An Underwater Superhighway - The EAC

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The Eastern Australian Current

by Sheree Marris

The East Australian Current (EAC) runs north to south from the top end of the Great Barrier Reef to the southern reaches of Tasmania. A staggering 100 kilometres wide and running over 500 metres into the dark depths of the ocean, the EAC spans the length of the east coast of Australia measuring around 4,000 kilometres.

Nemo's dad and his forgetful side-kick were sucked into this super underwater highway where they surfed the currents with some gnarly green turtles before being smacked in the face with a wall of jellyfish.  Made famous in the animated film, Finding Nemo, the East Australian Current (EAC) runs north to south from the top end of the Great Barrier Reef to the southern reaches of Tasmania. A staggering 100 kilometres wide and running over 500 metres into the dark depths of the ocean, the EAC spans the length of the east coast of Australia measuring around 4,000 kilometres .

It is like a massive underwater conveyer belt transporting 30 million cubic metres  of water per second with speeds that reach up to seven kilometres per hour To put that into perspective, this equates to over 16,000 Olympic swimming pools flowing along the coast every second, making it the largest ocean current close to the shores of Australia.

Generated by the equatorial winds of the South Pacific Ocean and the rotation of the Earth, the EAC, like all ocean currents, is more than a fast-flowing river of saltwater. "Currents are really unsung underwater heroes," says Associate Professor Neville Barrett, Marine Ecologist from the University of Tasmania.  "They're the ocean's air conditioning system, playing a vital role in distributing and transferring heat from the tropics to waters in the south, influencing productivity, biodiversity, and what grows where and when."

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Turtles are not the only ones to ride this sunken superhighway. The EAC attracts a huge range of marine species including tropical fish, sharks, corals, larvae and even marine algae. Humpback whales use the currents to migrate from breeding grounds on the Great Barrier Reef to cooler feeding grounds in the Southern Ocean.  And a bit like Nemo, larvae of many tropical species, as well as larger fishes and sea snakes frequently get carried further south than planned and end up stranded in colder Southern Ocean waters where they often perish.

Despite existing below the ocean's surface, it also shapes weather patterns above water by warming the air and adding extra moisture, impacting a large proportion of the Australian population and its economies. Although the mighty EAC may seem like an indestructible force too enormous for us to influence, the last 40 years have seen the strength of the EAC increase, and the temperature rise more than 2 degrees Celsius in the Tasman Sea along which it flows, as a result of human actions and climate change. This ocean artery has also extended its warm reach and southerly influence by 350 kilometres in the last 60 years.

Beginning in the Great Barrier Reef, the EAC carries a large amount of warm tropical water from the equator southward. This process is part of what allows the GBR to thrive, as it distributes nutrients and smaller organisms throughout the reef and surrounding areas. The GBR is one of the most complex natural ecosystems on the planet, a dynamic home to over 1,500 species of fish, 134 species of sharks and rays, and threatened species including sea turtles and marine mammals. The foundation of the reef and its fame come from the 40 plus species of hard corals that live there.

This dynamic is shifting with increasingly warmer waters being driven from the equator and carried by the EAC. Rising water temperatures on the reef will force mobile species to seek refuge further south, however, for the northern GBR, there are no species left to fill the void. Even surrounding areas such as the Timor Sea cannot replenish the life here. All thrive within the same temperate gradient. If it gets too warm, the impact on the GBR is profound. It is not great news either for southern coral reef systems of the GBR that would be displaced by the northern species. Their survival is dependent on the presence of hard rocky reef structures off the coast on which larval corals can settle and grow to establish coral reef ecosystems. Rocky reefs are either too rare or too deep to provide much refuge as conditions warm.

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As the EAC moves south, it flows along the exposed coastline of New South Wales, a transitional area for marine species. Unlike the south and the north, there are few endemic species here. As it sweeps past Sydney, it forms a number of ever-changing eddies, or giant whirlpools, that influence the marine life that finds its way into the Harbour. Some years, recruitment is dominated by tropical species such as the striated frogfish, while other years, temperate species such as silver drummer and crimson-banded wrasses call the harbour home.

These giant whirlpools spin eastward, nearly 800 kilometres off the coast of Sydney towards Lord Howe Island. The island is a volcanic remnant, an oasis that rises up from the depths, peaking at 875 metres above sea level and measuring only 10 kilometres long and 2 kilometres wide. What it lacks in size, it makes up for in diversity. 

It is a marine biological melting pot, a virtual United Nations of the oceans where tropical and temperate species and currents collide. The result is a rich biodiversity, dominated by sub-tidal coral reef habitats and scattered with lagoons, inlets, estuaries and fringing smaller islands.  Lord Howe Island, this tiny marine park in the Tasman Sea, is a world heritage site boasting the most southerly true coral reefs, over 80 species of corals, many of which are endemic to the area, and over 500 species of fish including its very own species of 'nemo', the Whitesnout anemonefish (Amphiprion mccullochi).

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As the current meanders southward into traditionally cooler climates, it clips the top east corner of Victoria past Cape Howe, bringing with it warm, low nutrient water, and never seen before species, which are increasing in frequency. The strength and temperature changes in the EAC are magnified at the extent of its southern reach in the waters of Tasmania. This island is considered one of the most biologically diverse marine environments on the planet, and is a bastion for temperate marine life.  Over 80% of the marine species that live in these waters are found nowhere else on Earth including the endangered red handfish, giant kelp forests, live bearing sea stars, sponge gardens and Maugean skates.

All are now under pressure from a number of threats that include rising sea temperatures, primarily driven by the increasing influence of the EAC, and competition from introduced species. Tasmania has already experience massive losses of its iconic giant kelp forests. Many of these cooler-water species are being pushed to the limit of their temperature threshold and there is nowhere further for them to go. There are no habitats, coastlines or shelters further south. It is literally the last stop. Species that cannot survive and adapt will stop existing. Modelling suggests many of the Tasmanian endemic species, from fish to sea stars to seaweeds, are unlikely to survive into the next century under even the most conservative climate predictions.

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Associate Professor Neville Barrett explained "The rate of warming off the coast of Tasmania has been nearly four times the global average over the past few decades. Almost no aspect of coastal biodiversity will be unaffected."

The EAC is a complicated moving marine behemoth that is difficult to research and predict, and there is still so much to learn. For years, it was assumed that the EAC only flowed in one direction, however it is a two-way street or underwater autobahn in this case. There is a push and pull of the currents that is also influenced by the wind.  This makes it unpredictable, and nowhere is this uncertainty more prevalent than in Tasmania when there is an interaction with other currents.

Understanding the key drivers for the EAC, how it functions, and the extent of its influence, are vital. That is why temperature trajectory models of the EAC are currently being developed in what scientists have termed "climate velocity" as it will help predict the movement of species by tracing pathways connected by climate. 


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In reality, the increasing influence of the EAC along the eastern coastline of Australia is a double-edged sword. Species that can migrate further south will be able to move and expand their ranges, resulting in an increase in biodiversity in some areas. Some species that have broader thermal tolerances such as Yellowtail kingfish and snapper will thrive under a changing climate because they are quick to adapt.

Marine ecosystems and diversity however, could become more uniform, as boundaries for tropical and temperate species will become similar in what scientists have described as "smearing" of biodiversity. When it comes to species diversity, there are really only three options: move, adapt or die.

The environmental alarm bells are ringing as hard as they can.  It might seem doom and gloom, and out of our power to make a difference especially when the issue is underwater, and out of sight.

But we need to reframe and look at this as an opportunity. Just like the East Australian Current and other powerful currents that shape our world, it is time for us to move and act--like corals. When considered cumulatively, the actions of tiny coral animals can be seen from space in the form of magnificent reef systems. Our combined actions can make a difference, create change, and shape the future that we want. As with corals, it needs to be a collective approach.

In the end, like Nemo, let us hope we too find our way back home by creating solutions to pressing environmental issues and figuring out how to support systems that sustain not only us, but all the life that inhabits them. 

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