coral reefs

Coral reefs cover less than 0.1% of the ocean floor but provide a protective habitat, nursery area, food and shelter for about 25% of marine life. There are thousands of species of corals in the ocean, found not just in warm and shallow tropical areas but also in polar areas and the cold and dark conditions which prevail at depths of up to 6,000 metres. Corals can be hard or soft: hard corals have a calcium carbonate skeleton and live in reef-forming colonies while soft corals resemble brightly coloured fans or feathers. Although they look like plants, corals are animals, belonging to the Cnidaria group of invertebrates, along with jellyfish and anemones.

Each coral animal, or polyp, usually hosts microscopic algae known as zooxanthellae. The zooxanthellae help the polyps survive by providing them with food resulting from photosynthesis and give the corals their vibrant colours. In return, the polyps provide the zooxanthellae with a protected environment and the nutrients they need to carry out photosynthesis, an example of a symbiotic relationship. The polyps use the energy provided by the zooxanthellae to take calcium carbonate from the water to build their exoskeletons through the process of calcification. When the polyps die, their skeletons harden and other polyps can live on top of them, the reef develops as this cycle continues. Under the right conditions, corals can live for thousands of years, growing between 10 and 20 cm a year in shallow waters or about 10 mm a year in deep-sea ecosystems. Polyps either exist alone as small, solitary organisms or work in large teams, meaning that colonies can range from just a few metres in length to complex structures formed over thousands of years and stretching over more than 2,000 km, as with the Great Barrier Reef. 

Not only do reefs host the highest concentration of marine life they also create a protective barrier for coastlines, reducing 97% of wave energy and so the effect of storms and flooding on the near 200 million people who live within 50km of reefs. Their economic importance has been valued in the billions, in terms of tourism revenues, reef fisheries and reduction of storm damage.

What is threatening them?

When water temperatures get too high (from 1 to 2 °C higher than normal), the polyps expel the algae within them. As the algae are lost from the tissue the white skeleton shines through the mostly transparent host tissue, giving the corals a whitish appearance. This phenomenon is known as coral bleaching. If the water temperature doesn’t drop and therefore allow the algae to return, the polyps starve, succumb to disease and die, the skeletons are then covered by a different type of algae, changing to a grey/green colour. Without live tissue, the reef is exposed to the eroding forces of the environment and the framework breaks down within a few years.

Coral bleaching has been seen in reefs around the world over the past three decades, beginning with the first mass bleaching event in 1997/1998. A new development in coral reefs has been observed recently where corals produce vibrantly coloured chemicals that act as sunscreens to protect themselves from the heat, creating a coral glow. This glowing is seen as a last cry for help from the corals.

In addition to warming temperatures, ocean acidification, overfishing, agricultural runoff, plastic pollution and sedimentation from urban development and land reclamation activities threaten the health of coral reefs around the world.

According to The Coral Reef Breakthrough, an initiative developed in collaboration with the UN Climate Change High-level Champions (HLCC), the International Coral Reef Initiative (ICRI), and the Global Fund for Coral Reefs (GFCR) with support from the Government of Sweden and the Principality of Monaco, 14% of our coral has already been lost. They are calling for resources and action “to stop local and global drivers of decline, and scale cost-efficient solutions to enable the survival and recovery of resilient coral reefs on a global scale”, aiming for investment in the amount of US$12 million of to secure the future of at least 125,000 km2 of shallow-water tropical coral reefs.

How can we protect and restore reefs?

• reduce ocean temperatures and acidification through reductions in greenhouse gas emissions;

• reduce urban development in areas near reefs;

• reduce land-based sources of pollution;

• protect marine areas from destructive fishing practices near reefs;

• increase resilience of coral reefs;

• restore broken reefs;

• grow new corals;

• reduce invasive species.

The most important step in protecting coral reefs is to reduce the temperature of the ocean. Current projections expect a 2.8 °C temperature rise by the end of the century so there is much to do. Even if the Paris Agreement’s goal to limit global warming to 1.5 °C is met we are still looking at the destruction of more than 90% of warm water coral reefs.

Many organizations dedicated to coral reef protection have established programmes around the world, working to both fight climate change and ocean warming but also to grow new corals. Coral gardens and nurseries have sprung up around the world, using a concept which can be traced back to Charles Darwin’s ideas from the 1800s on how coral reefs develop.

Healthy corals can be grown and replanted in the ocean. Coral restoration can be simple: collecting detached corals (broken fragments or even larger colonies of coal) growing them in nurseries and then replanting them, attaching them to reefs. A far more complex procedure involves collecting millions of naturally-produced eggs and sperm during natural spawning events and using in-vitro fertilization to breed new corals in a laboratory before placing them back on the reef.

Removing invasive species is also a key element. Cleaning corals by brushing or filing off non-native algae from their surface, which blocks light getting through to them, together with reintroducing natural predators to the area helps coral reefs to thrive.

A number of NGOs are working on coral reef restoration: the Coral Reef Alliance (CORAL) and the International Coral Reef Initiative (ICRI) work on an international scale, while the Coral reef alliance, Coral Gardeners, save the reef, secore international and Ocean Gardeners have a more regional emphasis.