One of the 50 million sponges along 1000 kilometres of Fiordland coastline that was bleached in the largest such event ever recorded. Photo: Supplied
A mass bleaching of more than 50 million sponges along 1000 kilometres of Fiordland coastline was the largest ever recorded globally - and the most devastating, with tens of millions of sponges killed.
A new study, published in the environmental journal Global Change Biology, estimates the 2022 bleaching cut the marine sponge population by close to half, making it the world's largest "sponge mortality event".
Lead author Professor James Bell - a marine biologist at Te Herenga Waka - said the bleaching occurred during an intense marine heatwave that lasted 259 days and pushed water temperatures to 4.4ºC above average
The mass bleaching saw the cup-shaped sponge Cymbastella lamellata turn from their usual chocolate-brown colour to bone white.
Nearly one year later, the researchers found almost 50 percent of sponges at sampling sites had died.
"Not only was this the largest sponge bleaching event seen globally, it was also the largest sponge mortality event that's ever been recorded," Professor Bell said.
One of the 50 million sponges along 1000 kilometres of Fiordland coastline that was bleached in the largest such event ever recorded. Photo: Supplied
As the climate warms, marine heatwaves are occurring more often and becoming more intense, with major effects on sea life.
"During the 2022 heatwave in Fiordland, more than 90 percent of C. lamellata sponges were bleached. These sponges are one of the most abundant species in Fiordland and play a crucial role in the ecosystem, including by providing habitat and recycling nutrients that support marine life.
"We discovered the sponges are key contributors to dissolved organic carbon production in the water column. This food source has the potential to fuel the microbial food chain and support organisms higher up the chain. The mass death of sponges after the 2022 heatwave means this potential fuel source has been reduced-we estimate by as much as 850 tonnes."
Professor Bell and his co-researchers discovered that some sponges were able to recover from bleaching, showing they may be able to adapt to heat stress.
"We detected changes in the microbial communities that these sponges play host to the tiny creatures they have a symbiotic relationship with. The changes we observed suggest the sponges are displaying a rapid evolutionary response to warming waters in an effort to survive."
He said the discovery gave scientists reason to be optimistic that sponges may be able to adapt to warming waters, but added that more research needed to be done.
"We need to... test this and monitor how more frequent and intense marine heatwaves may affect this vital species," .
One of the 50 million sponges along 1000 kilometres of Fiordland coastline that was bleached in the largest such event ever recorded. Photo: Supplied
Researchers also observed that the drought conditions and unusually low rainfall that coincided with the marine heatwave may have allowed more light to reach the usually dark waters of Fiordland, worsening the bleaching and mortality rate of the sponges.
"This event highlights the need to consider the impacts of compounding extreme events-such as coastal temperature and freshwater extremes-on marine ecosystems, and how the likelihood of such events may change in the future," said Dr Robert Smith, a physical oceanographer from the University of Otago and part of the research team.
The research was funded by Te Herenga Waka-Victoria University of Wellington, Department of Conservation, George Mason Charitable Trust, Fiordland Lobster Company, and The Lesley Hutchins Conservation Foundation.
