Understanding Coral Bleaching: The Great Barrier Reef and Beyond

Introduction to Coral Bleaching

When I taught marine zoology to postgraduate students at the Institute of Science Mumbai for over 25 years, I never imagined exploring the wonders of the Great Barrier Reef in Australia. However, during my visit to Australia in October-November 2019, I had the opportunity to snorkel in the Great Barrier Reef (GBR) near Cairns. This unique experience provided me with firsthand insight into the delicate ecosystem of these vibrant creatures.

Corals, often overlooked in their residential simplicity, are invertebrates that belong to the group of colorful and wonderful animals known as cnidaria, which also includes sea anemones. Each individual coral is a polyp, and these polyps can form colonies of hundreds to thousands of genetically identical polyps through a process called budding. These colonies are the basis of reef creation, providing homes for millions of species.

Types of Corals

Corals can be broadly classified into two types:

Hard Corals: Extract large amounts of calcium from the surrounding seawater to create hardened structures for protection and growth. These corals form extensive reef systems by millions of tiny polyps that live in symbiosis with zooxanthellae, single-celled algae. Soft Corals: These do not build reefs but are still vital components of the marine ecosystem, providing habitats and shelter for numerous species.

Roughly located off the eastern coast of Australia, the Great Barrier Reef is the largest coral reef system in the world, covering approximately 1600 miles or 2600 km. This reef ecosystem is home to over 1500 species of fish and 411 species of hard corals, contributing significantly to Australia's economy and supporting tens of thousands of jobs.

What Causes Coral Bleaching?

Coral bleaching occurs when coral colonies lose their color, either due to the loss of pigments by microscopic algae (zooxanthellae) living in symbiosis with the host polyps or because these zooxanthellae are expelled. Without these zooxanthellae, the stony skeletons of corals appear white, hence the term "coral bleaching."

Severe stressors like warmer water temperatures and ocean acidification are the main culprits behind coral bleaching. Temperatures exceeding 1 degree Celsius above the normal seasonal maximum can trigger widespread bleaching. Light intensity, nutrients, and other ecological parameters can also expel zooxanthellae, causing corals to turn white.

Great Barrier Reef and Coral Bleaching

Severe bleaching events have been documented in all three regions—northern, central, and large parts of the southern sectors—of the GBR. Notable instances of GBR bleaching have occurred in 2002, 2016, 2017, and again in 2020. These events underscore the urgency of addressing the ecological impacts of climate change on marine ecosystems.

The Great Barrier Reef suffers third mass bleaching event in five years, highlighting the repeated and escalating threat to this vital system. Coral reefs are the largest living structures on the planet, and their health directly impacts global biodiversity and ecosystem services.

Conclusion

Understanding and mitigating the causes of coral bleaching is crucial for the preservation of our oceans' most vibrant and vital ecosystems. From the colorful corals of the Great Barrier Reef to the vast and diverse marine life they support, protecting these fragile ecosystems is not just about maintaining beauty but ensuring the health of our planet.

References:

Everything you need to know about coral bleaching—and how we can stop it

Great Barrier Reef

Great Barrier Reef suffers third mass bleaching event in five years

What is coral bleaching?