NZ marine sponges ‘melted’ in temperatures not expected until 2100

CHRISTCHURCH. June 3: Marine heatwaves are becoming annual events in New Zealand and the resulting mass mortalities of marine creatures and the impacts on ecosystems rival those seen on Australia’s Great Barrier Reef

“Marine heatwaves can have concerning effects, with heat stress pushing marine organisms towards or beyond their thermal tolerance limits,” says Dr Vonda Cummings, Principal Scientist of Marine Ecology at New Zealand’s National Institute of Water and Atmospheric Research. (NIWA). “These events can upset the balance of marine food webs and disrupt ecosystems.”

It’s been estimated that 66 million cup sponges (Cymbastella lamellate) bleached and 30 million died in Fiordland, on the West Coast of New Zealand’s South Island, in 2022, as water temperatures climbed more than 4^oC above average. The heatwave affected 1000 km of Fiordland’s coastline, lasted 259 days and was the largest sponge-bleaching event ever recorded worldwide.

Cup sponges, vital to Fiordland’s rocky, benthic marine community because of their sheer numbers, died in their millions, says marine ecologist, Professor James Bell of the Victoria University of Wellington. A shocking and saddening sight he adds.

Around 10 percent of the sponges were lost in Pātea (Doubtful Sound), with more gone further south in Tamatea (Dusky Sound) and in Te Puaitaha (Breaksea Sound).  

“These sponges occupy, on average, around 15 to 20% of the available space, and have quite a significant impact on the carbon cycling. Although they are quite small, there are millions and millions and millions of them, and they’re circulating substantial volumes of water”

Fiordland’s marine ecosystems are unique. Tannins washed down the steep-sided bush-clad slopes by the annual 6 to 8m of rainfall mean Fiordland’s waters are permanently stained the colour of tea.

“These ‘mesophotic’ (middle light) ecosystems are basically animal-dominated and pretty much unexplored in terms of their biodiversity,” Bell told Cosmos. “Because of limited light penetration through the tannin layer, algal communities don’t develop anywhere near as well and animals dominate much closer to the surface.”

Out on the open coast you’d have to dive 100 mor more before you see black coral, in Fiordland you’ll see black corals at five or six metres because it’s so dark, he says.

So why did they die?

Sponges, like corals, host photosynthetic microalgal communities, and it’s those algae that give the animals their distinctive colours. Lose the algae, lose the colour, leaving the underlying skeletons of limestone or silica, respectively, which give the corals and sponges a bleached look. 

Coral rely on dinoflagellates called Symbiodinium, for nutrition, so loss of algae with rising temperatures can mean death by starvation. Bell says sponges host diatoms, microalgae with silica walls, which produce dissolved organic carbon that sponges probably feed on. “We don’t know that for sure, because we haven’t measured whether the sponge host consumes the carbon those diatoms fixed,” he adds.

Which doesn’t mean the microalgal carbon isn’t important to the ecosystem.

“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 nutrients from the wider marine ecosystem.”

Bite marks on bleached sponges suggests the sponges died through predation, he says.

“We think that because the sponges were bleached bright white, they were super obvious to the fish, whereas before, they were brown, blending in really, well with that the dark substrate and fiord”

In the Hauraki Gulf, a similar marine heatwave caused more of a physiological effect. The temperature caused the sponges to basically melt. They obviously exceeded their temperature tolerance, says Bell, then melted and just slid off the walls, in some cases. Necrosis followed and all that tissue was lost, he adds.

On a brighter note — some of the sponges may have survived the heatwave by changing their microbial communities. Diatoms found in some living animals after the event were different to those found before.

We don’t know if there is any functional significance to that, says Bell. Different microbes might provide different carbon cycling pathways, for example, or help with detoxification of cellular projects products that are produced as part of sponge stress, he adds.

Sponges have been around for more than 500 million years, surviving multiple mass extinctions. Perhaps the ability to change microbial numbers or types, depending on conditions, could account for some of that longevity?

“The heatwave was extreme,” says Bell. “Although there’s usually some kind of heat wave happening somewhere in New Zealand.”

“That particular year, everything on land was basically dried out and the ground was crispy. You know, there were no waterfalls or anything. It was wonderful doing field work, but terrible if you’re a tree.”

Marine heatwaves are now an annual event in New Zealand waters. Another in Fiordland in 2023, was not as severe, says Bell. That one lasted 270 days in the south of the country. The waters of North Island’s Bay of Plenty were in heatwave conditions for a year.  NIWA now issues monthly sea surface temperature reports.

A lot of time is spent thinking about IPCC scenarios. How warm it’s going to be by 2100. But the temperatures experienced by these sponges were warmer than those predicted as long-term increases, says Bell.

“So, these climate change impacts aren’t things that are going to happen in the future. They’re actually happening literally now, right now, in real time.”

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