Going, Going, Gone: Has the time for tropical coral reef recovery run out?
Tropical coral reefs are some of the most diverse marine habitats on earth, often described as ‘Rainforests of the Sea’. Coral reefs are composed of colonies of tiny polyps that live within a calcium carbonate skeleton, they have a symbiotic relationship with photosynthetic algae called zooxanthellae, these algae are what give the coral their colour. In recent years, coral reefs have been experiencing coral death due to the loss of the zooxanthellae because of environmental stress. The ability of coral to rejuvenate is a slow and tedious process therefore making these reefs a vulnerable and extreme habitat possibly on the brink of complete loss.
The value of coral reefs
Providing a habitat for approximately 25% of all marine organisms, an estimated 2 million species. Ecologically coral reefs are an integral part of the marine ecosystem, as they provide a natural barrier against weather events such as storms. The reef also acts a nursery ground for a quarter of fish species around 4,000 fish species, this is key in their development as it is an area full of nutrients required for juvenile fish and provides protection from predators. Some of the fish species that live off the reef habitat are key to commercial fisheries therefore coral reefs are not only ecologically but also economically valuable.
Healthy cauliflower coral. Source: http://oceanservice.noaa.gov/podcast/sep16/dd67-reef-resilience.html
The threats to coral reefs
In the last 50 years coral reef degradation has become a more prevalent issue, with significant loss occurring to reef systems globally at an unexpected and almost uncontrollable rate. It is estimated in some areas such as the Caribbean, 80% of their coral reefs have been lost in this time. Events as recent as April 2016 have seen a reported 67% of the coral on the Great Barrier Reef die. There are a number of different causes for coral reef degradation but without a doubt climate change is the main factor driving coral death.
Coral bleaching events have been the main cause of mass coral death over the last 20 years. The bleaching event in 1997-1998 saw 16% of the worlds coral reefs die, areas such as the Seychelles experienced 90% coral degradation in just a matter of months. Coral death occurs when the ocean temperature increases by 1°C or more above its mean summer maximum over a prolonged period. This temperature increase causes stress so much so the polyps expel the zooxanthellae, without which they struggle to get the nutrients required to survive and eventually the polyp dies just leaving a white calcium carbonate skeleton behind. With the time it takes for sea temperature rise to have a permanent effect on coral being as little at 10 weeks and the time it takes coral to heal and replenish being years, the likelihood of saving the coral reefs in time is low, now it may just be a case of mitigating the coral reefs that are left.