Temperate Australasia (TU)

Discovering Australia's and New Zealand's Great Southern Seas

By Shanti Plants – The Earth Provides

Stretching across the cool southern waters of Australia and surrounding the islands of New Zealand lies one of Earth's most extraordinary marine regions—the Temperate Australasia Marine Realm. While tropical coral reefs often capture the public's imagination, the temperate oceans of the Southern Hemisphere harbour an equally remarkable diversity of life. Beneath rolling swells and often cold, nutrient-rich waters exists an underwater world dominated by towering kelp forests, colourful sponge gardens, extensive seagrass meadows, rocky reefs, submarine canyons and deep-sea coral ecosystems that have evolved over millions of years (Spalding et al., 2007).

The Temperate Australasia Marine Realm encompasses the temperate coastlines of southern Australia, Tasmania and New Zealand. It forms one of twelve global marine realms recognised by the Marine Ecoregions of the World (MEOW) classification system, which groups coastal and shelf waters according to their shared evolutionary history and ecological characteristics (Spalding et al., 2007). Rather than being defined by political borders, this realm reflects a natural biogeographic region where ocean currents, climate, geology and evolutionary isolation have shaped distinctive marine communities.

Isolation has been one of the defining forces behind the remarkable biodiversity of this region. Australia separated from Antarctica approximately 45 million years ago, while New Zealand has remained isolated for even longer following its separation from the ancient supercontinent Gondwana. Over geological time, this isolation allowed countless marine organisms to evolve independently, resulting in exceptionally high levels of endemism—species found nowhere else on Earth. Southern Australia's marine flora is particularly distinctive, with more than 70% of its large seaweed species considered endemic, making it one of the world's most unique temperate marine floras (Spalding et al., 2007).

Life within the Temperate Australasia Marine Realm is profoundly influenced by the movement of the ocean itself. On Australia's east coast, the East Australian Current transports warm tropical waters southwards into temperate regions, while the Leeuwin Current performs a similarly unusual role along Western Australia's coastline by carrying warm Indian Ocean waters towards the Great Australian Bight. Together, these currents moderate coastal climates, transport larvae across vast distances and shape the distribution of countless marine species. They also contribute to some of the fastest rates of ocean warming observed anywhere in the Southern Hemisphere, creating both opportunities and challenges for marine life (CSIRO, 2024; Bureau of Meteorology, n.d.).

Unlike tropical reefs built by corals, much of the temperate coastline is structured by giant brown algae. Vast underwater forests formed by kelps such as Ecklonia radiata and the giant kelp Macrocystis pyrifera provide food, shelter and nursery habitat for thousands of marine organisms. These underwater forests perform ecological roles comparable to terrestrial rainforests by capturing carbon, supporting fisheries and protecting coastlines from wave energy. Their ecological importance has led many scientists to refer to Australia's southern coastline collectively as the Great Southern Reef, one of the planet's most significant yet least recognised marine biodiversity hotspots (DCCEEW; CSIRO).

Today, the Temperate Australasia Marine Realm supports globally important fisheries, internationally significant seabird breeding colonies, diverse marine mammal populations and some of the deepest and oldest coral communities on Earth. At the same time, climate change, marine heatwaves, invasive species and habitat degradation present unprecedented conservation challenges. Understanding this remarkable marine realm is therefore not only an exploration of biodiversity but also an opportunity to appreciate the ecological systems that sustain coastal communities, cultural heritage and ocean health throughout Australasia.

Quick Facts

Feature	Information
Marine Realm	Temperate Australasia
Global Classification	Marine Ecoregions of the World (MEOW)
Region	Southern Australia, Tasmania and New Zealand
Climate	Temperate oceanic
Dominant Habitats	Kelp forests, rocky reefs, seagrass meadows, sponge gardens, estuaries, deep-sea coral reefs
Major Ocean Currents	East Australian Current, Leeuwin Current
Biodiversity Highlight	Exceptionally high marine endemism, particularly seaweeds and reef fishes
Iconic Ecosystem	Great Southern Reef
Major Industries	Commercial fisheries, aquaculture, tourism, recreation
Key Threats	Climate change, marine heatwaves, invasive species, coastal development, ocean acidification

Where is the Temperate Australasia Marine Realm?

The Temperate Australasia Marine Realm occupies much of the southern coastline of Australia, extending from south-western Western Australia across South Australia, Victoria, Tasmania and New South Wales before encompassing the entire coastline of New Zealand. Offshore islands, continental shelves and submarine plateaus also form part of this diverse marine landscape.

Unlike terrestrial ecosystems, marine realms are defined by ecological and evolutionary relationships rather than visible geographical boundaries. Ocean temperature, current systems, continental shelf depth and species distributions collectively determine where one marine realm ends and another begins (Spalding et al., 2007).

To the north, tropical waters of the Indo-Pacific gradually replace temperate ecosystems, while to the south the Southern Ocean introduces colder Antarctic influences. This transitional position creates an extraordinary overlap between warm-water and cool-water species, making the region particularly dynamic and biologically rich.

Did You Know?

Australia's Great Southern Reef stretches for approximately 8,000 kilometres, making it one of the longest continuous temperate reef systems on Earth. Despite supporting thousands of species and contributing billions of dollars to Australia's economy, it remains far less well known than the Great Barrier Reef.

Why This Marine Realm Matters

Healthy oceans regulate Earth's climate, produce oxygen, absorb atmospheric carbon dioxide and support billions of people through food and livelihoods. Temperate Australasia plays a disproportionately important role in these global processes.

Its kelp forests and seagrass meadows are among the planet's most productive ecosystems, converting sunlight into biomass that fuels intricate food webs ranging from microscopic plankton to great white sharks and southern right whales. These habitats also function as important blue carbon ecosystems, storing carbon within marine sediments for centuries and helping to moderate global climate change.

Economically, the region supports valuable fisheries targeting species such as southern rock lobster, abalone, snapper, blue grenadier and hoki, while recreational fishing, diving and marine ecotourism contribute substantially to regional economies throughout Australia and New Zealand. Protecting these ecosystems therefore safeguards both biodiversity and the livelihoods of countless coastal communities.

Yet the future of these ecosystems is increasingly uncertain. Ocean warming around south-eastern Australia is occurring at roughly twice the global average in some regions due to changes in the East Australian Current. Marine heatwaves have already caused widespread losses of kelp forests and seagrass meadows, altering habitats that evolved over thousands of years (CSIRO, 2024).

Understanding the Temperate Australasia Marine Realm is therefore more than an exercise in natural history. It is an opportunity to recognise one of Earth's great centres of marine biodiversity and to appreciate the delicate balance between ocean currents, climate, evolution and conservation that continues to shape life beneath the waves.

The Geological Story of a Southern Ocean

Every landscape has a history, and beneath the waves of southern Australia and New Zealand lies a story that began hundreds of millions of years ago. The remarkable marine biodiversity of the Temperate Australasia Marine Realm did not appear by chance. Instead, it is the result of ancient continental movements, changing climates, evolving ocean currents and millions of years of biological isolation.

To understand why this marine realm supports such extraordinary life, we must first travel back to the age of the supercontinent Gondwana.

Around 180 million years ago, Gondwana began breaking apart. The landmasses that would become Africa, South America, Antarctica, Australia, India and New Zealand slowly drifted away from one another through the movement of Earth's tectonic plates (Geoscience Australia, 2023). Australia remained connected to Antarctica until approximately 45 million years ago, while New Zealand separated even earlier, around 80 million years ago (Te Ara – The Encyclopedia of New Zealand, n.d.).

These separations dramatically altered global ocean circulation. As Australia migrated northwards, a deep ocean passage opened between Australia and Antarctica. Eventually, this allowed the formation of the Antarctic Circumpolar Current (ACC)—the world's largest ocean current—which flows uninterrupted around Antarctica and fundamentally influences global climate (National Oceanic and Atmospheric Administration [NOAA], n.d.).

The establishment of the ACC isolated Antarctica from warmer tropical waters, allowing vast ice sheets to develop while Australia's climate became progressively drier. At the same time, the southern coastline remained influenced by cool, nutrient-rich marine waters, creating ideal conditions for the evolution of temperate marine ecosystems.

Unlike many marine regions that have experienced repeated glaciation or large-scale habitat loss, much of southern Australia's rocky coastline has remained relatively stable over millions of years. This environmental stability has provided countless opportunities for species to evolve into specialised ecological niches, resulting in one of the highest rates of marine endemism on the planet (Spalding et al., 2007).

Evolution in Isolation

Australia is often celebrated for its unique terrestrial wildlife, including kangaroos, platypuses and eucalyptus forests. Less widely appreciated is that similar evolutionary processes occurred beneath the ocean's surface.

Marine organisms with limited dispersal abilities, such as seaweeds, molluscs, sea stars and many reef fishes, became isolated along southern coastlines for millions of years. Without regular genetic exchange with distant populations, these species gradually evolved into entirely new forms.

Today, southern Australia is recognised as one of the world's leading centres of marine endemism.

Scientists estimate that approximately:

• more than 85% of temperate reef fishes are found nowhere else on Earth

• around 70% of large seaweed species are endemic

• numerous molluscs, echinoderms and crustaceans also occur only within this marine realm (Bennett et al., 2016; Spalding et al., 2007).

New Zealand tells a similar story. Surrounded by deep ocean and isolated for tens of millions of years, its coastal ecosystems have developed distinctive fish, marine invertebrates, seabirds and deep-sea coral communities found nowhere else.

This exceptional biodiversity makes the Temperate Australasia Marine Realm one of Earth's most important natural laboratories for studying evolution.

Did You Know?

Many marine species living along southern Australia are older evolutionary lineages than their tropical relatives. Some groups have survived relatively unchanged for tens of millions of years because the region escaped many of the environmental upheavals experienced elsewhere.

The Ocean is Always Moving

Unlike forests or deserts, marine ecosystems are shaped by water that is constantly moving.

Ocean currents transport heat, nutrients, oxygen, plankton and even the larvae of countless marine animals across enormous distances. They influence where species can survive, where fisheries thrive and how ecosystems respond to climate change.

The Temperate Australasia Marine Realm is influenced by several major ocean currents, but two dominate its coastal ecology:

• the East Australian Current

• the Leeuwin Current

  
  

Together, these currents make southern Australia unlike any other temperate coastline on Earth.

The East Australian Current

The East Australian Current (EAC) is one of the strongest western boundary currents in the Southern Hemisphere.

Originating in the Coral Sea, the EAC carries warm tropical water southwards along Australia's eastern coastline before turning east into the Tasman Sea (CSIRO, 2024).

While many Australians recognise the EAC from the animated film Finding Nemo, its real ecological significance is far greater.

The current:

• transports tropical fish larvae hundreds of kilometres south

• influences coastal temperatures

• affects rainfall patterns

• supplies nutrients to marine ecosystems

• connects coral reef species with temperate habitats.

In recent decades, the EAC has strengthened and extended further south due to climate change. As a result, many tropical species are expanding their ranges into temperate waters, while some cold-adapted species are retreating towards Tasmania (CSIRO, 2024).

This phenomenon, known as tropicalisation, is transforming southern reef ecosystems.

Species Spotlight

The Eastern Rock Lobster (Sagmariasus verreauxi)

The Eastern Rock Lobster depends on the East Australian Current during its early life stages.

After hatching, microscopic larvae drift within ocean currents for many months before eventually settling onto rocky reefs. Small changes in ocean circulation can therefore influence future lobster populations and commercial fisheries.

The Leeuwin Current

Running down Australia's west coast is one of the world's most unusual ocean currents.

Most eastern ocean boundaries transport cool water from polar regions towards the equator. The Leeuwin Current does the opposite.

Flowing from tropical waters near north-western Australia, it carries relatively warm, low-salinity water southwards before turning east along the southern coastline (Commonwealth of Australia, 2023).

Because of this warm current, coral species occur much further south than scientists would normally expect, while temperate ecosystems receive a steady supply of warm water throughout the year.

The Leeuwin Current also contributes to the extraordinary biodiversity of the Great Southern Reef, supporting habitats that might otherwise be far less diverse.

However, the current is highly variable. During years when it weakens or strengthens, marine ecosystems can experience significant ecological changes.

Did You Know?

The Leeuwin Current is considered one of the world's few major poleward-flowing eastern boundary currents, making it unusual among global ocean circulation systems.

The Great Southern Reef

Although many people have heard of the Great Barrier Reef, comparatively few know about Australia's other great marine ecosystem. Stretching approximately 8,000 kilometres from Kalbarri in Western Australia around the southern coastline to northern New South Wales, the Great Southern Reef is the world's largest interconnected temperate rocky reef system (Bennett et al., 2016).

Rather than being built by corals, the reef is dominated by forests of large brown algae, especially kelp.

These underwater forests create complex three-dimensional habitats where thousands of marine species feed, breed and shelter.

Scientists estimate that the Great Southern Reef supports:

• more than 5,000 known marine species

• hundreds of endemic fishes

• thousands of marine invertebrates

• globally significant fisheries

• millions of recreational visitors each year (Bennett et al., 2016).

Despite its ecological importance, many Australians remain unaware of its existence.

Ocean Productivity

One reason temperate marine ecosystems support abundant life is their high productivity.

Unlike the clear, nutrient-poor waters surrounding many tropical coral reefs, temperate waters frequently experience:

• seasonal nutrient mixing

• winter storms

• coastal upwelling

• strong tidal currents.

These processes bring nutrients from deeper water into the sunlit surface layer.

Microscopic algae known as phytoplankton rapidly convert these nutrients into organic matter through photosynthesis.

Phytoplankton form the foundation of nearly every marine food web.

Tiny zooplankton consume the phytoplankton.

Small fishes feed on zooplankton.

Larger predators consume those fishes.

Eventually, energy reaches sharks, seals, dolphins, seabirds and whales.

This efficient transfer of energy explains why temperate oceans often support productive commercial fisheries.

Seasonal Rhythms

Unlike tropical oceans where temperatures remain relatively stable throughout the year, temperate seas experience pronounced seasonal changes.

During winter:

• stronger storms mix ocean layers

• nutrients become widely distributed

• many kelps experience rapid growth.

In spring:

• phytoplankton blooms occur

• zooplankton populations expand

• fish spawning increases

• seabirds begin nesting.

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Summer brings warmer surface waters, greater biological activity and increased tourism, while autumn marks another period of nutrient replenishment before winter mixing resumes.

These annual cycles influence almost every aspect of marine life, from whale migrations to the flowering of seagrasses and the breeding seasons of seabirds.

Climate Change and Changing Currents

The same ocean currents that have sustained marine biodiversity for millions of years are now changing.

According to Australia's State of the Climate report, south-eastern Australia is among the fastest-warming marine regions on Earth (CSIRO & Bureau of Meteorology, 2024).

Warmer waters are already causing:

• more frequent marine heatwaves

• shifts in fish distributions

• declining kelp forests

• increased disease outbreaks

• altered breeding seasons

• expansion of tropical species into temperate ecosystems.

These changes affect not only wildlife but also fisheries, tourism, coastal communities and First Nations peoples whose cultures have long been connected to the sea.

Understanding these oceanographic processes is therefore essential for predicting how the Temperate Australasia Marine Realm may change during the coming century.

Looking Ahead

The geology of Gondwana, the isolation of Australia and New Zealand, and the powerful currents that flow around their coastlines have together created one of the world's richest temperate marine ecosystems.

Yet the ocean is far more than moving water. Beneath the surface lies an intricate living landscape built by giant kelps, seagrasses, sponges, corals and microscopic algae. These habitat-forming organisms create the underwater architecture upon which countless marine species depend.

In the next chapter, we explore these remarkable ecosystems in detail—from towering kelp forests and blue carbon seagrass meadows to deep-sea coral gardens and hidden sponge habitats that support one of the Southern Hemisphere's most diverse collections of marine life.

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References (APA 7)

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Bureau of Meteorology. (n.d.). Ocean currents. https://www.bom.gov.au/resources/learn-and-explore/marine-knowledge-centre/ocean-currents

Commonwealth of Australia. (2023). Australia's State of the Environment 2021: Marine and coastal. Department of Climate

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CSIRO. (2024). State of the Climate 2024. Commonwealth Scientific and Industrial Research Organisation & Bureau of Meteorology. https://www.csiro.au/en/research/environmental-impacts/climate-change/state-of-the-climate

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Spalding, M. D., Fox, H. E., Allen, G. R., Davidson, N., Ferdaña, Z. A., Finlayson, M., Halpern, B. S., Jorge, M. A.,

Lombana, A., Lourie, S. A., Martin, K. D., McManus, E., Molnar, J., Recchia, C. A., & Robertson, J. (2007). Marine ecoregions of the world: A bioregionalization of coastal and shelf areas. BioScience, 57(7), 573–583. https://doi.org/10.1641/B570707

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