World Oceans Day was celebrated on 8 June. The United Nations established it in 2008 to draw attention to the devastating impact of humans on marine ecosystems. Coral reefs are among the most endangered and threatened with extinction. Why and how soon will this happen? What can humanity do to avert disaster? Here’s the key takeaway from the UN Environment Programme’s latest report on coral reef restoration.

Where and how coral reefs form

Coral reefs can be found in the Pacific, Indian and Atlantic Oceans in waters of more than 100 countries. Most are located in the belt between the tropics of Cancer and Capricorn, but there are also coral reefs in regions further from the equator where warm tropical currents lead. Off the coast of Japan and the US state of Florida, for example. Coral reefs now cover 284,300 square kilometres around the world (almost as large as two Vologda regions, or one Italy).

Where are coral reefs?

Coral reefs are formed by colonies of coral polyps. These invertebrates secrete calcium carbonate from the sea water and deposit it underneath in layers that serve as the outer skeleton for an entire colony of polyps. This creates a reef framework that is complemented by many species of algae, sponges and molluscs.

In addition to reef-forming corals, there are also soft corals: they have no calcium base. They are found not only in warm waters, but also in the Sea of Okhotsk and in the Norwegian Sea. Soft corals look like whimsical plants, their clusters often look like colourful underwater gardens.

Why coral reefs exist?

Over 240 million years, coral reefs have evolved into one of the most complex ecosystems on the planet. Coral slime feeds bacterial plankton, which feed in clouds of sediment – fed by zooplankton and benthic invertebrates, which in turn feed on fish. Occupying less than 0.1% of the world’s ocean surface, coral reefs are responsible for more than 25% of all marine biodiversity, providing a habitat and food source for over 1 million marine animal and plant species.

Coral reefs protect coasts from erosion on the one hand, and human settlements and beaches from the ravages of ocean waves on the other. As more frequent and more intense storms are predicted around the world amid global warming, the importance of this role for corals is growing every year.

In December 2004, one of the most destructive tsunamis ever recorded occurred in the Indian Ocean: waves up to 30 m hit the shores of 18 countries, from South Africa to Thailand, killing 228 000 people. Some coastlines, however, were protected from the waves by massive coral reefs; on some shores there was no destruction or loss of life. In contrast, when sand and limestone mining destroyed reefs in the Maldives, the authorities had to build wave protection structures at a cost of nearly US$10 million per km.

The annual value of ecosystem services provided by coral reefs around the world is estimated at US$375 billion. The bizarre polyps and algae that build the structure support the livelihoods of at least 500 million people in more than 100 countries, primarily fishers and tourism workers. Fish and other reef dwellers are an important source of protein in the diet of nearly 1 billion people. Reef plants and animals produce substances used in the treatment of cardiovascular diseases, ulcers, leukaemia, lymphoma, skin cancer and many other ailments. More than half of all research into new cancer drugs is conducted on coral fauna and flora.

When and why coral reefs will disappear

In the past 30 years, the world has lost more than half its coral reefs: back in the 1980s, they covered almost 600 000 square kilometres. The World Resources Institute estimates that over 60% of coral reefs are at risk of extinction. This proportion could exceed 90% by 2030.

Which corals are threatened with extinction

The causes of extinction are many. First and foremost is the pollution of the ocean by sewage and rubbish, and the destruction of coral reef habitat. This is due to offshore construction and fishing methods such as explosive fishing or bottom trawling, which leave the seabed a lifeless desert. Finally, many reefs are mined for sand and limestone.

55% of the world’s coral reefs are negatively impacted by fishing practices which significantly reduce populations of herbivorous fish. This leads to excessive algae growth, causing coral polyps to die.

Added to this is the threat of global warming. Reef-forming corals are not well tolerated by rising temperatures – when the water gets too warm, they bleach and die. In 2014-17, for example, prolonged bleaching caused the death of one-third of the corals in the Great Barrier Reef stretching 2,300 km along the north-eastern coast of Australia.

Sea-level rise is another threat. Corals get less sunlight in deeper water, which reduces their growth rate and makes them less viable. Increased acidity in the water also prevents corals from growing their skeletons: colonies become more vulnerable to disease and seawater storms, among other things. The ocean, on the other hand, absorbs a quarter of all carbon dioxide from the atmosphere. Since the industrial revolution in the 19th century, CO2 levels in the air have risen by a third and are still rising – the seas are becoming more acidic.

The Intergovernmental Panel on Climate Change predicts that coral reefs will almost certainly disappear, even if global warming is kept within ‘safe’ limits of 2°C by the end of the 21st century.

How to save coral reefs

Saving them from extinction requires not only reducing greenhouse gas emissions, reducing ocean pollution and expanding protected areas, but also restoring endangered ecosystems.

One of the most ambitious projects since 2018 is being implemented by Australia’s Great Barrier Reef Marine Nature Park. The initiative is funded by the government and local tour operators. Park staff are relocating healthy corals to damaged areas of the reef, as well as releasing corals grown in nurseries on the seabed and in artificial pools to their natural habitat. By May 2020, the Australians have established 50 such nurseries. More than 17,000 corals have been stocked in the Big Barrier Reef’s six biggest tourist areas. The organizers intend to expand their project.

The effectiveness of underwater coral farming is evidenced by an experiment by employees of Israel’s National Institute of Oceanography. Coral reefs in the Eilat Bay of the Red Sea are suffering due to their proximity to naval bases, commercial ports and diving centres. To preserve the ecosystems, for more than 15 years, scientists have been releasing eight species of coral near Dekel Beach, grown in a marine nursery. Despite the harsh living conditions, the artificially grown corals are growing on the reef at the same rate as in captivity. By 2020, 1,400 colonies had been established. The planting of corals has provided new habitats for organisms associated with them (fish and invertebrates).

Another approach to coral rearing is the construction of artificial reefs that are attractive to polyp colonies. This is what the authorities of Guadeloupe (a French overseas territory in the southern Caribbean) have done. Ships entering the Bay of Deschêts would destroy the local reefs with their anchors. To eliminate the threat without interfering with navigation, the authorities have installed 40 mooring concrete blocks with an eco-friendly design and banned the use of bottom anchors. Vessels now moor to floating modules that are attached with cables to the concrete structures. They are covered with bumps and indentations characteristic of real reefs. Over a period of six years, these artificial ecosystems have been colonised by native coral species. Subsequently, 43 species of fish have taken over the ‘underwater cities’. By the way, only 25 of these species live on natural reefs in the same bay.

Large coral reef restoration projects are being carried out on the island of Okinawa in Japan, in Florida (USA), and in the island nations of Oceania (Fiji, Kiribati, Tuvalu, Samoa, Vanuatu, French Polynesia). In addition to relocating corals and constructing artificial reefs for them, other restoration techniques have been used. Among them – breeding and settling on the reefs of polyp larvae, the construction of artificial reef scaffolding, the supply of electric current on the reef area to stimulate the production of calcium carbonate by polyps. Finally, the breeding or conversely the removal of various algae and fish that affect the flourishing of polyp colonies.

Many of these approaches, especially their combinations, require a lot of money and the training of hundreds or even thousands of staff. Reef restoration is a long process, with the first results becoming visible years later. But this is the price of conserving marine biodiversity. It is likely that, before the end of the century, the only coral reefs left on the planet will be those cared for by underwater zootechnicians.