Coral Bleaching: can coral reefs recover?
A viral article recently announced that the Great Barrier Reef was dead, as of 2016. It is safe to say that this is not the case; preliminary assessments of damage suggest that 22% of the reef died as a result of the 2016 bleaching event, deemed the worst on record. While writing an obituary for the Great Barrier Reef might be premature, it cannot be denied that coral reefs are in trouble on a global scale.
Over 27 years, in the period between 1985 and 2012, global coral cover on reefs was found to decrease from 28%, to 13.8%. This represents a major decline in the global coral abundance; driven primarily by tropical cyclones, coral bleaching and predation by crown-of-thorns starfish. Each of these drivers has been linked to anthropogenic activity. Tropical cyclones and coral bleaching events are likely to increase in both frequency and intensity as the global climate becomes warmer, a result of human greenhouse gas emission. Outbreak frequency of crown-of-thorns starfish has been linked to increased nutrient input from rivers as a result of agricultural run-off. It is clear that humans are indirectly causing damage to global coral reef systems. In 1998, the worst coral bleaching event in history occurred, as the global oceans ‘overheated’ as a result of an El Niño event.
The animation below illustrates how the oceans warmed during 1998. Annual mean sea surface temperatures have been predicted to rise above the catastrophic, anomalous temperatures experienced in 1998 within a few decades. Leading some scientists to believe that such bleaching effects could be annual or biannual within 30-50 years. The question isn’t whether sea temperatures will rise, or will coral bleaching events become more frequent. The question is, can corals recover after these bleaching events?
Coral Bleaching: what is it?
Corals share an exceptional relationship with algae. Corals can catch their own prey to feed on but a substantial amount of their energy comes from symbiotic algae, called zooxanthellae, residing within their cells. These algal symbionts photosynthesise, producing energy for the coral, along with oxygen and glucose as byproducts. These corals are living at the edge of their thermal limits, meaning that small rises in temperature can upset the balance of this relationship.
Symbiotic cnidarians (the phylum corals belong to) often experience elevated levels of oxygen in their system due to the photosynthesising zooxanthellae. These conditions are deemed ‘hyperoxic‘. When hyperoxia occurs in conjunction with elevated temperature and the presence of UV light, active forms of oxygen are produced; leading to oxygen toxicity. Higher temperatures mean that more energy is available for chemical reactions to occur, which leads to the production of active oxygen (1O2); superoxide radicals (O2–) and hydrogen peroxide (H2O2). These active forms of oxygen will lead to tissue damage; thus, the corals are forced to find a way to reduce the amount of oxygen in their system, as they are unable to control the temperature of the water. They can do this in two ways; by reducing the amount of photosynthetic pigments within the zooxanthellae, or by the the direct expulsion of zooxanthellae from the cells. Both of these processes result in the whitening of the coral, hence the term ‘bleaching’. A process that can lead to the death of a coral. This is illustrated in the diagram below.
Can Corals Recover?
In short, corals can recover from bleaching. Such recovery has been observed on the reefs in the area around the Chagos Archipelago. The world’s largest ‘no take’ Marine Protected Area (MPA) in the world until 2015, representing an area the size of France. Here, the corals have been shown to have positive carbonate budgets. In 2015 carbonate budgets averaged 3.7 G (G = kg CaCO3 m−2 yr−1) implying that these corals are not just surviving, but growing. Critically, Acropora-dominated reefs demonstrated carbonate budgets averaging at 8.4G. Acropora being the species of coral most affected by the 1998 bleaching event. These carbonate budgets imply growth rates that would be keep pace with the projections of sea level rise, predicted by the IPCC.
The reefs of Chagos have demonstrated the ability of coral reefs to recover from bleaching events, as illustrated in the figure below. Coral cover in the Chagos reefs was dramatically reduced after the 1998 bleaching event. Coral cover was above 40% in reefs between 5 – 15m depth in 1996, but dropped to below 15% in 1999, after the bleaching event. However, over time, coral reefs showed recovery; reaching pre-1998 levels of coral cover at 10m and above 35% cover between 5 and 15m depth.
Can corals cope with climate change?
The reefs of the Chagos represent a unique situation whereby anthropogenic impacts are extremely reduced. Under these conditions, given time, the coral reefs have demonstrated the ability to recover. Not all coral reefs have the privilege of existing within a MPA, thus their ability to recover from bleaching is reduced. Chagos might be able to cope at this point, but the Great Barrier Reef has not shown any substantial recovery since the 1998 bleaching event.
Coral reefs can recover from bleaching events, if they have the right conditions for sufficient periods of time. However, the crux of the matter is that coral reefs are likely not going to have the time to recover. As mentioned earlier, coral bleaching events are likely to become much more frequent. If they are become an annual occurrence then corals would have to fully recover every year to survive. The coral reefs in Chagos took more than 10 years to recover after the 1998 bleaching event; without human disturbance. It might not be time to right their obituary, but if corals are to survive into the next century, they are going to have to prove to be much more resilient than they have shown thus far.