Sea sponges off New Zealand’s southern coastline have been found bleached bone-white for the first time, following extreme ocean temperatures.
A group of scientists from Victoria University of Wellington were alarmed to discover the sponges, which are typically a rich chocolate brown, were bleached in more than a dozen sites near Breaksea Sound and Doubtful Sound in Fiordland.
The university’s marine biology professor James Bell said in some parts as many as 95% of the sponges were bleached.
“Our initial estimates are there are at least hundreds of thousands of sponges likely to have been bleached and maybe even many more than that,” he said.
Bell’s team discovered the mass bleaching during a research trip in April. The species, which is common to the lower South Island, is one of 800 species of sponge found in New Zealand.
Sponge gardens dominate the sea floor around the country and hold an important role in creating habitats for fish and releasing carbon that other species feed off.
There have been reports of sponges bleaching, including off the coast of Tasmania earlier this year, but typically they tend to be more tolerant to oceanic changes compared with other species such as corals, Bell said.
“It’s a really unusual event,” he said.
“This just highlights the kind of climate crisis that we’re facing. There are so many species around New Zealand and we don’t know what their thermal tolerances are.”
Last year, the world’s oceans were the hottest in recorded history, due to climate change, which among other things can cause the oceans to acidify and degrade reefs and ecosystems.
New Zealand was no exception, with 2021 its hottest year on record. Temperatures in April were still unseasonably high, with coastal water temperatures up to 2.6 degrees above average, according to the National Institute for Water and Atmospherics.
In Fiordland, those temperatures were even higher, said Rob Smith, a University of Otago oceanographer, who works with the government-funded Moana Project researching marine heatwaves. He told RNZ the region recorded temperatures up to 5 degrees higher than usual.
“What we’ve seen this summer is the strongest marine heatwave on the west coast of the South Island in 40 years.”
Further research was needed to establish definitively whether ocean temperatures are causing the bleaching, Bell said. At this stage the researchers had an observation and a “very strong correlation” between the bleaching and a severe temperature spike.
Some bleached sponges could come back to life, Bell said, but the team would need to return to the site to see how they were faring towards the end of the month.
“There is some hope they may not die, but unfortunately some of them are a little bit manky and not very happy or healthy.”