Tasman Sea ecology

Category: Marine and Oceanic Biomes.

Geographic Location: South Pacific.

Summary: A biodiverse sea between Australia and New Zealand, the Tasman Sea is home to numerous unique marine species.

Nestled in the southwestern portion of the Pacific Ocean, the Tasman Sea is a saline body spanning the distance between the landmasses of Australia to the west and New Zealand to the east. The Tasman Sea is approximately 1,200 miles (1,931 kilometers) from west to east, and 1,700 miles (2,736 kilometers) north to south.

The Maori name for the Tasman Sea is Te Tai-o-Rehua, or the great sea of Rehua, the divine being who is the son of the sky god and earth goddess associated with immortality and various stars. The European name pays homage to Abel Tasman, the Dutch explorer who was the first European to discover New Zealand and the island of Tasmania, formerly called Van Diemen’s Land. Tasman’s voyage, in 1642-43, mapped substantial portions of the region; in the following century, British explorer Captain James Cook’s first voyage extensively navigated the Tasman Sea, producing a complete and substantially correct map of the New Zealand coastline.

Australians and New Zealanders commonly refer to the Tasman Sea as the Ditch, especially in the phrase crossing the Ditch, referring to passage across the sea in either direction.

Like other Australian waters, the Tasman Sea is generally nutrient-poor, and its ecosystem relies on processes such as upwelling to bring nutrients to the food chain. The mid-ocean ridge of the Tasman Sea was formed about 85 million years ago. Much later, Australia became drier, and Antarctica cooler when the island of Tasmania separated from Antarctica and changed the ocean currents around the major landmasses. As the rift valley sank, sediment accumulated, forming the Otway and Strzelecki ranges in southeastern Australia. The mid-ocean ridge is considerably closer to Australia than New Zealand, having formed essentially at the midway point between their continental margins.

Where the Otway ranges meet the Tasman Sea is Dinosaur Cove, a minor bay of seafront cliffs with fossil-bearing strata dating back over 1 million years. Many of the fossils show the kinds of dinosaurs that lived in the Tasman Sea region, including the taxa called the polar dinosaurs of Australia, which some paleontologists believe may have been warm-blooded.

Biodiversity

The seamounts of the Tasman Sea are extremely biodiverse, home to hundreds of species of fish, including the orange roughy, which can live for over a century. In 2004, the deep-sea research ship Tangaroa found more than 100 previously undiscovered species of fish during a four-week expedition of the Tasman Sea, as well as fossilized teeth of a megalodon, a prehistoric shark that was twice the size of the modern-day great white shark.

Seamounts, undersea mountains which are isolated from many disturbances, are good homes for coral reefs, which are both long-lived and slow-growing. One of the southernmost platform reefs in the world, Middleton Reef is home to the protected species Epinephelus daemeli, one of the fish known as black cod. Declines were noted in the early 1900s, and began to decline severely no later than the 1950s. The cod is vulnerable because of commercial interest in the fish and resulting overfishing, as well as its slow speed, large size, and territorial use of inshore habitats, making it an easy catch for spear fishers. Furthermore, some of the black cod populations in the Tasman Sea are believed to be non-breeding populations as a result of drifting larvae, slowing the replenishment of populations.

On the other side of a pass some 25 miles across is another platform coral reef, Elizabeth Reef. At low tide, most of the reef flat is above water; at high tide only one cay (Elizabeth Island) remains, just barely above sea level. Australia’s National Heritage Trust manages the Elizabeth and Middleton Reefs Marine National Park. In addition to black cod and a variety of other species, Elizabeth is home to a Galapagos shark (Carcharhinus galapagensis) nursery, large numbers of sea cucumbers (Holothuria whitmaei), and a recently growing number of crown-of-thorns starfish (Acanthaster planci).

The crown-of-thorns starfish is the second-largest starfish in the world, a solitary animal covered in venomous spines. Though it sometimes preys on brittle stars, the crown-of-thorns is principally a corallivore, feeding on the polyps of reef coral by climbing the reef, extruding its stomach onto the structure, and liquefying coral polyps in order to absorb nutrients from them. Its spines, which exude a strong neurotoxin, protect it from shrimp, mollusks, and large reef fish who attempt to prey on it.

The crown-of-thorns is best known as a threat to the coral reef ecosystem of the Great Barrier Reef elsewhere in Australian waters. A single crown-of-thorns consumes 65 square feet (6 square meters) of living coral reef surface each year, and the other factors that jeopardize a reef ecosystem exacerbate the effects of the crown-of-thorns’s predation. It may also promote the spread of coral diseases. So far, the crown-of-thorns is not seen in Elizabeth Reef in the same numbers as in the Great Barrier Reef, but the population has shown noticeable growth in the first decades of the twenty-first century.

Middleton and Elizabeth Reef are part of the same underwater plateau, the Lord Howe Rise, consisting of about 932,057 square miles (1.5 million square kilometers) of the Tasman Sea. The central-east area of the rise is known as the Lord Howe platform, and includes a seamount capped by Lord Howe Island and Ball’s Pyramid. Lord Howe Island is a crescent-shaped volcanic remnant about 6 miles (10 kilometers) long and 1 mile (1.6 kilometers) wide at its widest point, with a population of 348 in the 2006 census.

The island is a distinct terrestrial ecoregion and more than half of its plant species are endemic (found only here) to the island; many locals are involved in the Kentia palm industry, revolving around the endemic genus of Arecaceae Howea. The critically endangered creeping vine Calystegia affinis is native only to Lord Howe Island and nearby Norfolk Island. The island is also known for its banyans, pandanus trees, the spring-flowering bush orchid, and the glowing mushrooms (mycena chlorophanos and omphalotus nidiformis, both endemic to the island) that crop up in the palm forests after heavy rain.

Ball’s Pyramid is an eroded remnant of a shield volcano, consisting of an uninhabited volcanic stack. The most notable life on the Pyramid is a population of Lord Howe Island stick insects living among the growth of Melaleuca howeana shrubs that grow out of crevices in the rocks.

The central and southern Tasman Sea experiences a spring (September to December) bloom, with the maximum chlorophyll biomass occurring in early October. The surface of the sea includes a rich assortment of microscopic flora and fauna, including crustacea, pteropoda, heteropoda, radiolarians, dinoflagellates, foraminifers, polycaeta, chaeotgnatha, urochorda, vorticellids, and polychlads. Seabirds include the endangered kiwi species Apteryx haastii, A. rowi, A. owenii, and A. australis along the New Zealand coast, as well as the brown booby, brown tern, black-naped tern, bridled tern, crested tern, roseate tern, sooty tern, black noddy, red-tailed tropicbird, silver gull, wedge-tailed shearwater, and eastern reef egret.

Marine macrofauna include the humpback whale, the spinner dolphin, the dumbo octopus (which navigates with the help of two flaps, giving it the appearance of the Dumbo cartoon character), and the Pacific spookfish, which uses a long snout to detect the electrical impulses of hiding prey. Along the Fjordland coast, the Fjordland Crested Penguin nests.

Warming Threats

Climate change could affect the coral reefs with some intensity. As sea temperatures rise, the coral reefs of the Tasman Sea could be put at risk; those of Lord Howe Island are considered to be the most vulnerable. Furthermore, changes to ocean chemistry as the waters become acidified by absorption of increased greenhouse gas would hurt the shellfish of the Tasman Sea, making it more difficult for them to grow their shells, as well as the coral reefs.

Changes in water temperature have already been observed, as the East Australian Current reaches further south than before, bringing subtropical species into temperate waters. The long-spined sea urchin, previously found only off the shores of New South Wales, has recently been introduced to the western Tasman Sea. The European shore crab, introduced to Tasmania in the 1990s, has spread to the entire eastern coast, and other new species arrivals have impacted the ecosystem of Tasmania’s eastern coast.

Over the next 50 years, the temperature of the Tasman Sea is expected to keep rising incrementally. Phytoplankton are expected to push further south, and algal blooms may increase, while the phytoplankton populations of the temperate region will shrink. Temperature changes can also affect the sex of the embryos of turtles and other species, biasing the sex ratio toward females, which may lead to long-term fertility problems in some populations and overpopulation in others. A 2021 study published in the journal Nature Communications, also found that the warmer waters of the Tasman Sea are one of the primary drivers for the rising temperatures and ice melt on the Antarctic Peninsula.

Bibliography

Ayling, Tony and Geoffrey Cox. Collins Guide to the Sea Fishes of New Zealand. Auckland, New Zealand: William Collins Publishers, 1982.

International Hydrographic Organization. Limits of Oceans and Seas, Special Publication No. 23. Monte Carlo, Monaco: International Hydrographic Organization, 1953.

Longhurst, Alan R. Ecological Geography of the Sea. New York: Elsevier Science and Technology Books, 2006.

Sato, Kazutoshi, Jun Inoue, Ian Simmonds, and Irina Rudeva. “Antarctic Peninsula Warm Winters Influenced by Tasman Sea Temperatures.” Nature Communications, vol. 12, no. 1497, 2021, doi.org/10.1038/s41467-021-21773-5. Accessed 30 Aug. 2022.