Thriving in shallow sunlit waters across the equatorial belt, tropical coral reefs are among the most colourful, productive and diverse ecosystems on earth. Healthy coral reefs are major sites for fish nurseries and young fish development, which are important for maintaining regional commercial fish stocks, and are key attractions for tourists and divers.
Yet these finely balanced environments are at serious risk from an array of threats, including pollution, habitat destruction, coral bleaching, predation, over fishing, dynamite fishing, unsustainable tourism, the live aquarium trade, and predicted climate change. Despite this, new studies from Australia’s Great Barrier Reef could reveal some encouraging research that might help save this region and provide some hope for other endangered reefs with similar geographies.
Today, many of the world’s coral reefs are susceptible to damage, caused by both natural actions and human activities. The systems of the Caribbean are under particular pressure, with only 10% of modern reef areas now supporting living coral cover. Running adjacent to the Queensland coast of Australia, the Great Barrier Reef is also under threat, with a recent study from the Australian Institute of Marine Science revealing that around 50% of the coral cover in shallower reef regions has disappeared over the last three decades. It is believed that most of this damage was caused by storms (over the last seven years, six cyclones have impacted the region), as well as outbreaks of crown-of-thorns starfish (which feed on coral tissues), and coral bleaching. Crown-of-thorns starfish are prone to large population explosions, which are often due to an influx of nutrients into the seawater, caused by run-off from agricultural land and storm damage. The starfish extrude their stomachs to feed on coral tissue and during a population bloom can be responsible for consuming up to 90% of the live coral cover in a given area.
Although they resemble plants, corals are active feeders, ensnaring plankton and organic matter with their stinging tentacles. Related to jellyfish, corals are actually animals that spend the juvenile phases of their lifecycle suspended in the water column, as part of the zooplankton, before drifting to the seabed where they become polyps and cement their limestone skeletons (a process that eventually forms extensive reef systems).
Adult corals reproduce by releasing vast clouds of sperm and eggs into the surrounding water, which fertilise in the plankton before settling; this process, known as spawning, allows a coral to colonise other areas of a reef. Corals are also symbiotic organisms, with a relationship with an alga called zooxanthellae. This alga lives in the relative safety of the coral tissue and photosynthesises to manufacture sugars, which the coral polyps use to derive energy via respiration. This relationship benefits both organisms; yet for the algae to survive, it is typical for a coral to be positioned in clear, sunlit waters to obtain optimal light levels. However, new research from the Catlin Seaview Survey has discovered healthy coral habitats in deeper regions of the Great Barrier Reef, which could help to restore this ecologically important UNESCO World Heritage Site.
Specialised deep sea diving techniques and remote operated vehicles have located healthy growths of shallow water coral species at depths from 30 to 120 metres, including regions that have been impacted by cyclone damage in shallower areas. These finds are unusual because tropical reef forming corals are influenced by sunlight, which allows the symbiotic algae to photosynthesise (some other corals, including deep sea and polar water species have large tentacles for feeding, rather than relying on symbiotic algae). Accordingly, it is now thought that the deeper regions of a reef may provide a refuge for shallow water coral species, which will subsequently spawn, providing an opportunity for juvenile corals to settle in shallower waters, including areas that have been damaged by storms or starfish outbreaks. Once settled, these young corals would probably flourish due to the lack of competition from other corals and the optimal conditions. This could provide encouraging hope for the entire reef system and help repopulate shallower areas of the reef that have been damaged.
Hopes for the Future
The Great Barrier Reef is the largest collection of coral reefs in the world, which stretches for some 2,600 kilometres along the Northeast coast of Australia. Despite this size, half of this environment is already damaged by storm action, starfish predation and other phenomena, and there are further threats from climate change and ocean acidification that are also predicted. Accordingly, the future for this region, which generates around £3.2 billion a year from tourism, appears bleak. However, growing environmental awareness, sustainable tourism and proposed starfish population controls will help to conserve this ecologically rich area. Indeed, coral reefs have survived other natural threats in the past and nature is often more resilient than at first perceived. Yet, in a world changing at such a fast pace, can corals and other reef organisms adapt in time?
Despite these concerns, the discoveries made by the Catlin Seaview Survey will provide further hope for this region. What is more, these findings will be encouraging for other reef systems that are under threat, which may also have underlying coral growth in deeper waters, thriving in marine geographies that have yet to be explored, which can help to re-populate shallower, damaged reef systems.