The waters off the coast of New Zealand are usually filled with velvety brown sea sponges, which play a fundamental role in marine ecosystems, by providing food and shelter to various species of aquatic animals. In May 2022, scientists raised an alarm when they observed the bleaching of hundreds of thousands of sea sponges off New Zealand’s southern coastline – a phenomenon in which, due to environmental stressors such as prolonged heatwaves, sponges lose their cells, leaving behind a white, bare skeleton.
Further analyses revealed that there could be in fact tens of millions of these aquatic creatures currently dying in the “worst bleaching event on record.”
“We have never seen this before in New Zealand. Sponge bleaching on a small scale was reported in Tasmania four years ago, but nothing like the scale we have seen in New Zealand. It has also been reported in the tropics before, but again not at the scale we have seen,” said James Bell, a professor of Marine Ecology at Victoria University of Wellington. “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.”
Like coral, sea sponges rely on symbiotic organisms that photosynthesize inside them, providing food for the sponges and deterring predators. Although bleaching does not necessarily kill the sponges immediately, it evicts these symbiotic organisms, thus lowering the sponges’ chemical defenses and depriving them of food. While some sponge species can recover after bleaching, many others cannot.
This mass bleaching event was most likely caused by two severe marine heatwaves in New Zealand that have created record temperatures over the past months.
“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,” said Robert Smith, an oceanographer at the University of Otago. In some areas, this marine heatwave had begun in September 2021 and is only now concluding, after 213 days.
“Seeing these unusually warm temperatures last such a long period of time is the really unusual aspect,” Smith explained. “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.”
Although attributing any single heatwave to clear anthropogenic factors is problematic, climate change has recently led to a significant increase in the frequency, intensity, and duration of marine heatwaves all over the world. If urgent efforts will fail to address the climate crisis, such heatwaves will become even more severe in the future, leading to more devastating die-off events.
“What we are seeing now is a window into what our oceans are likely to look like for our kids and our grandkids,” Smith warned.