It is early afternoon by the time we motor to a stop in a bay on the Tasman Peninsula, south-east of Hobart. The water is an inky blue in the low winter sun; above us the heaped layers of dark cliffs rise precipitously. A few kilometres back we passed seals lazing in the water, their bodies dark and sleek as they rolled and tumbled; here the water is still and quiet, the only sound coming from the gentle swell against the rocks.
I am here with University of Tasmania marine ecologist Dr Scott Bennett and former fisheries biologist and co-owner of the Eaglehawk Dive Centre, Mick Baron, to visit one of several sites at which Bennett and his colleagues Drs Cayne Layton, Scott Ling and Craig Sanderson are working on a last-ditch attempt to save the forests of giant kelp (Macrocystis pyrifera) that once surrounded the Tasmanian coast. The collapse has been terrifyingly swift: only 20 years ago, these forests were so dense they had to be marked on maps to prevent boats from becoming snarled; two decades later, less than 5 per cent of them remain.
The reason for this is simple: as the oceans heat up, the East Australian Current, which flows down the eastern edge of the continent, is extending its range south, pushing warm, low-nutrient water into the waters around Tasmania. For the giant kelp it has been disastrous. “Kelp, and giant kelp in particular, doesn’t like hot water,” says Layton when I speak to him in Hobart. “It’s a physical intolerance, so as the water gets warmer it’s literally just melting away at a physiological and biological level.” This process has been accelerated by the arrival of long-spined sea urchins in Tasmanian waters. Formerly confined to the coast of the mainland, the urchins are voracious consumers of seaweed and, if left unchecked, can reduce whole stretches of seabed to bare rock.
The scale of the collapse is starkly apparent in the waters in which we have moored. Baron points across the water to a line of cliffs a kilometre and a half away, and then back towards a bluff a couple of kilometres behind us. “It was kelp forest right through here,” he says. “It started to decline around 2000, and by 2010 it was gone. There are a few areas where it hung on a bit longer, but basically, over a period of 10 years, give or take, it just disappeared.”
Baron, who has been diving in these waters for more than 40 years, says the disappearance of the kelp has led to a wider scale collapse of the ecosystem. “When the forest was here, you couldn’t put a net in it or drive a boat through it because it was so thick it just clogged everything up. So it was a natural nursery, with schools of trumpeter and all kinds of other fish. But once the forest is gone you can get access everywhere, and so the fish just got hammered by netting.”
In a world where ocean temperatures are rising fast, the fate of the kelp and the ecosystems it supports looks bleak. But over recent years Bennett and his colleagues have been working on a scheme to hold back its seemingly inexorable decline. “Although the kelp has lost about 95 per cent of its range, there’s still that 5 per cent that’s hanging on,” says Layton. “That raised the possibility that those individuals or populations were a bit more tolerant of warm water.”
Bennett, Layton and other members of the team took samples of some of these surviving populations to their laboratory in Hobart, where they exposed them to higher temperatures to identify the individuals with the highest thermal tolerance. These were then cultivated to produce juveniles capable of surviving in warmer waters.
After a series of trials that showed these thermally tolerant strains could be successfully reintroduced to the wild, in October 2022 the team began the next phase of the project. With the backing of fashion-designer-turned-climate-entrepreneur Sam Elsom’s Sea Forest Foundation, an initiative that hopes to help tackle the climate crisis by protecting and restoring kelp and seaweed, they began the process of planting giant kelp across more than half a hectare. “For the past few years we’ve been developing the methods,” says Bennett. “As a result we now know we can grow these plants. This current project is the first attempt at forest-scale restoration. And by that we mean planting it in a way that will result in something like the density of a natural forest.”
Thus far, the process has been a success: by late March 2023 the juveniles that Bennett, Ling, Layton and Baron spread across the site in October had resulted in somewhere between 250 and 300 plants, the largest of which had reached around 3.5 metres. Nonetheless, as we wait to enter the water, Bennett is nervous about what he will find. “When conditions are good, the kelp can grow extremely well, but at the same time, they’re really vulnerable because there are so many ways they can die.”
Finally we all step off the side of the boat. The water is surprisingly clear, and cold, and as we descend we pass over beds of golden kelp and crayweed, their honey-coloured leaves shifting in the light from above. Yet they do not prepare me for the sight of the giant kelp plants when they emerge out of the dimness. A dozen or so are gathered in one spot, each of them rising 5 metres or so from the seabed to the surface, their tops lost in the dance of light from above. Moving carefully, I approach the nearest. To my surprise the trunk is not a single thick stem. Instead, cord-like stipes are wound together to form a sort of rope several centimetres in diameter, from which sprouts the long, honey-coloured ribbons of the blades, and the brown, fruit-like forms of the air bladders, or pneumatocysts, that hold the kelp aloft.
Because of their height the kelp plants look like they must be rigid, but instead they hang suspended like an underwater version of the Indian rope trick. When I touch one it yields easily, and for a moment I can imagine what it must have been like to swim through a forest of such giants, their forms stretching upwards like the columns of a cathedral.
Back on the boat, Bennett is relieved and happy: the kelp is thriving, and has grown a metre and a half in the five weeks since he was here last. But despite the project’s success to date, Bennett admits it remains emotionally challenging work, and a reminder that “headlines about the impacts of climate change aren’t hypothetical; they’re happening right now in front of our eyes. I grew up with giant kelp, it was everywhere, and a part of our lives, and it’s disappeared in a decade or so. Seeing something like that happen on such a vast scale, it’s frightening.”
When I ask Layton if he ever worries that rising ocean temperatures will overwhelm even the team’s thermally tolerant kelp strains, he looks away and pauses before responding. “It’s terrifying. It’s no exaggeration to say it keeps me up at night. These ecosystems are so underappreciated and so threatened. So, yes, emotionally it’s tough.”
Nonetheless these worries do not detract from their belief in the project’s significance. “These warm-tolerant genotypes aren’t a silver bullet,” says Layton. “They don’t mean you’ll suddenly be able to grow giant kelp in Sydney Harbour. But it’s buying us time while we solve the overall challenge of reducing emissions.”
Bennett agrees. “We’re facing the loss of kelp forests in Australia within a decade or two. That could still happen, even with what we’re trying to do. But if we do nothing, you can guarantee they’ll be gone. So I don’t think we have the luxury of not trying.”
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