Model Answer
0 min readIntroduction
Coral reefs are vibrant ecosystems, often called the ‘rainforests of the sea’, supporting approximately 25% of all marine life. However, these delicate ecosystems are increasingly threatened by a phenomenon known as coral bleaching. Coral bleaching occurs when corals, stressed by changes in their environment, expel the symbiotic algae (zooxanthellae) living in their tissues, causing them to turn completely white. This isn’t coral death, but it leaves them vulnerable and significantly increases mortality risk. Understanding the reasons behind this stress is crucial for effective conservation efforts.
Understanding Coral Bleaching
Corals have a symbiotic relationship with microscopic algae called zooxanthellae that live in their tissues. These algae provide the coral with up to 90% of its energy through photosynthesis, and also give corals their vibrant colors. When corals are stressed, they expel these algae.
Reasons for Coral Stress & Bleaching
1. Rising Sea Temperatures
Thermal stress is the most significant cause of coral bleaching globally. Even a 1-2°C increase above the usual maximum temperature can trigger bleaching. This is directly linked to climate change and the absorption of excess heat by the oceans. The Great Barrier Reef has experienced several mass bleaching events in recent years (2016, 2017, 2020, 2022) due to record-breaking sea temperatures.
2. Ocean Acidification
Increased levels of carbon dioxide (CO2) in the atmosphere are absorbed by the ocean, leading to a decrease in pH – a process called ocean acidification. This reduces the availability of carbonate ions, which corals need to build their skeletons. Weaker skeletons make corals more susceptible to stress and bleaching.
3. Pollution
- Nutrient Pollution: Runoff from agricultural fertilizers and sewage introduces excess nutrients into the water, promoting algal blooms. These blooms can smother corals and reduce water quality.
- Chemical Pollution: Exposure to pollutants like pesticides, herbicides, and heavy metals can directly damage coral tissues and impair their ability to function.
- Sedimentation: Increased sediment runoff from deforestation and construction can cloud the water, reducing light penetration and hindering photosynthesis by zooxanthellae.
4. Extreme Weather Events
Hurricanes and cyclones can cause physical damage to coral reefs, breaking coral structures and increasing sedimentation. Heavy rainfall can also lead to freshwater runoff, reducing salinity and stressing corals.
5. Diseases
Coral diseases, such as white band disease and black band disease, can weaken corals and make them more vulnerable to bleaching. These diseases are often exacerbated by environmental stressors.
6. Changes in Salinity & Light Exposure
Sudden changes in salinity, often due to heavy rainfall or freshwater runoff, can stress corals. Similarly, excessive or insufficient light exposure can disrupt the symbiotic relationship with zooxanthellae.
Regional Variations
While global warming is the primary driver, regional factors play a role. For example, localized pollution is a major concern in the Caribbean, while sedimentation is a significant issue in Southeast Asia.
Conclusion
Coral bleaching is a complex issue driven by a combination of global and local stressors. While rising sea temperatures pose the most significant threat, addressing pollution, promoting sustainable land management practices, and mitigating climate change are crucial for protecting these vital ecosystems. The future of coral reefs depends on urgent and concerted action to reduce these stressors and enhance coral resilience.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.