BLEACHING – A CATASTROPHIC NATURAL DISASTER
Is it being ignored or is it just proving too sensitive to discuss?
Wolfgang Grulke and Terri Martin - April 2000
From January to May 1998 a subtle change in the water temperature occurred around the Seychelles islands. A rise of a mere two to three degrees resulted in the decimation of more than 95% of all coral animals in the archipelago. It destroyed an ocean ecosystem on which hundreds of species of fish, shells and crabs are dependent. Much of this ocean warming has been linked to El Nino.
Like the tropical rain forests, coral reefs house some of the most complex and diverse ecosystems on the planet. More than a quarter of all marine species live on coral reefs.
What if 95% of the Kruger Park’s animals had suddenly been destroyed? Had it been the doe-eyed ‘warm and furry' mammals that had been destroyed, rather than the tiny coral animals, it would have made the front page of every popular magazine and newspaper. It would have been considered a natural (and economic) disaster of unprecedented proportions. Somehow the ‘wet and slimies’ of the coral reef don't attract the same kind of media attention!
On the Seychelles islands of Praslin and La Digue, the weathered pink granite cliffs tumble dramatically into azure coral seas. It was during a recent diving trip to these islands we noticed that the shallow seas appeared unusually murky, and discovered that most of the coral beds situated off our hotel beachfront were dead, washed white and with little visible form of life. Previously these coral gardens teemed with life, exquisite patterns and vibrant colours. The word 'bleaching' was tentatively proffered by the locals as the cause. We later realised that this was just one small example of a major ecological catastrophe, the scale of which we had totally underestimated.
Sea temperatures around the Seychelles usually range from 25 - 29 degrees Celsius but for a few months at the beginning of 1998 a hot spot developed over the Indian Ocean and temperatures rose two to three degrees. On some days temperatures of 32 degrees at 20 meters were recorded. It was this heating of the water that has decimated much of the hard coral.
It now appears that most of the coral reefs which dominate the world's shallow tropical and subtropical waters have also been affected, and this includes the full spectrum of coral species. Across the Indian Ocean, from the east coast of Africa to southern India, 70 - 80 percent of the coral appears to have died. We have observed comparable decimation and whitewashing around the tiny islands off the Malagasy coast. Many of the beaches are littered with broken coral pieces washed ashore by waves and currents. This is an occurrence not seen before.
Published records of bleaching date back to 1870, so while the phenomenon is not totally unheard of or uncommon no report equals the current devastation. Scientists suspect that a swing from episodic events to chronic levels of bleaching has occurred.
Is the world paying sufficient attention to this widespread and disastrous situation?
While the coral reefs of the world occupy less than one percent of the earth's surface, they are the oldest and most biologically diverse and intricate type of ecosystem in the world. Coral reefs are to the oceans what tropical forests are on land. They are the largest living things on Earth and the corals of the Great Barrier Reef are the only structures created by living creatures that are visible from space, and overshadow anything built by man. A quarter of all the ocean species are reef-dwellers, including at least 65 percent of marine fish species.
Corals are very simple animals that belong to a large group of organisms called coelenterates. Massive coral reefs result from the accumulation and cementation of the skeletons of these tiny, anemone-like animals. The existence of these reefs is the result of a delicate relationship between the coral polyp and the minute single-celled algae which live symbiotically within the cells of the polyp.
Within the tissues of most reef-building corals are millions of single-celled algal plants known as zooxanthellae. Safely protected inside the tissues of the polyp, zooxanthellae are able to photosynthesise, using the energy of the sun to convert carbon dioxide and water into carbohydrates and oxygen, thereby providing the coral polyp with a supply of oxygen in its tissues for respiration. The carbohydrates produced by the zooxanthellae also leak out of the algal cells and are used by the coral polyp as food.
The coral kingdom is roughly divided into ‘hard’ and ‘soft’ corals. The crucial difference with the so-called hard corals, or reef-building corals, is that the animal, the coral polyp, secretes a hard stony cup, or skeleton, in which it lives. Typically, hard corals are colonial animals and the numbers in their colonies can be in the millions making up large aggregations in the shapes of domes (e.g. brain corals Leptoria sp.), fields of lettuce (e.g. Turbinaria mesenterina) or fields of pink-tipped spiky antlers (e.g. stag horn corals Acropora sp.). Each generation grows on the dead shoulders of its predecessors building enormously hard structures. It is these hard corals which have been particularly hard hit.
DELICATE NATURE: BLEACHING MEANS DEATH
Corals are extremely vulnerable. In order for the relationship between the simple algal plant and the coral animal to thrive, a stable environment is required. They have limited resistance to outside influences such as changing light levels, water clarity, salinity, sedimentation or extremely high sea surface temperatures. As corals are carnivorous animals they also require constant currents to provide food in the form of tiny planktonic creatures. Any change in any of these conditions, however slight, can be fatal.
Coral species live in temperatures ranging from 16 to 32 degrees Centigrade but ideal growth takes place in a much narrower range, perhaps between 21 to 28 degrees. In unusually warm waters the coral polyps become stressed and appear to reject the zooxanthellae algae that live within its tissues. The coral then turns white, the so-called bleaching effect. Coral usually recovers from a short bout of bleaching but if the pattern persists it is generally fatal as the coral depends on the algae to help feed it through photosynthesis. A bleached coral reef soon turns into something of a graveyard. The bright colours and sharp edges of a living coral reef are gone. Instead, a vast monotone Hiroshima-like landscape presents itself to the snorkeller or diver. The creatures that feed on the corals are gone, too. Almost everywhere, solitary coral disks lie abandoned; everything appears neglected, covered in dust.
These un-lived-in coral skeletons become extremely fragile and it is characteristic of a bleached landscape to see vast fields of staghorn and similar coral collapsed in on themselves, often accelerated by the careless tourist or dragged boat anchors.
Coral bleaching sets off a chain of events that leads to increased degeneration of the reefs.
Butterflyfish, whose pointed faces
delight so many snorkellers, are specifically adapted to feed on live coral
polyps and thin out dramatically when their food source dies. So it goes with
parrotfish. We noticed a massive decline in the number of butterflyfish and
parrotfish in both the Seychelles and Madagascar.
Conversely, immediately the coral
community begins to die off, an ecological niche is created for other forms of
life to develop. However, the long-term effects of this imbalance will lead to
further degeneration of the ecosystem.
A sure sign of the current imbalance on the reef is the emergence of filamentous green algae in small bright clumps scattered over the ‘bleached’ coral skeletons. This strange new algal growth attracts new kinds of predators. The black long-spined sea urchin, Diadema sp., is evident everywhere. Certain species of shells proliferate to graze on the algae. Everywhere, new visitors start boring into the deserted reefs, further increasing their instability.
The crown-of-thorns starfish is a coral marauder that is munching its way through reefs in the Red Sea, in the Mozambique Channel, the Aldabra Group, the Maldives, Indonesia, Australia and throughout much of the Pacific. Over a period of a year, one adult crown-of-thorns can consume 13 square meters of coral. Their movement is usually contained by the presence of staghorn coral but with them bleaching and dying, the starfish move more easily to other palatable 'massive corals', making them more vulnerable than before. It is not clear if coral bleaching reduces the number of crown-of-thorns starfish by killing off their staple diet, or if the starfish merely find other food sources.
From Chile in the south to British Columbia in the north, when sea temperatures rose, fish that normally lived in tropical or subtropical waters either migrated or were displaced polewards.
Following altered sea-level rises at Christmas Island in the mid-Pacific seabirds abandoned their young and went in a desperate search for food. By the time conditions returned to normal, some 24% of the adult fur seal and sea lion population had died, as well as virtually all that year's pups of these two species.
Reef structures themselves are being jeopardised by the death of coral. About one-sixth of the world's coasts are shielded by reefs and many support some of the densest human populations in the world. If reefs surrounding land break down, the erosion of land by wave action will increase through diminished protection. Coastlines are already being threatened by rising seas due to global warming and many low-lying island archipelagos could disappear altogether.
Eventually, as the grazers destroy
enough of the green algae to encourage a new kind of balance, it appears that
the right conditions can be created in which new coral growth can gradually be
seeded as currents bring in new settlers on these barren reefs.
Surprisingly, the coral reefs least stressed as a result of the 1998 disaster were those in the most polluted and pressurised marine reserve in the Seychelles. According to John Collie, managing director of the Seychelles Marine Parks Authority, the reefs of the St Anne Marine Park, situated immediately outside the harbour of Seychelles’ capital Victoria, took the least beating. Already stressed, it appears they have developed an immunity to further shocks!
Off Praslin, we saw a few small clumps of staghorn coral regenerating amongst acres of bleached coral formations. In places some soft octocorals, and the soft white polyps of Tubipora musica, the organ pipe coral, were flourishing again. At the same time, the land reclamation work being done off Mahe’s east coast has enabled much surviving coral to be transplanted to nearby marine parks. This pioneering endeavour has proved 40% successful. A drop in this ocean perhaps, but every little bit helps.
Bleaching itself is not necessarily a death sentence: given time and protection from other dangers, corals can recover.
It is estimated that about 10 percent of the world's coral reefs may have already been degraded beyond recovery. It is the opinion of Chris Bright, senior editor of World Watch, that if we can't find a way to ease the reef's afflictions, nearly three-quarters of the ocean's richest biome may have disappeared 50 years from now.
In John Collie's view, it will take another 25 years or more for the damaged reefs of the Seychelles to return to their former glory. Ove Hoegh-Guldberg of the Sydney University’s Coral Reef Research Institute sounds a dire warning: global warming could eliminate coral reefs from most parts of the world by 2010 if no bold action is taken, and that coral bleaching events could, within 20 years, become an annual occurrence. Moreover, he estimates that coral reef recovery could take as long as 500 years!
Can the corals re-generate themselves and how long will this process take? Can we find a way to ease the reef's afflictions? There is much more at stake here than just having beautiful coral under the sea to look at. Is anyone counting the economic and social costs of the reef deterioration?
Perhaps, with an increase in cosseted and well-managed marine protection parks, a new generation of tourists, snorkellers and divers will witness the rebirth of the coral reefs around the world, and wonder at the renewed explosion of colour and glory.
Your “Last Chance to See!” is no more – it is already too late to see most of the world’s coral reefs in their prime! Whatever happens, it will take between 50 and 500 years to return to the profusion we delighted in during the 1970s and the early 1980s.
Whenever you have the chance to see a real live coral reef, take it!