New Zealand is experiencing the largest-ever bleaching of sea sponges after extreme ocean temperatures turned millions of aquatic creatures white, scientists say.
The discovery comes after researchers sounded the alarm in May when bleached sea sponges were first found off New Zealand’s south coast.
Researchers originally estimated that hundreds of thousands of the sponges had been bleached – but last month scientists conducted surveys at coasts across the country and found millions – possibly tens of millions – had turned bone white.
“As far as we know, this is the largest magnitude and number of sponges bleached in a single event reported anywhere in the world…certainly in cold waters,” said marine ecologist Prof James Bell of the Victoria University.
When members of Bell’s team first detected the May bleaching event in Fiordland, they informed the Department of Conservation and other charter vessels in the area to see if it had been detected in other sounds.
“They reported the bleaching pretty much everywhere they went,” he said. The team now believes “there are at least millions of sponges, perhaps many millions of sponges, that have undergone this bleaching.”
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Like coral, sea sponges rely on symbiotic organisms to photosynthesize inside them, provide food for the sponge and sometimes deter predators.
While bleaching doesn’t necessarily kill the sponges immediately, it drives these organisms away – lowering the sponges’ chemical defenses and depriving them of food. While some species can recover from severe bleaching, Bell said others don’t.
The oceanographer Dr. Robert Smith of the University of Otago said two marine heatwaves in New Zealand had caused record ocean temperatures – in some areas rising five degrees hotter than normal.
“Across New Zealand’s northern and southern borders, we have experienced the longest and strongest marine heatwave in 40 years since satellite sea temperature measurements began in 1981,” he said.
Smith said in some areas the ocean heatwave started in September last year and has only just ended – lasting 213 days.
“To see these unusually warm temperatures persist for so long is the really unusual aspect,” Smith said.
“Some organisms will be fine with above-average temperatures for a day or a week — but once you start accumulating that heat stroke… we’re going to start to really feel the effects.”
Smith said it’s difficult to attribute a single heatwave event to the man-made climate crisis, but ocean temperatures are rising around the world.
“What we can say is that the frequency, duration and intensity of ocean heatwaves has increased significantly around the world over the past century,” he said — and that projections indicate these heatwaves will become even more extreme and prolonged in the future.
“What we’re seeing now is a window into what our oceans are likely to look like to our children and our grandchildren.”