The Southward Stampede Pt.1
A few years ago, a group of marine scientists came up with a new way of thinking about Australia’s cool temperate reefs. They looked at the vast stretch of reef habitats that curl all the way around the southern half of the continent and asked themselves – why isn’t all of this one gigantic ecosystem? From Kalbarri in the west, across the Bight and around Tassie then up to the NSW/Queensland border, this massive sweep of inshore reefs, dominated by kelps, have since been studied as a single entity – the Great Southern Reef (GSR).
The moment such an understanding is applied, large-scale aggregated data starts to reveal that which isolated studies couldn’t frame. One of the startling revelations of GSR thinking relates to “endemism”, the measure of geographic uniqueness of a creature. Because Australia has been vastly isolated during most of its evolutionary history – at least the past 23 million years – an estimated 77% of the 565 species of red seaweed found on the GSR are not found anywhere else on Earth. And that’s just one example… it’s likely that tens of thousands of species within the GSR system are yet to be identified.
Giant Trevally, Great Barrier Reef. Photo: Craig Parry.
Dr Scott Bennett is a surfer and marine ecologist at the University of Tasmania. His work has taken him from Townsville to WA, South Africa and now Spain, where he’s studying the Mediterranean and the Arctic. According to Bennett, “oceanographic processes like the poleward flow of the East Australian Current down the east coast, and the Leeuwin Current that wraps down the west coast and along the south coast” are vital in distributing larvae and ‘propagules’ (like spores in the wind) throughout the GSR. Thanks to those flows, the GSR is more or less defined everywhere by one humble organism: the habitat-forming species of kelp Ecklonia radiata (“common kelp”).
Bennett says kelp forests are the GSR’s “biological engine”, supporting an abundance of temperate marine species. But his research shows that the forests are dying in the north and growing in the south. You’re not imagining it when you feel that winter dips aren’t as cold as they used to be – isotherms (lines of temperature) are shifting poleward at a rate of 20 to 50km per decade. Because of the east-to-west alignment of Australia’s southern coast, he says, there will come a point when continued warming means “there is no more southerly habitat to which (the kelps) can retreat.”
As the East Australian Current pushes warm, nutrient-poor water southward into eastern Tasmania, we’re seeing dramatic losses of giant kelp (Macrocystis pyrifera) forests. These beautiful structures have been habitat and hiding place for everything from fur seals to lobsters and whelks. Their destruction is a silent and unseen disaster. Eight years ago, the Australian giant kelp forests became the first marine community to be listed as endangered under the Environment Protection and Biodiversity Conservation Act. But the losses continue unabated.
Another cause of kelp loss is the southward march of a large purple menace; the Longspine or “Black” sea urchin (Centrostephanus rodgersii), resulting in bare patches called “urchin barrens”, and reduced fisheries productivity. These urchins are native to New South Wales, but temperature changes have enabled them to get a foothold in Tassie. At up to 30 centimetres in diameter, and with an appetite to match, they’re a serious threat to Tasmania’s kelp forests. Once they've destroyed the larger marine plants, they'll scrape micro-algae off the rocks and live quite happily in their own barrens. Studies have shown that large crayfish feed on the urchins, and help maintain healthy kelp forests; in a sense, keeping marauders out of their own homes. Trouble is, southern rock lobsters are themselves under significant pressure from overfishing.
Climate change has warmed the upper oceans at rates of between 0.09 to 0.13°C per decade since 1970, causing a poleward migration of isotherms (temperature lines) along the ocean surface at rates averaging 21.7km per decade. Species, meanwhile, have been heading poleward too, averaging 72km per decade. Australian waters are warming at about two times the global average in south-west Australia and four times this in south-eastern Australia.
That’s the long-term picture. But attention is increasingly turning towards sudden blasts of warmth in our oceans: “marine heatwaves”. In unusually strong La Niña conditions in 2011, a once-in-200-year marine heatwave affected south-western Australia for 60 days. The Leeuwin Current went into overdrive, pushing tropical water with peak temperature anomalies of up to 6.5°C poleward. At the same time, the exchange of heat between air and sea was unusually high, and both processes were superimposed onto a decades-long warming trend.
The result was extensive damage to the GSR. Particularly at Kalbarri, scientists watched kelp forests contract virtually overnight. South of the township, Scott Bennett and his colleague Thomas Wernberg saw “100% loss of kelp across 38km2 of reef.”
“We thought we were in the wrong spot,” Bennett told The Atlantic at the time. “It was like someone had bulldozed the reef. It was like a moonscape underwater – scungy, brown, and empty.” And those reefs haven’t recovered. The natural north-to-south flow of the Leeuwin Current encourages tropical species migration down the coast and inhibits the reverse. As kelps vanish, herbivores like parrotfish and rabbitfish have come to dominate because of the new abundance of turf algae. The divers found a 400% increase in the biomass of scraping and grazing fishes, a characteristic of coral reefs, not kelp forests. And their extraordinary grazing rates prevent the recovery of kelp. “We tried to transplant some kelp back onto these reefs to see what would happen and if these tropical fish would actually eat it. They smashed around 40kg of kelp within 48 hours.”
Intertidal kelp, Great Australian Bight. Photo: SA Rips.
The serious damage to WA’s kelp forests tells us a bit about what the future might hold for Australia’s temperate marine environment. For the Kalbarri coast, the change is probably irreversible. “The probability of prolonged cool conditions that could… facilitate the recovery of kelp forests is becoming increasingly unlikely,” said Scott in 2015, “while the risk of more heatwaves… is increasing.”
How right he was. In 2015-16, south-eastern Australia experienced an unprecedented 251-day marine heatwave, and peak anomalies of up to 2.9°C above long-term records. Research shows that marine heatwaves have increased in frequency and duration over the last century, attributable to increases in mean ocean temperature. It turns out that WA’s experience wasn’t once-in-200 years at all.
In 2018, the IPCC (Intergovernmental Panel on Climate Change) produced a Special Report on the impacts of global warming of 1.5°C above pre-industrial levels. Keep that line in mind – it’s the measure for everything. Each and every assertion of fact that the IPCC makes, it grades in terms of how confident it is. So global warming is likely to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate: this can be said with high confidence. Keeping global warming to 1.5°C rather than 2°C would minimise increases in ocean temperature and acidity, and decreases in the ocean’s capacity to hold oxygen levels.
The wild Southern Ocean. Photo: SA Rips.
Even the best-case scenario is a tragedy. Even if we can keep warming to 1.5°C, coral reefs are going to disappear by a further 70-90%. And the losses will be greater than 99% at 2°C. A recent study backed by major climate science organisations around the world has found commitments to reduce emissions must be at least tripled and possibly increased fivefold if we are to meet the goals of the 2015 Paris Climate Agreement – that is, to keep global mean warming to 1.5°C. The report found that current plans – or in Australia’s case, the current absence of plans – would put global temperature rises into the range between 2.9°C and 3.4°C by 2100. That range is less studied than the band between 1.5°C and 2.0°C. Why? Because it’s beyond our current understanding. It would be catastrophic.
Much of this discussion is about surface temperatures, but it’s a three-dimensional problem – you have to think of ocean warming horizontally and vertically. The impacts of warming will also be felt at depth. “Just like on mountains, where plants and animals can move upslope to escape warming, marine plants and animals can theoretically move deeper to cooler temperatures,” says Scott Bennett. “For some species it’s a good strategy. Along Australian coasts, moving ten metres deeper is the equivalent of moving 10-100 kilometres poleward. But not all species can move deeper. Kelps, seagrass and corals that depend on light can only go so far before they hit the base of their photic zone. And in southeast Australia isotherms are deepening particularly fast: around 30 metres per decade. This means that in one to two decades, warm surface temperatures will penetrate down to depths of more than 50 metres, beyond the depth that photosynthetic organisms can escape to.”
Warming is attacking south-eastern Australia both horizontally and vertically, in other words. There’s only one place left to go. Ultimately it will push organisms south… off the edge of the Australian continental shelf.
Banner image – Australian Fur Seals, South Australia. Photo: Craig Parry.