A mass bleaching event which impacted more than 50 million sea sponges off the coast in Fiordland was the largest event of its kind ever recorded worldwide, according to new research.
Victoria University of Wellington researchers said the bleaching was due to the impact of an intense marine heatwave in 2022 that saw water temperatures rise to above average levels in the area for 259 days.
"There are moderate heatwaves, strong heatwave and extreme marine heatwaves. So, it was above that long term average for about 259 days and that peak it reached at its very maximum intensity was 4.4C … so it wasn't at that temperature the whole time," said lead researcher and marine biologist James Bell.
He said water temperatures on Fiordland's coast were pushed to 4.4C at the heatwave's peak, which researchers said correlated with when nearly half of the cup-shaped sea sponge Cymbastella lamellata turned from the usual chocolate-brown colour to a bright white.
Bell told 1News his researchers first sent him images of "white patches all over the reef" in May 2022 which they then realised were the sponges that were usually brown.
Nearly one year later, the researchers found nearly 50% of sponges at sampling sites along Fiordland's coastline had died, which wwas the "largest sponge mortality event that's ever been recorded".
"Firstly, they're what we call filter feeders, so they're responsible for processing really large volumes of water in the marine environment. And as they do so, they feed on all the tiny particles that are in the sea water and then produce a bunch of waste products that things on the sea floor eat. So they kind of recycle that material in the water," said Bell.
He said the mass death of sponges following the heatwave meant the potential fuel source had been reduced by approximately more than 850 tonnes, and would have a wider impact on marine life within the environment.
"They're really important for providing kind of structural complexity and three dimensionality to the sea floor that provides a place for lots of other things to live.
"People may not realise that there are over 800 different species of sponge in New Zealand and the ones that bleached weren't the only ones that were impacted in New Zealand during the the marine heat wave," he said.
The researchers also observed that 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.
However, Bell said the research was "not all bleak", as researchers discovered the sponges able to survive the bleaching mostly recovered, and in fact presented different microbes inside them than those that were killed by the heatwave.
"The death side of the sponges is obviously, you know, really concerning and quite a worry given the ecological impact of these sponges. But the really interesting thing we found is that some did survive, lots did survive. And those ones that survived had changed.
"What we are suggesting in our paper is that potentially this represents a kind of acclamation mechanism ... When these sponges are exposed to more marine heatwaves in the future, they better might be better able to deal with them and are kind of a bit preconditioned to future marine heatwaves," he said.
Bell said keeping sea sponges healthy is "certainly key to keeping the overall ecosystem healthy".
"Most of the research current research says that marine heat waves are going to become more frequent and they're going to become more intense, where the temperatures are going to get even warmer over time. So it means we're going to see the impacts of these events probably more often and they're probably gonna be more widespread in the future."
The study was published in the journal Global Change Biology.
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