Bleaching in marine sponges in temperate waters off Tasmania’s east coast has been observed for the first time, with scientists warning the discovery could be an indicator of climate change in deeper reef systems.
Researchers at the University of Tasmania’s Institute of Marine and Antarctic Studies used seafloor surveys to discover the bleaching of cup sponges in the Flinders commonwealth marine reserve after heatwave events.
The sponges, which are ordinarily reddish-purple in colour, live in a mesophotic ecosystem – the twilight zone at depths between 30 metres and 150 metres – that bridges brightly lit shallow waters and the darkness of deep ocean.
Images of the bleached cup sponges were captured in 2017 but not analysed until later. The research findings were recently published in the journal Climate Change Ecology.
“There seems to be a pattern going on with increased bleaching through time,” Dr Nick Perkins, the study’s lead author, said.
Perkins said the bleaching of cup sponges could act “as a canary in the coalmine” to indicate the impact of climate change on rocky reefs in temperate waters.
“We haven’t seen any mass mortality of the sponges so far … but it is really important to track what is happening on these deeper reefs.”
Sea temperatures off Tasmania’s east coast have risen at nearly four times the global average. The region experienced intense marine heatwaves in both 2015-16 and 2017-18.
Neville Barrett, an associate professor who led the seabed monitoring research, said the discovery pointed to the likely impacts of climate change on biodiversity.
“As things do warm up with climate change over the next century or two, a lot of these deeper species will move south,” he said. “The biggest problem is that whatever endemic species we’ve got here in Tasmania now, they’ve got nowhere to go as things warm up.”
Barrett said sponges played an important role in drawing nutrients from seawater and “converting them into material that feeds reef production”.
Off Tasmania’s east coast, the cup sponges support fisheries of rock lobster and striped trumpeter, Barrett said.
“Once you get down below the algal zone of 30 to 40 metres, it’s all sponge-dominated seabed. There’s no kelp there.”
Images of the cup sponges were captured by autonomous underwater vehicles during surveys between 2009 and 2017. The sponge bleaching was later identified by Phoebe Gallagher, a marine science student, when analysing the imagery.
The researchers said more work was necessary to precisely establish the threshold temperature – and duration of warming – that would result in bleaching, as well as how long the bleaching persisted for.
