Coral bleaching

Coral polyps, which are tiny, individual animals, form colonies that build the intricate structures of coral reefs. These polyps live in a mutualistic relationship with microscopic algae called zooxanthellae. The algae reside within the coral's tissues and perform photosynthesis, converting sunlight into energy. This energy, primarily in the form of sugars, is then transferred to the coral, providing it with vital nutrients for growth, calcification, and survival. In return, the coral provides the algae with protection and access to compounds like carbon dioxide, essential for photosynthesis.

When environmental conditions deviate from a coral's optimal range, particularly when ocean temperatures rise even by 1–2 °C (1.8–3.6 °F) above the average summer maximum, this delicate symbiosis can break down. Other stressors include extreme drops in temperature, overexposure to sunlight, increased sedimentation, pollution, and changes in ocean chemistry (ocean acidification). Under stress, the photosynthetic processes within the zooxanthellae become impaired, leading to the production of harmful reactive oxygen species. In response, the coral expels the zooxanthellae from its tissues.

The expulsion of these pigmented algae leaves the transparent coral tissue to reveal the underlying white calcium carbonate skeleton, giving the coral a bleached appearance. While bleached corals are still alive, they are severely weakened and vulnerable. Without their primary food source, they begin to starve. If the stressor is removed and favourable conditions return within a few weeks to months, corals can sometimes reacquire zooxanthellae and recover. However, if the stressful conditions persist, the corals will eventually die from starvation or become susceptible to disease, leading to widespread reef degradation.

Encountering coral bleaching during a liveaboard trip can be a profound experience, offering a direct view into the challenges facing marine ecosystems. While divers naturally hope to see pristine, colourful reefs, observing areas affected by bleaching highlights the global impact of environmental stressors and underscores the importance of marine conservation. It provides an opportunity to understand the resilience of corals, as some areas may show signs of recovery or harbour more resilient species, while others may be severely impacted.

Divers often travel to remote and pristine locations accessible only by liveaboard, where reefs may have a better chance of recovering from bleaching events due to less localised human impact. However, even these remote areas are vulnerable to global changes like rising ocean temperatures. Seeing bleached corals can prompt discussions with dive guides and marine naturalists on board, who can often provide valuable insights into local reef health, recovery efforts, and the broader context of coral conservation.

As a diver, your actions can contribute to reef health. Practising excellent buoyancy control to avoid contact with corals, using reef-safe sunscreen, and supporting responsible tourism operators are all crucial. While liveaboards provide access to incredible dive sites, it's essential to recognise that these ecosystems are delicate. Witnessing bleaching can be a powerful motivator for divers to become advocates for ocean protection, contributing to the long-term health of the reefs they love to explore.

Myth: Bleached corals are dead corals. Fact: Bleaching means the coral has expelled its symbiotic algae and is under severe stress. While severely or prolonged bleached corals often die, they are still alive and have a chance to recover if environmental conditions improve quickly and they can reacquire algae.

Myth: All corals bleach at the same temperature. Fact: Coral species have varying tolerances to temperature changes. Some species are more resilient than others, and even within the same species, individual colonies may react differently based on their specific micro-environment or past exposure to stress.

Myth: Bleaching is a natural process that reefs recover from easily. Fact: While minor, localised bleaching can occur naturally, the widespread, severe, and frequent bleaching events seen today are primarily driven by human-induced climate change and are far beyond natural recovery rates. Reefs are struggling to adapt and recover between events.

Myth: Divers cause coral bleaching. Fact: Divers do not cause coral bleaching directly. The primary drivers are global stressors like rising ocean temperatures and ocean acidification. However, divers can contribute to localised stress through poor buoyancy control, touching corals, or using non-reef-safe products, which can exacerbate the impact of bleaching or hinder recovery.