What Is Coral Bleaching?
Coral bleaching occurs when corals expel the symbiotic algae — called zooxanthellae — that live within their tissues. These microscopic algae are responsible for the vibrant colours of healthy coral and, more critically, for producing up to 90 percent of the energy corals need to survive through photosynthesis. When environmental stress causes the coral to eject its algae, the transparent tissue reveals the white limestone skeleton beneath, giving the coral its characteristic "bleached" appearance.
Bleached coral is not dead — at least not immediately. If the stressor is removed within a few weeks, zooxanthellae can recolonise the coral tissue and the colony can recover. However, if the stress persists for too long, the coral starves and dies. Dead coral is quickly colonised by algae, and the complex three-dimensional structure that supports thousands of species begins to erode and collapse.
The primary trigger for mass bleaching is elevated sea surface temperature. When water temperatures exceed the local average summer maximum by just 1 to 2 degrees Celsius for several weeks, widespread bleaching can occur. Other stressors — including high solar radiation, changes in salinity, pollution, and sedimentation — can compound the effect, but temperature is overwhelmingly the dominant factor in the large-scale bleaching events that have struck the Maldives.
The 1998 Bleaching Event
The 1998 mass bleaching event was the first truly catastrophic blow to Maldivian coral reefs in recorded history. Driven by one of the strongest El Nino events of the twentieth century, sea surface temperatures across the Indian Ocean spiked well above normal levels for months. In the Maldives, water temperatures rose to 33 to 34 degrees Celsius in shallow lagoons — far beyond the 29-degree threshold that local corals could tolerate.
The results were devastating. Surveys conducted in the aftermath estimated that roughly 90 percent of shallow-water corals across the archipelago bleached, and around 60 to 70 percent of those died. Entire house reefs that had taken centuries to grow were reduced to algae-covered rubble within months. Branching Acropora species, which had dominated many reef crests and slopes, were virtually eliminated from large areas.
The ecological consequences rippled outward. Fish populations declined as their habitat degraded. Reef sharks and other predators that depended on healthy reef structure saw reduced prey availability. Even the beaches were affected — dead reef produces less sand, and without the wave-breaking function of a healthy reef crest, shoreline erosion accelerated on some islands.
The 2016 Global Bleaching Event
The 2015-2016 global bleaching event, fuelled by another powerful El Nino combined with the background warming trend of climate change, hit the Maldives hard once again. Sea surface temperatures remained elevated for an extended period, and this time many reefs that had partially recovered from 1998 were struck before they had fully matured.
Research stations and resort marine biologists documented severe bleaching across all atolls. Some sites lost 70 to 80 percent of their live coral cover in a matter of months. The event was particularly damaging because it affected deeper corals as well — reefs at 15 to 20 metres depth, which had largely escaped the 1998 event, showed significant bleaching in 2016.
Coral species responded differently. Massive Porites corals, often called "boulder corals," showed higher survival rates than branching species. Some Acropora colonies in areas with good current flow survived, suggesting that water circulation helps moderate thermal stress. These differential survival rates are reshaping the composition of Maldivian reefs, potentially making them less structurally complex but more heat-tolerant over time.
El Nino and Climate Change
El Nino is a natural climate pattern in which warm water accumulates in the central and eastern Pacific Ocean, triggering changes in weather and ocean temperatures worldwide. In the Indian Ocean, El Nino events typically cause elevated sea surface temperatures — the direct trigger for mass coral bleaching in the Maldives.
However, El Nino alone does not explain the increasing severity and frequency of bleaching. Global ocean temperatures have risen by approximately 1 degree Celsius since the pre-industrial era due to greenhouse gas emissions. This background warming means that even moderate El Nino events can now push temperatures past bleaching thresholds that corals could have tolerated decades ago. Scientists warn that if global warming continues at the current rate, mass bleaching could become an annual occurrence in the tropics by mid-century, leaving no time for recovery between events.
The Maldives, with an average elevation of just 1.5 metres above sea level, faces a double threat from climate change: the loss of its protective reef ecosystems and the prospect of sea level rise inundating its islands. This makes the Maldives both one of the most vulnerable nations on Earth and one of the most vocal advocates for global climate action.
Recovery and Resilience
Despite the severity of past bleaching events, Maldivian reefs have demonstrated a remarkable capacity for recovery. Following the 1998 event, coral cover on many reefs gradually increased over the next 10 to 15 years. Fast-growing species like Acropora recolonised rubble fields, and fish populations rebounded as habitat complexity improved.
Several factors influence how quickly a reef can recover. Reefs with good water flow and low local stressors tend to bounce back faster. The presence of herbivorous fish — particularly parrotfish and surgeonfish — is critical because they prevent algae from dominating the substrate and smothering coral recruits. Marine protected areas that restrict fishing help maintain these herbivore populations.
Coral restoration projects are also playing a growing role. Several resorts and NGOs in the Maldives operate coral nursery programmes, growing fragments of resilient coral species on underwater frames and transplanting them to degraded reefs. While these projects cannot restore reefs at the scale of natural recovery, they can accelerate regrowth in key areas and serve as powerful tools for education and engagement.
What Visitors Should Know
If you visit the Maldives, you may see signs of past bleaching — patches of dead coral framework overgrown with algae, or areas where the reef appears less colourful than photographs from decades past. This does not mean the reefs are not worth visiting. Even recovering reefs teem with life, and many sites have regained impressive coral cover and diversity.
There are several things you can do as a responsible visitor. Avoid touching, standing on, or collecting coral — even dead fragments, as these provide substrate for new coral growth. Use reef-safe sunscreen that does not contain oxybenzone or octinoxate, chemicals linked to coral stress. Choose snorkel and dive operators who follow sustainable practices and support conservation programmes. And consider the carbon footprint of your travel — the single most important factor in the long-term survival of coral reefs worldwide is reducing greenhouse gas emissions.
Many resorts now offer guided reef ecology tours where marine biologists explain the state of the house reef, point out signs of recovery, and describe the ecosystem's interconnections. These experiences can turn a holiday into a genuinely educational encounter with one of the planet's most urgent environmental challenges.