Now that the floodwaters have settled, what is the fate of Queensland's Great Barrier Reef?
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On 11 January 2011, three quarters of Queensland, Australia was flooded by heavy rainfall, causing rivers to overflow. Climate change event La Nina was credited for the disaster by Australia's Bureau of Meteorology (BOM).
In the days that followed, floodwaters began draining into the oceans through and marine biologists grew increasingly concerned. Because lying in the path of the fresh floodwater was the Great Barrier Reef - the world's largest coral reef system.
Within days post-disaster monitoring began, in a collaborative effort lead by the Australian Institute of Marine Science (AIMS) with the involvement of institutions such as James Cook University (JCU) in Northern Queensland and the state government's Department of Environment and Resource Management (DERM) to check the changed environmental conditions, such as temperature or salinity.
Reef ecosystems are adjusted to a stable, marine environment, where conditions do not greatly fluctuate. With flooding, great volumes of freshwater can dramatically rock a system by changing the conditions.
Freshwater can reduce salinity levels by diluting seawater, and after passage through land, floodwaters can fill with terrestrial soils or sediments, which can be deposited on the reefs. Corals are highly sensitive to all of this.
So far, only minor changes have been detected for the majority of reefs. "The first thing that must be stressed is the relatively tiny proportion of coral reefs that have been exposed to immediate impact of these floods," says Angus Thompson, co-ordinator of near-shore coral reef monitoring with AIMS. "The vast majority (>99%) of reefs have not been impacted."
This is because most reefs lie hundreds of kilometres offshore. Damage for reefs closer to the mainland depends on their proximity to flooded areas, particularly draining rivers.
Tipped to receive the most damage are the Keppel Island reefs. According to Angus, they lie directly in the way of the Fitzroy River's floodwaters, which previously flooded the city of Rockhampton.
What's worse, is that floodwaters can be constricted by Keppel Bay, which the islands surround. This leaves the area's coral under the direct threat of prolonged freshwater exposure.
Though they form one small part of the Great Barrier Reef, they are still a concern. The Keppel Island reefs are considered 'inshore reefs', which form at the base of islands about 20-30 km off the coast. Of the Great Barrier Reef’s 3000 individual reefs, there are about 700 inshore reefs. Despite their smaller number, they hold high significance as they are more accessible for tourism and fishing.
Just days after the flood, monitoring teams began reporting cases of bleached coral to team leader Britta Schaffelke, of Water Quality and Ecosystem Health at AIMS. Bleaching is caused by the loss of symbiotic algae that contribute up to 90% of coral diet as well as their colour. It is also indicative of stressed coral.
Alarmingly, bleached corals were also observed in reefs north of the Keppel islands, signalling that floodwaters are now dispersing towards the north. Depending on wind intensity and direction, they could reach the offshore reefs. This has happened before in a previous flood.
"The last comparable flood of the Fitzroy River occurred in 1991," says Angus. Back then, 100% of corals perished up to 1.3 m deep, on reefs facing the shore. The depth measured from surface of the reef flat, level surfaces formed by corals which end in drops extending deep into the sea.
