Corals are colonies of individual coral polyps, marine invertebrates which secrete an exoskeleton. Although they catch some food on tentacles, most get the majority of their energy from zooxanthellae, which live symbiotically in their tissues and give coral their colour. Under normal conditions, these zooxanthellae photosynthesise, providing their host coral polyp with energy.
Coral bleaching occurs when high temperatures stress corals, causing them to expel their symbiotic zooxanthellae, leaving the corals with greatly limited energy intake. The bleached coral is not dead yet, but if temperatures don’t decrease quickly enough, the coral will starve.
For many reefs systems around the world, 2024 was the worst year for bleaching in recorded history. From the Great Barrier Reef to the Mesoamerican reef, record summer heat and an el Niño climatic oscillation that exacerbates ocean warming coincided to wreak havoc on reefs globally. This was the Great Barrier Reef’s fifth mass bleaching event in eight years. Over 75% of coral colonies were bleached, and the effects of heat stress were felt down to 18 m, where corals are normally sufficiently buffered from changing conditions.
Yet Wakatobi’s reefs emerged relatively unscathed – there was no systematic bleaching. In fact, over the last 25 years, Wakatobi’s reefs have proved extremely resilient to climate change-induced bleaching. This resilience can be attributed to a combination of:
- Evolved heat tolerance
- Reef protection
- Ocean current mixing.
Local seasonal warming has driven the evolution of heat tolerance in Wakatobi’s corals. Changing conditions underwater at Wakatobi are driven by local weather systems much more than they are by global el Niño/la Niña oscillations. Local climate drives a regular seasonal fluctuation in water temperature from lows of around 25˚C to highs of up to 31˚C.
Reefs at Wakatobi have been exposed to this dramatic seasonal change, with high summer peaks, for at least recorded history, and likely long before that. They have had centuries to evolve to withstand the relatively rapid and prolonged warming associated with seasonal temperature highs. This is a stark contrast to reefs elsewhere, which have grown accustomed to moderate temperature fluctuations, only recently experiencing warming, and have not evolved strategies to cope.
There are a number of mechanisms by which corals can withstand higher temperatures.
Much of a coral’s heat tolerance is determined by the type of symbiotic zooxanthellae that the coral hosts, with corals in warmer waters evolving to host heat-tolerant symbionts. Corals evolve other physiological strategies, such as antioxidant production to combat stress. Under heat stress, a coral’s symbiotic zooxanthellae start to produce reactive oxygen species, which are toxic to the coral host. Some corals exposed to regular warming have evolved to produce antioxidants to neutralise this threat, and enable the coral to keep their zooxanthellae instead of expelling them.
Alongside evolved resilience, the protection of Wakatobi’s reefs also plays an important role in thermal tolerance. Stress affects reefs in much the same way as it affects people. A well person is in a good position to cope with life’s trials and tribulations. When subjected to a persistent stressor, though, a person becomes less resilient. The neighbour’s new puppy keeps you up all week, barking every night. Sleep deprived, an important deadline at work is much more stressful than it should be and by Friday you’ve picked up a cold. Continual pressure from one stressor has impacted your ability to tackle other stressors.
A reef is just the same. A ‘well’ reef is able to cope with small perturbations. However, when continually impacted by one stressor, the resilience of the reef is reduced, and it suffers disproportionately under additional stressors. Reef health is therefore an important factor in reef climate resilience. If we are able to shield reef ecosystems from direct pressures such as destructive fishing methods, pollution, and the introduction of invasive species, they fare better through warming events.
Even when Wakatobi’s waters warm up, they are often not warm for long, on account of powerful mixing. Wakatobi is located at a confluence of currents from the Banda Sea, to the East, and the Flores Sea, to the West. At the nexus of East and West-going currents, Wakatobi’s water is never still for long. Upwellings also bring water up from cool depths, and outgoing currents draw warm water off the reef-tops. This means that when reefs are exposed to warm spells, of 30 or even 31˚C, they are never warm for long.
At Wakatobi, a unique combination of evolved heat tolerance in response to local weather conditions, reef protection and extensive ocean mixing come together to protect reefs from the effects of climate change. As the majority of reefs worldwide suffer under bleaching events of increasing frequency and intensity, resilient reefs become more valuable in their biodiversity-supporting capacity.