by Jasper’s slow-moving progress across the Coral Sea has exposed up to 20% of the Great Barrier Reef to waves high enough to break apart corals, according to modeling by the Australian Institute of Marine Science (AIMs).
Scientists are also concerned that floodwaters from Post-Tropical Cyclone Jasper draining into the reef’s lagoon waters could impact corals and near-shore seagrass meadows.
Related: Military helicopters arrive to evacuate residents of flooded Wujal Wujal in north Queensland
Dr Marji Puotinen, a research scientist at Aims who specializes in studying the impact of cyclones on coral reefs, said wave heights of four meters or more were thought to be enough to uproot parts of coral colonies and cause major damage. make your reefs.
She said that while the modeling showed the impact could be widespread, the actual damage was likely to be patchy with some parts of the reef likely to experience the full force of the waves while others were likely to be spared.
Reefs off Cairns probably suffered the most severe damage from the waves, she said, as the modeling had damaged wave heights of up to eight meters in some places and persisted for as long as 36 hours.
“The damage zone stretches from Cooktown to Mackay,” Puotinen said, a distance of more than 750km.
“The reef will have crossed it from when the cyclone crossed the Great Barrier Reef and also when it was further out to sea.”
She said that although the cyclone was only a category two when it made landfall, the fact that it had moved so slowly across the open Coral Sea in the previous days meant that it was able to push the height of the waves up the hundreds of kilometers from the city center. cyclone.
“I wouldn’t expect all the reefs to be damaged, but I would say there’s a huge area where we could get a lot of damage,” she said.
She said there was not always a correlation between the strength of a cyclone and the amount of physical damage it could cause. But cyclones that were wide and slow had a better chance of whipping waves over larger areas.
As Cyclone Jasper was downgraded to a tropical storm system, it took almost five days to cross northern Queensland, leaving many rivers flooded – with some areas not registering more than a meter of rain in a few days.
Flooding in rivers that flow into the Great Barrier Reef can damage corals and seagrass meadows as they add nutrients and sediment that promote algae growth and expose them to 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 places and it will be very dependent on the weather over the next few days if [flood] smoke is dispersed or not.”
She said there had been monitoring during floods since 1991 “and we know that nutrient and sediment concentrations will be well above water quality guidelines over large areas”.
Dr Christina Howley, a Cooktown-based scientist with the Cape York Water Partnership, a non-profit group that works on water quality in freshwater and marine environments, was still overwhelmed by the devastation from the cyclone’s deluge.
She had seen photographs of large clumps of grass 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 sediment levels that were “about three times higher than we’ve measured in previous floods”.
“We are worried about what is going into the reef and the sea meadows. There will be many reefs and seagrass meadows under these floods.”
Dr Britta Schaffelke, senior scientist at Aims, said she was concerned about the cumulative effects of the cyclone system on the reef.
She said reef scientists were preparing for a hot summer and the possibility of a major coral bleaching event.
She said she was also concerned about the cyclone’s potential impact on remote reefs in the Coral Sea, which modeling suggested were exposed to potentially damaging waves for at least 12 hours.
She added that the flooding coincided with a time of year when sea starfish were feeding on coral spawning, which had the potential to trigger outbreaks.
The Great Barrier Reef Marine Park Authority said its thoughts were with communities affected by Tropical Cyclone Jasper.
“When it is safe to do so, the Great Barrier Reef Marine Monitoring Program will conduct reactive flood monitoring in the area as part of their normal flood response,” he said in a statement.
The statement said the program would use “satellite and remote sensing tools to identify the extent of floodplains, measure water quality conditions” and “look at exposure to the reef”.
“Any impact on the habitats (including coral reefs) will be better understood as model outputs, observations and survey data are made available through existing mechanisms and in-field activities,” he said.
