The Importance of Coral Reefs

23 11 2009

I was inspired to write this article after reading about a recent deep sea expedition off the coast of Australia in New Scientist. The expedition revealed new species of weird and wonderful marine life never seen before. It’s true that we know more about the moon than we do the oceans, and as a Scuba-fanatic, I think that’s incredible.

The Importance of Coral Reefs

[Edit 3rd December 2010: Sadly, Our Green Earth no longer exists but the owner has very kindly handed back copyright of my articles to me. Here, for your reading pleasure, is The Importance of Coral Reefs…]

The Importance of Coral Reefs

In May 2008, I was lucky enough to be staying on the Indonesian island of Gili Trewangan. This tiny island, just 4 miles in circumference, lies off the coast of Lombok and is a veritable Scuba diver’s paradise. As a self-confessed Scuba addict, I spent most of my four-day stay 30 metres under the water marvelling at the beautiful array of corals, fish and turtles. When I spotted a poster advertising that the local dive schools were organising a free “Earth Day” dive, I simply couldn’t resist and signed right up. Myself and around 30 other divers, plus snorkellers and landlubbers, took part in a clean-up operation to rid the reef and surrounding shoreline of litter. I’m pleased to say that my net was half empty when I surfaced as this particular reef was rather unpolluted but I dare say that this is not the case in reef systems elsewhere on the planet, and litter is certainly not the only problem that faces the world’s corals.

What is coral?

The structure we know as coral is in fact made up of thousands of tiny animal organisms called polyps. Polyps are closely related to jellyfish and have soft bodies complete with miniature tentacles. They have been in existence for over 200 million years (1) and have evolved to adapt to relatively nutrient-poor waters by forming a symbiotic relationship with another species of organism called zooanthellae.

A mutually beneficial relationship

Zooanthellae are single-celled algae that live inside the body of the polyps. By living in this way, polyps protect the zooanthellae from being eaten by fish and other grazers, and in return, the zooanthellae provide the polyps with some of its food, created by the process of photosynthesis, which stores carbon compounds by utilising the energy from the sun. As the polyps grow and reproduce they secrete a hard skeleton of calcium carbonate that helps the coral to maintain their position within the top 100m or so of sea level so as to afford the zooanthellae the best position for photosynthesis (1). Over many years, this skeleton builds up to form a coral reef ecosystem.

The importance of coral reefs

Of course, coral is not the only thing that makes up a coral reef ecosystem. Many other organisms depend on the coral reef for shelter, food and breeding sites and complex webs of biotic and abiotic interactions help to maintain the reef’s health. Humans too, use the reef as an important resource for generating income from tourism and fishing, amongst other things. It is estimated that approximately 15% of the world’s population rely wholly or in part on coral reefs for their livelihood. In Australia, for example, tourism of the Great Barrier Reef generates $1 billion USD per annum in revenue, and in developing countries, 25% of the fish caught annually comes from fisheries associated with coral reefs, thus providing a significant proportion of the protein required for a healthy diet (1).

Coral bleaching

Coral reefs are an incredibly delicate ecosystem in balance (1). The average species of coral polyp has adapted to survive within very narrow limits of temperature such that changes of just 1ºC can cause the polyps to respond to this stress by releasing their zooanthellae in a process known as “bleaching” (1). The polyp itself is often a translucent, colourless organism; the bright colours of coral so often seen in spectacular underwater photography come from the photosynthetic pigments found in the zooanthellae. When the polyp releases its complement of zooanthellae, the coral turns white; hence the name “bleaching”. Once bleached, the coral often dies.

Too much salt is bad for you (and coral)

Salinity is another parameter to which corals have become very finely tuned; mass bleaching can and does occur after heavy rains that dilute the saltiness of the ocean only very slightly (1).

The impact of humans on coral reefs

Coral bleaching is a reaction of the coral to stress. This stress can occur as a result of natural cycles, an example of which is the El Niño phenomenon, which when it strikes, can cause mass bleaching. However, research increasingly shows that this is a problem with anthropogenic roots that will only increase in severity as human activities continue to increase (2). Climate change as a result of increased atmospheric carbon dioxide (CO­2) and the depletion of the ozone layer has already seen sea levels and sea temperatures rise, leading to bleaching of some of the world’s most beautiful reefs. Not only bleaching, but mining of corals, over fishing and irresponsible tourism means that today, not a single coral reef on the planet remains unaffected by human activity (3), and 50-70% of them are deemed as at risk (1).

Coral reefs as a biodiverse ecosystem

The depletion of coral reefs has many serious implications for both humans and the ecosystem at large. Coral reefs are characteristically renowned for their vast biodiversity of millions of different species of fish, mammal, plant and algae. Loss of coral would result in the subsequent loss of not only those organisms such as coral-grazing fish that rely directly on the biomass of the coral, but also those of complex webs of myriad organisms. Biodiversity is important in itself from a genetic point of view as it increases the gene pool from which natural selection can continue to occur. It is also important for the survival of individual species that have carved a specialised niche for themselves that cannot be replaced by any other.

Coral as a primary producer

The sheer size of coral reefs means that they are an extremely important primary producer. Primary productivity is the storage of inorganic carbon into organic compounds, most often done via the process of photosynthesis. We are now familiar with the concept of carbon offsetting by planting a tree to neutralise the CO2 released by our activities, but corals as well as plants also play a part in reducing our carbon footprint, fixing as much as 40g of carbon per square metre of reef per day (1). Although attempts are underway in various parts of the world to create new coral reefs, mass bleaching and the death of corals is a rapid end for a very slow-growing organism so the practice of reef planting is always in debt.

Coral as a food source

Carbon fixing by coral is not only important for reducing the effects of our carbon footprint but is also an invaluable source of food. Polyps and zooanthellae form part of the food chain for a whole host of organisms, and these in turn are the food source for higher animals, ending ultimately in humans ourselves.

Intensive fishing

As the human population continues to increase, intensity of fishing also increases at the same rate, and for coral reefs, this is becoming unsustainable. Removal of the top fish predators from the food chain means that fishermen will need to hunt further down the chain, eventually fishing for juvenile species that do not have a chance to breed and replenish their numbers. Removal of the larger grazing and predatory fish has also been shown to result in an increase of coral-grazing invertebrates such as the Crown of Thorns sea star, which voraciously feeds on coral at a destructive rate if left unchecked, and fleshy brown algae that competes for light (4).

A vicious cycle

Not only this, but the death of coral itself removes the safe haven of the breeding and nursery ground altogether such that even those eggs that are laid will not survive to adulthood because they are exposed to predators. This vicious cycle impacts on other organisms that feed on young or larval seafood and leads to an overall reduction in biodiversity.

No-take areas

The establishment of “no take areas” (NTAs) will go some way towards maintaining sustainable fish stocks, but these must be effectively policed and become more numerous if we are to cope with the problem at large (3).

Coral as a source of tourism income

As previously mentioned, tourism of coral reefs provides a major source of income for many countries associated with the reefs. I for one am hopelessly in love with the colour and richness of life beneath the waves and there is no shortage of dive operators willing to exploit this fascination that we have with the sea.

Irresponsible diving

Unfortunately, as with many other forms of tourism, not all companies or their clients are as scrupulous or as concerned with the welfare of the reef as others. Irresponsible diving practices including hunting, coral mining or even simply a fin kicked in the wrong direction can damage the infrastructure of the reef. This in itself can stress the coral enough for it to bleach, but also encourages visiting species of fish and mammal to find new feeding and breeding grounds and disrupts the balance of the ecosystem.

Who wants to dive in a dead reef?

Coral bleaching by any of the other means previously mentioned also reduces the appeal of the reef to tourists. People Scuba dive because they want to see the glorious array of colours and animals that they’ve seen on TV; not uninteresting, crumbling skeletons of dead coral. Without the income from reef tourism, many countries would suffer and without the interest from the public that reef tourism creates, much needed scientific research programs would find a lack of funding. Clearly, tourism is both help and hindrance to the reef and needs to be kept in balance, like the reef itself.

The future of coral reefs

Coral reefs are beautiful ecosystems that we can enjoy for our own pleasure as well as being a source of food, money, building materials and even a source of new drugs and biochemicals. They create storm barriers for our coastlines and help to set up the environmental conditions needed for the health of other crucial ecosystems such as mangroves and sea grass beds (1). It is imperative that we use coral reefs responsibly and sustainably, and protect them carefully if they are to survive in the future.

  1. Hoegh-Guldberg, O, Climate change, coral bleaching and the future of the world’s coral reefs, Marine Freshwater Research 50:839-866, 1999
  2. Vidal-Dupiol, J., et al, Coral bleaching under thermal stress: putative involvement of host/symbiont recognition mechanisms, BMC Physiology 9: 14, 2009
  3. Hughes, T.D., et al, Climate change, human impacts and the resilience of coral reefs, Science 301: 929, 2003
  4. Scheffer, M., et al, Catastrophic shifts in ecosystems, Nature 413: 591-596, 2001
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