A bleached sea sponge in Fiordland, where sea temperatures recently soared to 5C above normal. Image / Victoria University
A bleached sea sponge in Fiordland, where sea temperatures recently soared to 5C above normal. Image / Victoria University
Ongoing marine heatwave conditions likely caused a mass die-off of sea sponges in Fiordland, where local sea temperatures have soared as high as 5C above normal over summer.
Last year, scientists were alarmed to discover millions of sea sponges there had turned from velvet-brown to bone-white – making for one of the worst bleaching events documented among sponge species anywhere.
That followed the most severe marine heatwave ever observed in Fiordland, with another major event playing out again this season.
When a research team returned to the region last month, they found that, while many sponges had recovered, there’d also been large-scale loss.
“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,” Victoria University marine biologist Professor James Bell said.
“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 per cent, based on video and photographic data.”
Early reports from the Southern Fiordland Initiative suggested the situation might be even worse in Dusky and Breaksea Sounds.
The initiative, co-led by 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 were also recent reports from Breaksea Sound of newly bleaching sponges, Bell said.
Near the end of their trip to Doubtful Sound, Bell and colleagues Dr Valerio Micaroni and Francesca Strano 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 5C warmer than normal.
This map shows sea surface temperatures around New Zealand by anomaly (left) and temperature (right) as at January 17. Amid a widespread marine heatwave, sea temperatures off the South Island's West Coast have been running as warm as - and in some places hotter - than those observed in the upper north. Image / NOAA
While the bleaching itself didn’t appear to kill the sponges directly, it did seem 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,” he said.
“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.”
The bleaching and sponge deaths could also pose 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,” he explained.
“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 was under way 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 late next month.
Recent research carried out through the collaborative Moana Project has shown that, for the last couple of years, marine heatwaves have persistently impacted New Zealand’s coastal waters.
That’s come with myriad knock-on effects, including mass kelp die-offs, fish species like kingfish and snapper drifting further south, and worsening melt rates on southern glaciers.
What toll this heatwave ultimately wrought on glaciers would be clearer when scientists carried out their annual aerial snowline survey at the end of the summer season.
“Overall, if the situation exceeds 2018 then I’m expecting the highest snowlines on record,” Niwa climate scientist Dr Drew Lorrey told the Herald last month.
In any case, he said, the marine heatwave was “really bad news” for the glaciers this year.
Our largest salmon exporter, New Zealand King Salmon, was also recently forced to close farms in Pelorus Sound after a heatwave led to 1300 tonnes of dead salmon being sent to landfill and local job losses.
Recent research has suggested that, by 2100, the 40-odd marine heatwave days we currently see in a normal year will increase to between 80 days in a low-emissions, best-case scenario – and a worst-case 170 days.
For some regions, such as the southern tip of the South Island, there was also a high chance that marine heatwaves began to last more than a year.
