Coral disease and its causes

Corals, like all animals, can be affected by diseases. It may be caused by infections from microbes such as bacteria or viruses or by external stressors including extreme temperatures, toxins and nutrients. A combination of these factors can also result in disease.

Studies have also shown that high coral cover, particularly where the species is more susceptible to disease, can increase the opportunity for disease transmission between colonies.

Around the world more than 20 different coral diseases have been described by scientists.  Of these, less than half have been due to an infection, leading scientists to believe that environmental conditions are just as likely to cause coral disease.

The environmental conditions that can increase the risk of disease in corals include:

Extreme sea temperatures:

Prolonged, high sea temperatures can lead to coral bleaching. Even if corals recover from bleaching, the weakened corals are often more vulnerable to disease. On the Great Barrier Reef, the Australian Institute of Marine Science recorded a substantial increase in coral disease following the 2002 mass coral bleaching event.

Excess nutrients:

Nutrient levels on the Reef can fluctuate due to flood run-off. Elevated nutrients may increase the susceptibility of corals to infection as well as the rate of progression of disease. Elevated nutrients (particularly nitrogen and phosphorus) may also increase the growth rates of some disease-causing microbes.

Bird guano is rich in these nutrients. This may account for recent Great Barrier Reef research findings that found elevated levels of coral disease around permanent structures that attract large numbers of roosting birds.

Increased levels of coral disease have also been observed following major flood events in Queensland over the past five years. These outbreaks often follow freshwater bleaching events and can result in localised damage to inshore reefs.

Physical damage:

Storms and cyclones can damage corals over large areas, and increase turbidity, nutrients and algal blooms, all of which can increase the susceptibility of corals to disease.

What is the difference between coral disease and coral bleaching?

Technically, coral bleaching is a disease, though when it is caused by temperature (an environmental stressor) it is often described differently. The white coral seen during coral bleaching is not bare skeleton. The coral tissue is still alive, however it has lost the organism (zooxanthallae) that live in side it that provide its food and colour. If extreme temperatures continue, the bleached coral will eventually die and the bare skeleton will be rapidly colonised by algae. Even if temperatures come down and the corals regain their zooxanthallae, the recovering corals are more susceptible to disease.