Barrier reef braces for flood impact

As floodwaters in the Queensland capital Brisbane begin to recede, scientists are casting their eyes out to sea and wondering whether the region’s greatest natural feature, the Great Barrier Reef, will be scarred by the experience.

This is the world’s largest reef system – in fact, the largest thing on Earth made by living organisms, stretching for 2,600km along the coast.

Its myriad of islands and tendrils teem with fish, also supporting dugongs, dolphins, turtles and shellfish – and because of all that, a tourist trade worth several billion dollars per year.

The flood waters emerging from Brisbane itself are not a major concern, as the reef lies further north.

But northern rivers are also seeing flow rates way above normal.

As the water floods into the seas west of the reef, it inevitably freshens the environment around the reef; which is not good news.

“Freshwater kills corals, and there is nothing we can do about it,” says Katharina Fabricius, principal research scientist with the Australian Institute of Marine Science.

Queensland is a heavily agricultural state. And this means that the floodwater brings with it another threat.

Pesticides, herbicides, fertilisers and sediment – mud – are washed off the farms, into the rivers and then onto the reef.

The fertilisers do in the sea what they do on land – stimulate the growth of plants.

But here, that is a problem, as the marine plants cover growing coral, choking it to death.

The sediment also hurts the reef, blocking sunlight and covering the coral fronds.

This run-off degrades the reef at the best of times; but in flood conditions, it becomes much more serious.

“The young corals are highly sensitive to exposure to organically enriched sediments,” Dr Fabricius tells BBC News.

“That retards the ability of reefs to recover from the freshwater damage.

“My own research has also shown that increasing levels of nutrients can lead to more seaweed – up to a five-fold increase – and reduce coral biodiversity, with half of the coral species potentially lost from the exposed sites.”

Local issue

The sites at highest risk are reefs close to the shore and close to the mouths of rivers discharging floodwater.

For example, the Keppel group of islands lies about 10km from the shore, in the path of water rushing from the mouth of the Fitzroy River.

Scientists monitoring coral there say they have already seen indications of coral damage, but that is is too early to tell how big the impact is likely to be.

As the floodwater spreads further from land, it dissipates in the seas – but can still have major consequences.

“The waters discharging from the Fitzroy River are moving hundreds of kilometres north and 50-100km offshore,” says Michelle Devlin, a coral reef ecologist from James Cook University in Townsville.

“There is the potential for large areas of the reef to experience river plume water, with potentially damaging levels of nutrients, sediments and pesticides.”

Recovery hopes

On their own, the floods would not necessarily be a significant threat.

The region has had them regularly down the years. 1991 saw major damage to inshore coral – but it recovered.

“The 1991 flood was extremely hard for the reef – pretty much most of the corals were wiped out down to about six to eight metres of depth, and it took about 10 years for them to recover,” says Alison Jones from Central Queensland University in Rockhampton, through which the Fitzroy flows.