A study by the Institute for Marine and Antarctic Studies (IMAS) has found bleaching in sea sponges for the first time in Tasmanian waters — and scientists fear it could be another sign we are reaching a "tipping point: in the warming of the Earth.
Reports of marine die-offs and bleaching are common in coral reefs and other shallow ecosystems, but evidence of bleaching in deeper temperate waters off Tasmania's east coast has just been discovered.
Between shallow, well-lit waters and dark, deeper parts of the ocean marine researchers have found bleaching in the 'twilight zone', known in scientific terms as the mesophotic zone.
While light is still able to penetrate depths over 30 metres below the ocean's surface, the mesophotic zone is home to thriving communities of coral, sponges and red algae and is vital to the ecology of marine life.
From 2009 to 2017, IMAS researchers monitored areas within the Flinders, Freycinet and Huon Marine Parks to determine the impact heatwaves and extreme weather events are having on the region.
Using robotic and remotely operated vehicles to collect images and videos, lead author Dr Nick Perkins said it's the first time deep shelf reefs have been surveyed.
However, something that did catch the attention of researchers but was not properly examined in the last round of surveys was disruption to the reproductive output of small sponge species.
"What we've seen with the size structure of the sponge through time is [that] no new small sponges [have] come in over the time period where bleaching has occurred."
While the link between bleaching and the lack of sponge cup reproduction requires further research, there is some good news, he said.
According to Dr Perkins, we're yet to see a mass die-off of cup sponges, unlike 95 per cent of Tasmania's kelp forests which have been wiped out.
"It doesn't appear that there's been mass mortality [but that's] definitely one of the things we'll be looking at with the latest round of surveys."
Tasmanian waters near 'tipping point'
IMAS Associate Professor Neville Barrett has spent two decades researching Australia's temperate reef systems.
He said while many people are familiar with the bleaching of shallow reefs such as the Great Barrier Reef off Queensland's east coast, there was little understanding of what's happening in areas further off shore.
"What [the study] is telling us is that this warming climate that we're seeing in-shore is also found further across the continental shelf."
While it's probable that warming ocean temperatures are impacting deeper marine life, Professor Barrett said the study was the first of its kind to gather hard evidence.
"These deeper reefs are known to be biodiverse zones and could contain species not yet described by science. We still know very little about these systems below diving depths — how they function and how they may respond to climate change," he said.
"The main thing with the greater number of heatwaves is simply that the base temperature that we're operating from has increased.
"We're now starting to reach that tipping point where we're seeing this kind of bleaching... that wouldn't have happened previously."
