Cyclone Jasper’s slow-moving progress across the Coral Sea exposed as much as 20% of the Great Barrier Reef to waves high enough to break apart corals, according to modelling from the Australian Institute of Marine Science.
Scientists are also concerned flood waters from ex-Tropical Cyclone Jasper that drained out into the reef’s lagoon waters could damage corals and seagrass meadows close to shore.
Dr Marji Puotinen, a research scientist at Aims who specialises in studying the impact of cyclones on coral reefs, said wave heights of four metres or more were considered enough to rip off parts of coral colonies and cause severe damage to reefs.
She said while the modelling showed the potential for widespread impacts, the actual damage would probably be patchy, with some parts of reefs bearing the full force of waves while others would probably have been spared.
Reefs off Cairns were likely to have faced the most intense wave damage, she said, because the modelling suggested damaging wave heights were up to eight metres in some places and were sustained for as long as 36 hours.
“The damage zone extends from Cooktown to Mackay,” Puotinen said, a distance of more than 750km.
“The reef will have copped it both from when the cyclone crossed the Great Barrier Reef and also when it was further out to sea.”
She said while the cyclone was only a category two when it made landfall, the fact it had moved so slowly across the open Coral Sea in the days before meant it was able to push up wave heights for hundreds of kilometres away from the centre of the cyclone.
“I would not expect all the reefs to be damaged, but I would say there’s a huge area in which we might find severe damage,” she said.
She said the strength of a cyclone did not always correlate with the amount of physical damage it could do. But cyclones that were wide and slow moving had greater opportunity to whip up waves over larger areas.
As Cyclone Jasper was downgraded to a tropical storm system, it took almost five days to cross the north of Queensland, leaving many flooded rivers – with some areas registering more than a metre of rain in just a few days.
Floods in rivers that flow into the Great Barrier reef can damage corals and seagrass meadows because they add nutrients and sediment that promote the growth of algae and starve them of light.
Dr Jane Waterhouse, a water quality expert at James Cook University’s TropWATER research group, said: “The risk of impact is quite high in some locations and it will be quite dependent on the weather over the next few days if [flood] plumes get dispersed or not.”
She said monitoring during floods had been ongoing since 1991 “and we know concentrations of nutrients and sediments will be well above water quality guidelines over large areas”.
Dr Christina Howley, a scientist based at Cooktown with the Cape York Water Partnership, a not-for-profit group working on water quality in freshwater and marine environments, was still overwhelmed at the devastation from the cyclone’s deluge of rain.
She had seen photographs of large clumps of seagrass that had been ripped from the ocean floor and washed up on nearby beaches – a sight she had never seen before.
But she said data from a remote instrument at the mouth of the Annan River near Cooktown showed levels of sediment that were “about three times higher than we have measured in previous flood events”.
“We are concerned about what’s going into the reef and the seagrass meadows. There will be multiple reefs and seagrass meadows inundated by these flood plumes.”
Dr Britta Schaffelke, a senior scientist at Aims, said she was concerned about the cumulative effects the cyclone system was having on the reef.
She said reef scientists had been bracing themselves for a hot summer and the potential for a major coral bleaching event.
She said she was also concerned for the potential impact the cyclone may have had on remote reefs in the Coral Sea, which modelling suggested were also exposed to potentially damaging waves for at least 12 hours.
She added that the flooding had also coincided with a time of year when coral-eating crown-of-thorns starfish were spawning, which had the potential to promote outbreaks.
The Great Barrier Reef Marine Park Authority said its thoughts were with communities affected by ex-Tropical Cyclone Jasper.
“When it is safe to do so, the Great Barrier Reef Marine Monitoring Program will conduct reactive flood event monitoring in the area as part of their usual response to flooding,” it said.
The statement said the program would use “satellite and remote sensing tools to identify the extent of flood plumes, measure water quality conditions” and “look at exposure for the reef”.
