic interventions and their ensuing repercussions.

The ongoing trajectory of climate modification engenders a relentless upsurge in worldwide oceanic temperatures. Over the preceding century, there has been a discernible augmentation in sea temperatures, amassing an incremental gain of 0.13 degrees Celsius per decade on average. The concomitant elevation of thermal content within aquatic environs augments concentrations of atmospheric carbon dioxide, engendering a cascade of oceanic acidification as marine realms assimilate carbon dioxide to uphold chemical equilibrium.

Presently, coral ecosystems confront unparalleled thresholds of aqueous acidity, precipitating a curtailed accessibility of calcium carbonate, a vital constituent essential for the fortification of coral skeletal architecture.

Metamorphoses in solar irradiance and the contour of coastal infrastructural progress also contribute to the burgeoning phenomenon of coral bleaching. The escalating incursion of plastic contamination has burgeoned into a substantial exigency, imperiling oceanic realms and maritime ecologies in recent times. The alimentary consumption of plastic debris by coral organisms and their entanglement in derelict piscatorial entrapments constitute a pressing quandary.

Diverse plastic entities, inclusive of microscale plastic particulates, engender a suffocating shroud upon coral formations, thereby encumbering photonic ingress. Parallelly, undertakings such as deep-sea mineral extraction coalesce in a compounding nexus of deleterious ramifications.

Notwithstanding, the paramount impetus behind the phenomenon of coral bleaching remains contingent upon the crescendo of thermal ascent. Notably, the Great Barrier Reef is confronted with an escalated susceptibility to episodic bleaching in each successive summer, a testament to the amplified atmospheric elevation of Australia by 1.4 degrees Celsius postdating the Industrial Revolution, which surpasses the global mean elevation.

Effects of Coral Bleaching

Helicopter ride over the Great Barrier Reef at the Whitsunday Islands, Australia.
Helicopter ride over the Great Barrier Reef at the Whitsunday Islands, Australia. Image: Flickr.

Drawing from prevailing temperature escalation patterns and the escalating phenomenon of ocean acidification, scientific projections posit the complete eradication of coral reefs by the temporal juncture of 2050.

Owing to the swift augmentation in sea surface temperatures, global coral formations underwent a deleterious decline of 14% within the temporal ambit spanning from 2009 to 2018. This decrement eclipses the expanse of the entire extant coral reef system in the precincts of Australia, encompassing a staggering aggregate of 11,650 square kilometers. The worldwide coral expanse experienced a substantial contraction of 32.5% during the expansive bleaching episode that transpired in the year 1998. Although the planetary state of coral vitality evinced amelioration in 2009, each successive incidence of bleaching augments the arduousness inherent in the coral’s regenerative endeavors.

It is pertinent to emphasize that coral reefs function as sanctuaries for a quarter of the diverse array of marine species; consequently, a decline in the diversity of coral reef ecosystems results in the loss of essential species. This decline will inevitably reduce the available space for hundreds of thousands of organisms, leading to harmful consequences for their survival and populations, with significant impacts on the overall balance of entire ecological systems and trophic interactions.

The worsening trend of climatic fluctuations will create an environment marked by increased volatility and more frequent occurrences of severe weather events, resulting in excessive discharges of nutrients or sedimentary debris. Without the protective barrier provided by coral reefs, coastal areas will face an elevated range of risks from storm surges. Furthermore, it is crucial to acknowledge the economic significance inherent in coral reefs, contributing to various industries, including food production, tourism, and pharmaceuticals, generating a substantial $2.7 trillion in accumulated revenue.

Concisively, the evanescence of coral reefs stands poised to bequeath profound ecological and economic repercussions, thereby exerting a deleterious influence upon marine biodiversity, coastal safeguarding, and other sectors reliant upon resources tethered to reef-centric domains.

The Future of Coral Reefs

Colony of Tubastraea micranthus partially affected by bleaching.
Colony of Tubastraea micranthus partially affected by bleaching, Koh Phangan, Tailandia. Image: Chaloklum Diving.

The sixth instance of extensive coral bleaching is presently confronting the Great Barrier Reef. In order to mitigate the progression of global warming, which induces an escalation in both oceanic acidification and thermal elevation, it becomes imperative to diminish our reliance on fossil fuels. Nevertheless, the circumstances do not exist in an entirely desolate state. As per a report from the year 2020, a considerable number of coral reefs possess an inherent capacity for autonomous recuperation, contingent upon the favorability of ecological parameters. Demonstratively, the global expanse of coral reefs has reverted to levels akin to those preceding 1998 within the preceding decade. Furthermore, the year 2019 witnessed a surge of 2% in coral reef dimensions.

Furthermore, there is a differential resilience among assorted coral reefs. Illustratively, coral formations situated in the utmost northern reaches of the Gulf within the Red Sea have evinced heightened resistance to augmented temperatures in juxtaposition to the majority of their coral counterparts. Additionally, a coral reef of newfound origin, situated adjacent to the coastal periphery of Tahiti within the expanse of the South Pacific Ocean, has recently manifested an absence of bleaching indications. This alludes to the latent presence of numerous undisclosed and plausibly intact coral reefs within the oceanic milieu.

Though challenges endure, these instances instill optimism, suggesting that through purposeful endeavors directed towards climate modulation and safeguarding maritime ecosystems, the potential for coral reefs to reinvigorate and flourish in the midst of persistent environmental exigencies remains viable.