New Zealand records largest ever bleaching of sea sponges

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New Zealand is experiencing the largest bleaching of sea sponges ever recorded, scientists say, after extreme ocean temperatures turned millions of the aquatic creatures white.

The discovery comes after researchers raised the alarm in May, when sea sponges off New Zealand’s southern coastline were found bleached for the first time.

Initially, researchers estimated hundreds of thousands of the sponges had been bleached – but over the past month, scientists conducted investigations at coastlines around the country, and found that millions – possibly tens of millions – had been transformed bone-white.

“As far as we’re aware, it’s the largest scale and largest number of sponges bleached in one event that’s been reported anywhere in the world … certainly in cold waters,” said Victoria University marine ecologist Prof James Bell.

When members of Bell’s team first spotted the May bleaching event in Fiordland, they put the word out to the department of conservation and other charter vessels around the region to see if it had been spotted in other Sounds.

“They pretty much reported the bleaching everywhere they went,” he said. The team now believes “there are at least millions of sponges, maybe many millions of sponges that have undergone this bleaching”.

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Sea sponges, like coral, rely on symbiotic organisms that photosynthesise inside them, providing food for the sponge and sometimes deterring predators.

While bleaching does not necessarily kill the sponges outright, it evicts those organisms – lowering the chemical defences of the sponges 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 ocean temperatures – in some areas rising to five degrees hotter than normal.

“At the northern and southern limits of New Zealand, we’ve seen the longest and strongest marine heatwave in 40 years, since satellite based measurements of ocean temperature began in 1981,” he said.

Smith said in some areas, the marine heatwave had begun in September last year, and was only just concluding – lasting 213 days.

“Seeing these unusually warm temperatures last such a long period of time 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 accumulating that heatstroke … we’re going to start to really feel the effects.”

Smith said that attributing any single heatwave event to the human-made climate crisis was difficult, but ocean temperatures were rising around the world.

“What we can say is that there has been a significant increase in the frequency, the duration and the intensity of marine heatwaves globally over the past century,” he said – and that projections indicated those heatwaves getting more extreme and longer in future.

“What we are seeing now is a window into what our oceans are likely to look like for our kids and our grandkids.”

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