3.0 The nature and rate of change which affect The Great Barrier Reef ecosystem's functioning
Change affects the Great Barrier Reef on a variety of scales, from daily tidal fluctuations to seasonal changes and the longer-term changes associated with changes in sea level. In recent years, human-induced changes have accelerated the nature and rate of change in the Great Barrier Reef ecosystem.
There are numerous types of changes that occur on the Great Barrier Reef but we will focus on three.
1. Impact of Sea Levels on The Great Barrier Reef
The Nature of Change (what's it like?)
Reefs grow when sea levels are high and die when sea levels are so low that they are exposed (see optimal growth factors - depth of water).Historically, sea levels stabilised at the present level around 6200 years ago. The modern Great Barrier Reef started growing between 8000 and 8500 years ago over older reef structures. Scientists have drilled and have discovered that periods of reef growth and decline are linked to the rise and fall of sea levels.
The Rate of Change (how fast or slow?)
Over the past hundred years, Australian seas appear to have risen an average of 12 to 16 centimetres. Global warming may be a cause, but earth movements can also cause rises and falls in sea levels. If the rate of sea level change occurs slowly in the Great Barrier Reef, its corals may be able to keep growing to keep pace with the rising water levels. But if the sea level changes occur rapidly in the Great Barrier Reef, its coral may not be able to grow fast enough and there may be destruction in some areas
2. Crown-of-thorns Starfish Infestations
The Nature of Change
The crown-of-thorn starfish is considered a boom or bust organism (i.e. an organism that is either found in small numbers or plague numbers). The crown of thorn starfish alters the reef by eating the coral polyps. When it is in plague proportions it severely alter the Great Barrier reef by damaging it and creating conditions for other organisms to invade the reef.(see biogeographical process- Invasion
The Rate of Change
Scientist have found that there have been periodic infestations of crown-of-thorn starfish during the past 6000 and 8000 years. Therefore some see the outbreaks as a natural phenomenon. Some believe that the starfish help regulate the overgrowth of coral.
However, in recent times the outbreaks have occurred more frequently.>Some argue this is due to human induced modifications to the Great Barrier Reef ecosystem through such things as:
- Shell collectors removing the starfish’s predators (crabs love eating the starfish)
- Sediment runoff from eroded banks caused by humans
- Excessive nutrient runoff from farms that flow into rivers and out to sea.
- Overfishing, trawlers catching many unwanted species as part of their by catch which may remove many natural predators of the starfish when they are in their larvae stage.
The problem with the change of rate is that the coral in the Great Barrier Reef has low elasticity (rate of recovery) sometimes it takes 12 to 15 years for coral to reach its pre infestation levels. If reinfestation occurs within the elasticity period it could alter the Great Barrier Reef significantly
3. Coral Bleaching
The Nature of Change
Coral bleaching occurs when the water temperature drastically changes, for a period of several weeks that it exceedes the coral polyps tolerance (usually around 30 degrees Celsius) . The polyps eject their symbiotic zooxanthellae before they die. This allows the white limestone exoskeleton to show and hence are said to have bleached.
The severity of bleaching can vary substantially according to water depth, location and species of corals.
The colouration of healthy coral (left) is due to microscopic algae called zooxanthellae (centre) living within its tissue. When bleaching occurs, the coral loses its zooxanthellae, leaving the white skeleton starkly visible through the transparent tissue (right).
The Rate of Change
If stressful conditions continue long enough, bleached corals will die. However, if stressful conditions fade away, then the bleached corals can recover their symbiotic algae and return to their normal, healthy colour.
If stressful conditions continue in particular regions, large areas of coral reef can be affected in what is known as a mass-bleaching event. The Great Barrier Reef (GBR) suffered widespread coral bleaching in the first half of 1998. Indeed, bleaching occurred in almost all tropical oceans in 1997/98. High water temperature appears to have been the main cause. Although some reef provinces suffered extensive coral death (up to 90% in the Maldives, Sri Lanka, Kenya, Tanzania and Seychelles), the GBR as a whole escaped with relatively little long term damage.
(see a recent timeline of bleaching events)
Bleaching of some GBR inshore reefs was evident from the air during the mass bleaching event of 1998.
A Greenpeace-sponsored scientific study written by professor Ove Hoeghguldberg, one of the world’s leading experts in coral bleaching, concluded that if climate change is not stopped, coral bleaching is set to steadily increase in frequency and intensity all over the world until it finally occurs on an annual basis between 2030 and 2070. The report claimed that the Great Barrier Reef could be severely affected by sea temperature rise within the next twenty years.
