Coral reef bleaching is a stress response where corals lose their vibrant colors and turn white. These underwater structures are colonies of small animals called polyps. Bleaching occurs when corals expel the microscopic algae living within their tissues, which reveals the coral’s underlying white calcium carbonate skeleton. A bleached coral is not immediately dead but is weakened and more susceptible to starvation and disease.
The Biological Process of Bleaching
The brilliant colors of healthy corals come from a symbiotic relationship with microscopic algae called zooxanthellae. This partnership is mutually beneficial; the algae perform photosynthesis, providing the coral with up to 90% of its energy. In return, the coral provides the algae with shelter and necessary compounds for photosynthesis.
When corals experience environmental stress, this symbiotic relationship breaks down. The stress causes the zooxanthellae to produce reactive oxygen species, which are toxic to the coral. As a survival mechanism, the coral expels the algae from its tissues, leading to the bleached appearance. Some corals may produce bright fluorescent pigments when stressed, appearing yellow or blue, as a protective mechanism similar to sunscreen.
Primary Causes of Coral Bleaching
The primary cause of mass coral bleaching is rising ocean temperatures driven by global climate change. Corals are highly sensitive to temperature fluctuations and live within a narrow thermal range. A sustained increase in sea surface temperature of just 1-2°C (1.8-3.6°F) for a few weeks, often during marine heatwaves, can trigger widespread bleaching.
While ocean warming is the main trigger, other environmental stressors can cause or worsen coral bleaching. Pollution from land-based runoff, including agricultural chemicals and sediment, can stress corals. Overexposure to solar radiation, especially during high water temperatures, can induce bleaching in shallow-water corals. Extreme low tides that expose corals to the air can also lead to bleaching.
Ecosystem-Wide Consequences
Bleached reefs no longer serve as vibrant habitats, leading to a loss of shelter and food for many marine species and a significant decline in biodiversity. Thousands of species, including fish, invertebrates like crabs and shrimp, and sea turtles, depend on coral reefs for spawning, protection from predators, and sustenance.
These consequences also have direct impacts on human communities. Coral reefs act as natural breakwaters, and their degradation diminishes protection for coastal areas from storm surges. Communities that rely on reefs for fishing and tourism face economic and food security challenges, as the loss of fish populations affects both local food sources and the global fishing industry.
Coral Recovery and Resilience
If the environmental stressor is short-lived, a coral can survive a bleaching event. Over weeks to months, corals can regain their zooxanthellae from the surrounding water, recovering their color and food source. The ability of a reef to recover is a measure of its resilience.
However, if the stress is severe or prolonged, the coral will starve and die. Without their algae, corals become more vulnerable to disease. Following death, the coral’s skeleton crumbles, leading to the loss of the reef structure. The increasing frequency of mass bleaching events gives reefs less time to recover between these stressful episodes.