Professor James Bell diving to photograph Fiordland's black coral. Photo: Matteo Collina
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The lights of the remotely operated vehicle, or ROV, scan the cliff-like reef of Doubtful Sound, illuminating coloured blobs, cup-shaped sponges and branched corals hidden in the gloom.
Aboard DOC's research vessel Southern Winds, Professor James Bell of Victoria University of Wellington is hunkered over the controls, in full concentration mode.
Out of the darkness, a large tree-like black coral appears. Misleadingly named, these corals have black or brown skeletons with bright white feathers erupting off their branches. These white filamentous structures are the coral's polyps.
This coral is a few metres across and likely many years old, as they are very slow growing. Some dead branches are adorned with colourful sponges and sea squirts (ascidians). Fishes dart and school among the coral's 'foliage'.
The ROV is used to take footage of corals at depth. Photo: James Bell
Providing habitat is one of the key ecosystem roles of these black corals, says James. "They are like little oases of life and biodiversity."
This is one reason why he and his team are studying these strange organisms.
Fiordland's black corals
Typically, black corals live in much deeper parts of the ocean around Aotearoa. However, Fiordland's unique ecosystem means they are found in the sounds at much shallower depths.
Fiordland's abundant rainfall sweeps through the rich leaf litter of the steep forest-clad slopes, collecting tannins - a type of chemical found in woody plants and wine. This tannin stains the freshwater brown like tea, and once it reaches the fiord, it forms a layer on top of the denser seawater, essentially blocking out some of the light.
Doubtful Sound. Photo: Claire Concannon / RNZ
In these medium-light conditions, seaweed struggles to grow and instead there are 'animal-dominated' reefs. The steep rocky sides of the fiords are littered with slow-growing filter-feeding creatures including sponges, black and red corals, sea pens, sea fans, and sea squirts.
Since 2018, a team led by James has been documenting the habitat of the outer, middle and inner areas of some fiords. The work, contracted by DOC, aims to discover what lives there, and how things are changing over time.
Down to 25 metres depth is the domain of divers equipped with cameras. They take a series of photographs along the reef that are later analysed to identify the species and figure out how much of the area they are covering. For deeper spots, down to 200 metres, the researchers use the ROV.
The team use an ROV to survey deeper habitats. Photo: Matteo Collina
And on this trip to Doubtful Sound, PhD candidate Miriam Pierotti is also taking samples of black corals from different areas, for her research into coral resilience.
Recreating black coral family trees
Dry suits are the gear of choice in Fiordland. After donning merino layers, and pushing the air out of her sealed suit, Miriam grabs some plastic baggies and bright yellow labels.
With her dive partner, Eva Ramey, she uses the labels to mark corals she is sampling - snipping a 'branch' from the large structures to place in a bag. Black corals are protected under the Wildlife Act, but Miriam has a permit to do this work.
Miriam Pierotti, Matteo Collina and Dr Alice Rogers talk pre-dive. Photo: Claire Concannon / RNZ
Then, she uses tinfoil to create a makeshift lab to process her samples at the back of the boat. She carefully slices small fragments of each coral sample and stores them in vials for future genetic analysis.
While Miriam works, the next dive team has gone below. Matteo Collina and Dr Alice Rogers take hundreds of pictures of the marked corals, and precise GPS markings. By stitching together the photos they can recreate a 3D model of the black corals as they appear on the reef - a process called photogrammetry.
Miriam Pierotti processes coral samples onboard. Photo: Claire Concannon / RNZ
By combining the genetic analysis and the photogrammetry, Miriam hopes to recreate a 'family tree' of the corals. This will help her understand how the next generations are faring and how far coral larvae could be travelling within the fiords. Both are important for understanding how resilient the black corals might be to local disruptions, like landslides.
Marine heatwaves
Another potential disruption of interest is ocean warming.
By taking live black coral samples back to the Victoria University of Wellington Coastal Ecology Lab, PhD candidate Amber Kirk is investigating how they respond when the temperature is ramped up.
In 2022, a long-lasting marine heatwave resulted in sea temperatures 4.5C above average for the sounds. This team documented [www.rnz.co.nz/news/national/524348/mass-bleaching-of-nz-marine-sponges-largest-ever-recorded-globally mass sponge bleaching events] as a result, and some subsequent sponge death. But it's unclear how the black coral coped.
Amber Kirk monitors the black corals in the lab. Photo: James Bell
In the lab, when Amber ramped up the heat to the same temperatures experienced in that heatwave, some of the corals died, but others hung on.
She's now investigating how the black corals responded physiologically, and whether the microbes associated with them have changed - something they have observed in certain sponge species in response to heat.
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