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New Zealand is experiencing the biggest bleaching of sea sponges on record, scientists say, after extreme ocean temperatures turned millions of aquatic creatures white.

The discovery comes after researchers sounded the alarm in May when sea sponges off the southern coast of New Zealand were found bleached for the first time.

Initially, researchers estimated that hundreds of thousands of sponges had been bleached – but over the past month scientists have been carrying out surveys of the country’s coasts and have found that millions – possibly tens of million – had been turned into bone white.

“As far as we know, this is the largest scale and number of bleached sponges in a single event that have been reported anywhere in the world…certainly in cold waters,” said Professor James Bell, a marine ecologist from Victoria University.

A group of bleached sponges photographed by researchers. Photograph: Provided by Professor James Bell

When Bell team members first spotted the May bleaching event in Fiordland, they let the Department of Conservation and other chartered vessels in the area know to see if it had been spotted in other Sounds.

“They pretty much flagged the laundering everywhere they went,” he said. The team now thinks “there are at least millions of sponges, possibly several million sponges that have undergone this bleaching”.

Sea sponges, like coral, depend on symbiotic organisms that perform photosynthesis inside, providing food for the sponge and sometimes deterring predators.

Although bleaching does not necessarily kill sponges instantly, it does expel these organisms, lowering the sponges’ chemical defenses and depriving them of food. While some species can recover from severe bleaching, Bell said others do not.

University of Otago oceanographer Dr Robert Smith said two marine heatwaves in New Zealand had created record-breaking ocean temperatures – in some areas reaching five degrees above normal.

“On the northern and southern edges of New Zealand, we have seen the longest and strongest marine heatwave in 40 years since satellite measurements of ocean temperature began in 1981,” said he declared.

Smith said in some areas the marine heat wave started in September last year and was only just ending – lasting 213 days.

“To see these unusually hot temperatures go on for so long is the really unusual aspect,” Smith said.

“Some organisms are going to be OK with a day or a week above average temperatures – but once you start racking up that heat stroke…we’re going to start to really feel the effects.”

Smith said it was difficult to attribute a single heat wave to the man-made climate crisis, but ocean temperatures were rising around the world.

“What we can say is that there has been a significant increase in the frequency, duration and intensity of marine heat waves around the world over the past century,” he said. he said – and that projections indicated that these heat waves would become more extreme and longer in the future.

“What we see now is a window into what our oceans are likely to look like for our children and grandchildren.”

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