Why are the world's coral reefs declining, and what can be done about it? Sally Best investigates
Coral reefs are a vital part of the natural world and have a net global worth of £6trn a year. They host 25% of known oceanic life despite making up just 1% of the ocean floor, and protect coasts by reducing wave impacts along coastline. They also generate billions of pounds of fishing and tourism revenue.
However, a combination of stresses have caused the loss of 50% of reef cover over the past 30 years, with projections suggesting 90% of reefs will be under threat by 2030 if no change is made.
What is a coral reef?
Coral reefs are animal, plant and mineral. The animal constituent consists of thousands of individual corals called polyps. These invertebrates secrete calcium carbonate, which provides a exoskeleton for corals to live on. Many species also contain a photosynthetic algae called zooxanthellae. The algae provides the coral with the food that it synthesises, while the coral gives the algae a place to live.
Sexual reproduction, known as 'broadcast spawning', generally occurs once a year. The embryos drift for a few days before developing into larva, which settle on the sea floor. These events are highly dependent on multiple factors, such as lunar cycles, sea temperature cycles and time of day.
Asexual reproduction occurs via 'budding' or fragmentation – whereby the coral divides into clones within the same colony. Fragmentation occurs where bits of coral are broken off through wave action, storm events
or even animal intervention.
A trio of threats
Pollutants such as oil, agricultural runoff and chemical spills can disrupt coral systems' growth and reproduction, and can also cause disease and mortality. Many of these pollutants are a result of human development in coastal areas.
Corals are also highly attuned to water temperature. A 1°C rise can cause the polyp to expel its algae, meaning it no longer has a food source; it also appears white or 'bleached'. The symbiosis can be reformed if temperatures reduce and
it is quickly re-established; otherwise, the coral will die.
“Increased CO2 prompts ocean acidification, degrading the coral reef exoskeleton“
During the past 30 years, the sea's surface temperature has consistently been at a record high – and warming events are happening more severely and frequently. A major bleaching between 2014 and 2016 harmed more than 70% of global reefs. Scientists are still trying to understand the causes and ramifications.
Increased CO2 also prompts ocean acidification. Sea water absorbs excess CO2 from the atmosphere, which reacts with water to form carbonic acid. This dissolves calcium carbonate, degrading the exoskeleton and leaving no home
for the polyps. Recent estimates have calculated that ocean pH could decrease from 8.2 to 7.8 by the end of this century, meaning ocean acidity would be the highest it's been in the past 20m years.
These stressors decrease reef health and increase susceptibility to disease. Stony coral tissue loss disease, for example, attacks a reef's hard 'framework' corals, and affects more than 20 species in the Caribbean. The cause is a suspected pathogen that can spread rapidly over and between reefs. Efforts to monitor it are under way, in the hope of identifying the cause.
Reef restoration
Bleaching and disease have highlighted the need for novel management strategies such as Marine Protected Areas, catchment management to improve water quality, the ending of destructive fishing practices, and global governmental consensus for reducing greenhouse gas emissions. Restoration is also crucial; this includes growing asexually or sexually derived corals in land-based or ocean nurseries and then outplanting them onto degraded reefs.
The use of asexually derived corals mimics the 'fragmentation' processes by which corals reproduce. Scientists on the Great Barrier Reef are taking cuttings from healthy corals that survived the bleaching of 2014-2016. These are attached to coral tree frames in ocean-based nurseries and then, once grown, re-attached to damaged areas of the reef. The hope is that any propagated coral colonies will be more resilient to any higher water temperatures.
However, at present, outplanted corals have a relatively low survival rate. It also takes a while for species that are supported by the reef to return. And although this method increses colony tolerance, it does not promote genetic diversity. SECORE, an organisation concerned with coral reef conservation, is focusing restoration efforts on the use of sexually reproduced coral juveniles. This involves collecting coral gametes during spawning events and mating them via in-vitro fertilisation to produce genetically unique larvae. These may be reared in the ocean or in land-based nurseries. Resulting growths are placed back onto an existing reef, adding new genes and so increasing genetic diversity and resilience. These new genotypes within the reef may cope better with variable environments.
SECORE has also recently begun developing coral settlement substrates that self-attach to an existing reef, meaning large numbers of coral growths can seed onto a reef without having to be transplanted by hand.
Building stress tolerance
Some researchers are using 'assisted evolution' to enhance traits that will increase reef tolerance. This involves identifying coral sites with high resilience to environmental stressors. It is then possible to create hybrids between this resistant coral and other less tolerant coral from other sites, outplanting the offspring onto the less tolerant reef. This introduces different genes into struggling coral ecosystems.
Conditioning involves exposing adult colonies to stressors in order to increase the tolerance and fitness of offspring and their zooxanthellae. Prolonged stress should cause the coral to express genes that enable it to cope better in altered environments.
The restoration of reefs is cost and time-intensive, and limited in scale. Outplanting fragments requires manual transplantation – an arduous task that may not keep up with the rate at which reefs are declining. Restoration techniques need to be upscaled urgently. Past reef restoration science seems to have primarily focused on short-term experiments that evaluate the initial establishment phase of outplanted coral. Future experiments should aim for a long-term assessment.
There is a small window in which we can stop the devastation of the remaining 50% of reefs and the benefits they provide. Concerted efforts to reduce greenhouse gas emissions will be crucial if we are to halt the warming and acidification of the oceans. Additionally, efforts must focus on increasing the scale of restoration in this race against time.
50% More than 50% of coral reefs have been lost during the past 30 years
£6trn Globally, coral reefs have a net global worth of £6trn a year
70% A global bleaching event between 2014 and 2016 harmed more than 70% of coral reefs
Sally Best is an environmental biologist and scientific journalist.
