Mud now covers 98 per cent of Moreton Bay’s floor, choking out plants and animals, UQ researcher finds

Mud now covers 98 per cent of Moreton Bay’s floor, choking out plants and animals, UQ researcher finds
  • PublishedMarch 25, 2024

When a strong easterly wind blows in Moreton Bay, its crystal waters can turn quickly the colour of mud.

University of Queensland Associate Professor Alistair Grinham said it was a very bad sign.

His paper, published today in the Science of the Total Environment, found 98 per cent of the floor of Moreton Bay had become covered in a thick layer of mud, smothering the underwater plants and animals that used to live there.

Fifty years ago there was 442 square kilometres of clean sand remaining in Moreton Bay, but the paper found there was now only 30 square kilometres left.

The civil engineer said this was largely due to human activity causing severe erosion all along South East Queensland’s catchments and creek banks.

Dr Grinham said these eroded channels, combined with heavier rainfall due to climate change, caused enormous volumes of fine sediment to be washed into the bay.

He said some of the mud had travelled as far as Toowoomba, so widespread was the degradation further inland in the upper catchments.

Brisbane River flood plume March 2022
When the Brisbane River flooded in 2022 it left Moreton Bay brown and turbid.(ABC Supplied: University of Queensland)

Dr Grinham said the muddy “dead zone” grew significantly during Brisbane’s 2022 floods, when plumes of sediment were washed into the bay.

“I don’t feel great [about it] — you want to try to leave something for your kids that’s better than what you started with,” Dr Grinham said.

“On a good day the bay is just a magical place. You have these mangrove systems, you have seagrass, you have coral, and it’s a wonderful environment.

“Once it’s gone, it’s very difficult to come back.”

He said the destruction of seagrasses meant a loss of food and habitat for the fish, microorganisms, and vulnerable dugongs and turtles that lived in the bay.

A long grey marine mammal with a fish tail, fins and dog-like snout swims through brightly lit blue water.
A dugong swims in Moreton Bay.(Reuters: stringer)

Dr Grinham said dugongs could migrate to other waters in the eastern bank, but the mud zone was increasingly contaminating those areas, too. 

He said without seagrasses to anchor the floor, more mud would be lifted and mixed into the surface water, cutting light from the rest of the plants below.

Over 300 million cubic tonnes of mud has washed into Moreton Bay over the last 50 years, enough to fill 300 Suncorp Stadiums, according to the scientist.

How plastic is polluting our plates

A separate paper, also published today in the Science of the Total Environment, estimated there were at least 7,000 tonnes of microplastics in Moreton Bay.

Lead author Elvis Okoffo, from the University of Queensland, said this was the equivalent of 1.5 million plastic bags, or enough to fill three Olympic swimming pools with plastic.

A man in a research laboratory
Elvis Okoffo says there’s enough microplastic in the bay to fill three Olympic swimming pools.(Supplied: University of Queensland)

The most common pollutants were polyvinyl chloride (PVC), used in pipes and building materials, and polyethylene (PE), which is used in single-use items such as bottles and food wrappings.

Dr Okoffo said microplastics were harmful to coastal ecosystems, and to the humans who ate seafood from Moreton Bay.

“Plastic can impact marine organisms, they impact the ecosystems, they end up on our tables, and we end up eating the plastics,” Dr Okoffo said.

“The plastic particles can also leach chemicals and organic compounds into the bay environment, contaminating the marine habitats.”

Dr Grinham said it was not too late to reverse some of the man-made damage inflicted upon Moreton Bay.

A man holding a slab of mud on a pier
Alistair Grinham says it’s not too late to heal the bay.(ABC Radio Brisbane: Kenji Sato)

He said it was possible to rejuvenate the upper catchments by restoring plants and reducing the angle of the creek banks, slowing down the flow of water.

Dr Grinham said if the catchment’s inland creeks were not restored, Moreton Bay would eventually become unable to bounce back from another flood.

In his view, it was less of an engineering challenge and more of a political one.

Dr Grinham said the political challenge was persuading councils and inland authorities to spend money restoring their catchments, even though much of the benefits would be felt downstream.

“I think soft skills are far more important when you’re dealing with very tricky situations when you’ve got to try to convince people to invest outside the area they own,” he said.

“That’s an extremely difficult thing to justify for them … but I think it’s the only way of doing it though, unfortunately.

“I think one of the most important things would be trying to return the channels back to approximately something that they were in the past.”


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