New report reveals crucial role New Zealand sea sponges play in filtering our oceans

A new report into sea sponges reveals the little organisms play a major role in filtering millions of cubic metres of our oceans.

The creatures clinging to New Zealand's reefs are not just decorative, they're a critical part of our food chain - filtering carbon and nutrients for fish to feed on.

Victoria University marine biologist Professor James Bell told Newshub the sponges are very efficient recyclers.

"A lot of that recycling could be really important for fish species, the things we like to eat," he said. 

Over the past several years millions of the sponges have been bleached and killed in marine heatwaves, sparking further research.

"What we're interested now in knowing why that particular species was impacted, but a whole range of other species weren't impacted," said Prof Bell.

Sea sponges are an important part of the food chain.
Sea sponges are an important part of the food chain. Photo credit: Prof James Bell / Supplied

So he's ramping up his Fiordland research - leading a team of scientists into its depths, studying the many different impacts of climate change on our vulnerable ecosystems.

And after years of work, his latest study has calculated the organisms cover 15 percent of Fiordland's rocky underwater areas. In Doubtful Sound alone they're pumping half a million cubic metres of water every minute.

"They're sucking in vast quantities of water and stripping out all the tiny food particles - things like bacteria and phytoplankton in the water, they remove 5 percent of that food from the water column," Prof Bell said. It's then transformed into waste, or sponge poo.

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"Some of that sponge poo is fed on by other organisms living on the sea floor. These are then feed on by things higher up the food chain including things like fish, but in Fiordland also potentially crayfish. Sponges here are really important in that cycling of nutrients," he said.

The professor is now looking at what happens to the carbon as some of the sponge waste products are released and eaten by organisms like hermit crabs and worms.

To gather the data, scientists are using a remotely operated vehicle, funded by the George Mason Charitable Trust. It reaches depths of more than 100 metres, grabbing samples to bring back to the surface for analysis. 

The ROV, Remotely Operated Vehicle, with a sponge sample in its grasp.
The ROV, Remotely Operated Vehicle, with a sponge sample in its grasp. Photo credit: Newshub

The researchers have a permit to take dozens of samples, and among them is marine biologist Manon Broadribb who is taking bits of sponges back to the Wellington laboratory for testing. As part of her PhD, she's studying deeper mesophotic reefs, which includes the role of sea sponges.

"They're ecosystem engineers, they filter a whole lot of water and create habitats, so it's important to know how they might look in the future," she said.

To better understand why some of Fiordland's sponges survived the marine heatwaves, she's testing the organisms' response to antibiotics.

"I'm going to treat them with antibiotics, to see if I can disrupt their microbial community. The microbiome of the sponge changes in the natural environment, we're trying to predict how that might affect their resistance to heatwaves," Broadribb said.

A sea sponge community in Doubtful Sound, Fiordland.
A sea sponge community in Doubtful Sound, Fiordland. Photo credit: Prof James Bell / Supplied

It will help them determine temperature thresholds.

"From some of our initial results, it looks very likely that just a few degrees more could have impacted many more sponges. If we lose all these sponges as marine heatwaves get more intense, we could see major impacts on other organisms," said Prof Bell.