Marine heat wave linked to death of Fiordland sponges

Warming waters may already be taking a toll on marine sponges.

Marine sponge with patches of white
A sponge in Te Puaitaha—Breaksea Sound with patches of white, showing the start of bleaching. Sponges are normally dark brown. Photo credit: Charlie Bedford.

Warming waters in Fiordland could be responsible for the loss of up to 10 percent of one of the most abundant marine sponges in Pātea—Doubtful Sound. More sponges may have been lost further south in Tamatea—Dusky Sound and Te Puaitaha—Breaksea Sound.

Widespread bleaching of sponges (Cymbastella lamellata) was discovered in May 2022 by researchers from Te Herenga Waka—Victoria University of Wellington. This bleaching was linked to an extreme marine heatwave and affected millions of sponges.

Marine biologist Professor James Bell led a return trip to Fiordland this summer with Dr Valerio Micaroni and PhD candidate Ms Francesca Strano to investigate what had happened to the sponges. They found mixed results.

“At the six sites we surveyed in Pātea—Doubtful Sound, we found nearly all the sponges had recovered their colour and were no longer bleached—that’s the good news. However, some sponges are likely to have died. It’s hard to say how many have been lost but we think it could be 5 to 10 percent, based on video and photographic data,” Professor Bell says.

Early reports from the Southern Fiordland Initiative suggest the situation might be even worse in Dusky and Breaksea Sounds. The Initiative, co-led by Professor Bell, was launched in 2021 to monitor environmental changes in the fiords.

“Initial surveys in Breaksea Sound by Katherine Mitchell from the Southern Fiordland Initiative suggest a much lower number of sponges than seen last year and many of the sponges have not fully recovered from the bleaching.”

There are recent reports from Breaksea Sound of newly bleaching sponges, Professor Bell says.

Near the end of their trip to Doubtful Sound, the researchers also found some sponges were showing signs they might be starting to bleach again. Fiordland experienced another severe heatwave in late December and through much of January, with temperatures in some areas more than 5ºC warmer than normal.

Professor Bell says the bleaching itself does not appear to kill the sponges directly, but it seems to make them more susceptible to being eaten by fish.

“When we visited Fiordland in May last year, we saw sponges with lots of large bite marks in them. We didn’t see this on our latest trip, suggesting these sponges have died or been eaten, since it’s unlikely they could have regenerated the tissue they lost in such a short period of time—and we didn’t see sponges with deformities from regrowth.”

The researchers have discovered the bleaching causes the sponges to lose their photosynthetic diatoms—tiny symbiotic organisms that live inside the sponges.

“We believe it’s the loss of these diatoms that either makes the sponges more palatable to fish or makes them more visible to fish,” Professor Bell says.

The bleaching and sponge deaths may have other flow-on effects on the local marine environment.

“We’ve found evidence that sponges get food from their symbiotic diatoms and there’s a very strong likelihood the sponges release excess carbon produced by these diatoms. This carbon will be eaten by other organisms, particularly microbes in the seawater, fuelling local food webs. However, this food source isn’t available when sponges are bleached, removing a potentially important nutrient from the wider marine ecosystem.”

Further work is now underway at the University’s Coastal Ecology Laboratory to assess the effects of rapidly warming waters on sponges in the Fiordland region.

“Back in the lab, we’re going to replicate the conditions the sponges experienced in May 2022 so we can gauge what impact marine heat waves have on a wider range of sponge species.”

The team will be revisiting Dusky and Breaksea Sounds in late March to assess the extent of the decline in sponge populations.

Their visit to Doubtful Sound in January 2023 was on board the Department of Conservation (DOC) vessel Southern Winds. Richard Kinsey, marine senior ranger for DOC, was also on board assisting Professor Bell.

“The work that James is doing is vitally important to help us understand what future impacts might occur in the fiords so we can try to manage them. He’s the expert so it is great for us to be able to provide a vessel for him to work off so we can see what’s happening first-hand,” Mr Kinsey says.

The research work is supported by DOC, the George Mason Charitable Trust, the Fiordland Lobster Company, and The Leslie Hutchins Conservation Foundation.