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Coastal shoreline

by  Maggy Wassilieff

Along New Zealand’s coast, with its exposed cliffs, windswept dunes and tidal rock pools, a highly specialised group of plants and animals have made their home. Some – such as the brilliant red pōhutukawa tree, or wheeling gulls – are easy to spot. Others, from tiny periwinkles to scuttling crabs and the native black beetle, can be discovered as you explore the environment at the water’s edge.


Cliffs and beaches

Whether it is the gentle wash of sand-bearing waters against estuarine plants, or storm waves attacking crumbling cliffs, constructive and destructive forces are constantly at play on the coast. Some parts of the coast are being eroded by waves. Others are growing as the sea deposits sediment on beaches or in subtidal bars. It is convenient to divide the coast into two sections, based on these different processes:

  • rocky coasts that are eroding
  • beaches developing from deposited material.

Rocky coasts

Much of the New Zealand coastline is steep cliffs. Some, composed of soft mudstones and jointed sandstones, are eroding rapidly. Dramatic examples are to be found on the east coast of the North Island. As the waves cut away at the cliff face, it retreats landward, leaving wide tidal platforms and rockfalls.

In contrast, the resistant granite and gneiss rocks of Fiordland in the deep south have been little affected by the regular pounding of rough seas. Steep cliffs plunge directly into the sea, with no beaches or only narrow shore platforms.

Disappearing coast

Sea levels in New Zealand have risen on average 1.6 millimetres each year since the start of the 20th century. This amounts to a strip of land about 15 centimetres wide being lost from around the coastline since 1900. Scientists predict the rise in sea level will be two or three times greater this century as global warming increases.

Beaches

About a third of New Zealand’s coastline consists of beaches. Sandy beaches are well represented on the North Island’s west and north-east coasts, at the top and bottom of the South Island, and on Stewart Island. Mixed sand and gravel beaches make up nearly a quarter of the coastline and are typical of the Canterbury and Hawke’s Bay coasts.

The composition of the beach reflects the energy of the waves depositing the material. Mixed shingle and gravel beaches form along exposed coasts where waves have enough energy to throw up sizeable stones. Fine silty muds accumulate in sheltered bays with gentle waves. Occasionally, storm waves may deliver large stones and boulders high up the beach.

How beaches form

The sediment that builds up beaches comes from three sources:

  • Eroding coastal cliffs. For example, the Nelson Boulder Bank has formed from rocks falling from Mackays Bluff, some 13.5 kilometres from the bank’s southern tip. Powerful currents transport eroded rock material southwards.
  • Rivers. Glaciers and rivers carry huge amounts of sediment to the coast. Some is carried offshore and deposited beyond the reach of breaking waves, and some is swept up in coastal currents and transported onto beaches. The latter process has produced the distinctive black-sand beaches on the west coasts of both main islands. Rivers flowing from the volcanic rocks of Mt Taranaki (Mt Egmont) and the central plateau volcanoes supply black ironsand (titanomagnetite) onto the west coast beaches of the North Island. The South Island’s black sand is mainly ilmenite, carried down rivers from schist rocks in the Southern Alps.
  • Offshore sediments. During the last glacial period, about 20,000 years ago, sea level was 120 metres below the present-day level, and more of the continental shelf around New Zealand was exposed. Rivers, glaciers and wind carried eroded debris from the hills and mountains onto the shelf. These ancient deposits have been swept ashore for thousands of years by prevailing currents, creating sandy beaches and barriers. Farewell Spit, a 24-kilometre sandbar at the north-western tip of the South Island, is a prime example.

Uplifted beaches

Large earthquakes have shaped parts of the coast in a dramatic fashion. A series of uplifted beaches is preserved at Turakirae Head on the southern coast of the North Island. The youngest was raised 6.4 metres in January 1855.


The rocky coast – upper shore

Vertical zones

There are recognisable vertical patterns or zones of life at the land’s edge. Plants and animals are specially adapted to conditions at different heights on the shore – from the cliffs down to the low-tide zone.

Cliff vegetation

Exposed coastal cliffs with their nooks and ledges support an open vegetation of grasses, hardy ferns, coastal flax (Phormium cookianum) and sprawling succulents such as horokaka, also known as Māori ice plant (Disphyma australe).

Gentler coastal slopes support lowland coastal forest and scrub. If they are cleared of original cover, pasture grasses and invading shrubs like gorse and mānuka will grow.

Pōhutukawa (Metrosideros excelsa), the distinctive red-flowered coastal tree of northern New Zealand, anchors itself to cliffs with sturdy roots, sending out a number of stems from its base. It is one of the few trees able to colonise bare lava, and forms the dominant cover on volcanic Rangitoto Island. Tolerant of salt winds, fast growing, and spectacular in summer, it is a popular ornamental tree, grown far beyond its natural southern limits in Poverty Bay and Taranaki.

Lichen zone

Just above the splash zone – the area constantly exposed to salt spray – flowering plants and ferns fail to thrive. Colourful lichens such as bright yellow-orange Xanthoria, white encrusting species of Pertusaria, and grey-green Ramalina, dominate this area. Lichens are composite organisms, consisting of a fungus in a partnership with algae or bacteria. Beneath the colourful lichens lies a distinctive black band of Verrucaria lichen.

Periwinkles galore

The hardiest maritime animal is the banded periwinkle (Austrolittorina antipodum), a tiny snail with a blue and white conical shell that can withstand days without water. Thousands congregate just above the high-tide zone, where they graze on thin films of seaweed and Verrucaria lichen.

Barnacle band

Barnacles occupy the next zone down, around the high-tide level, where they receive a twice-daily dowsing of sea water. Although having the outward appearance of shellfish, barnacles are filter-feeding crustaceans, relatives of crabs and crayfish.

Three shore species are commonly encountered. Column barnacles (Chamaesipho columna) are small, about the size of a rice grain, and occupy the largest section of the barnacle zone. The larger brown barnacle (Chamaesipho brunnea) can be found on very exposed northern shores. The ridged barnacle (Elminius plicatus) is the largest of the common barnacles, about the diameter of a 5-cent piece.

Home invasion

Ornate limpets (Cellana ornata) usually return to the same spot on the rocks after their nightly feed. A marked limpet was observed moving onto its neighbour’s spot. When the resident limpet returned it pushed against the intruder for 20 minutes, then settled some distance away. During the next high tide the offending limpet moved off to feed and the ousted limpet returned directly to its home site, staying put without feeding for the tidal cycle. The intruder returned to its own spot after feeding.

Grazing shellfish

The middle shore is the domain of grazing shellfish (molluscs). When the tide is high during the night, molluscs such as spotted top shell (Melagraphia aethiops), pūpū or cat’s eyes (Turbo smaragdus), limpets (Cellana species) and snakeskin chitons (Sypharochiton pelliserpentis) feed on the algae clothing the rocks. The common cat’s eyes have a dark green ‘trapdoor’ that closes when the animal withdraws into its shell.

When the tide is out during the day, young molluscs shelter in crevices or under rocks, but larger limpets and chitons will withstand full exposure to the elements by clinging to the rock surface with their muscular foot.


The rocky coast – lower shore

Animals and seaweeds become much more diverse at the low and sub-tidal zone. Compared to the upper shore, this habitat is more protected and predictable: environmental fluctuations are small, organisms are not in danger of drying out, and there is a constant supply of dissolved nutrients to support a rich array of red and brown seaweeds.

Pink paint and coral turf

Like the black lichen above them, some red seaweeds resemble a grand-scale paint job. Corallina officinalis and related seaweeds begin their life as flat sheets that extract calcium from the sea and deposit it as lime within their cell walls, forming a pink crust on the rocks, known as pink paint. Later they develop into a tufted form that resembles coral fans.

Coralline seaweeds extend from mid-tide level to well below low tide, where they provide an important habitat for sponges, shellfish such as young cat’s eyes, Cook’s turbans (Cookia sulcata), pāua (Haliotis species) and kina (Evechinus chloroticus), a sea urchin or sea egg. They are also known to produce chemicals that influence the development of the larval stages of many marine animals.

Brown seaweeds

Large brown seaweeds dominate the low-tide zone. Neptune’s necklace (Hormosira banksii) often accompanies Corallina in tidal rock pools throughout New Zealand, but does not extend into the subtidal zone. A few other large brown seaweeds form extensive underwater forests that support an extraordinary diversity of animals and smaller seaweeds. Common kelp (Ecklonia radiata) and flapjack (Carpophyllum maschalocarpum) are the key species of northern coasts. Bull kelp (Durvillaea antarctica) and bladder kelp (Macrocystis pyrifera) are the dominant southern browns.

By the armload

The reef star (Stichaster australis), a starfish about the size of a man’s hand and with 10–12 orange arms, lives on the west coast mussel beds, tearing mussels off the rock before prising them open to eat.

Mussels and oysters

Mussels and oysters are filter-feeding shellfish that cement themselves to tidal rocks. They extract microscopic particles of food from the water that passes over their gills. Beds of edible mussels often develop on exposed coasts between the mid-tide and low-tide zones. Blue mussels (Mytilus galloprovincialis) favour colder southern waters and green-lipped mussels (Perna canaliculus) are the common northern species. These two species grow to over 10 centimetres and should not be confused with the little blue-black mussel (Xenostrobus pulex), which is a quarter their size.

Pacific oysters (Crassostrea gigas) and rock oysters (Saccostrea cucullata) are also found at mid-tide levels on northern coasts. Pacific oysters arrived in New Zealand around 1950, quickly spreading around northern coasts and reaching the top of the South Island in 1977. They are twice the size of the native rock oyster and grow rapidly, spawning several times a year. They were an attractive proposition for oyster farmers who now cultivate this species rather than the native rock oyster.


Rock pools and boulder shores

Rock pools and surge channels are good places to observe life in the low-tide zone. At first glance, animals may be difficult to detect, for many are camouflaged, or hide under rocks and in crevices. The most mobile are small fish, of which there are at least 40 species. Cockabullies or common triplefins (Forsterygion lapillum) swim about in search of any small animal to eat. The nearly transparent shrimp (Palaemon affinis) is also a fast mover when it detects a scrap of food.

Found crawling about the sides and bottom of the pool are whelks (Cominella species), common predators of other shellfish. A whelk shell has a distinctive spindle shape, but appearances can be deceptive. Empty shells are a favoured home for kāunga, the hermit crab (Pagarus novaezelandiae), and a rapidly moving whelk shell signifies crab ownership. Other crabs commonly found in this habitat include the purple shore crab (Leptograpsus variegatus) and the smaller rock crab (Hemigrapsus edwardsii).

Encrusting or attached animals live in rock pools and the lower tidal zone. Red sea anemones (Isactinia tenebrosa) survive out of water as a blob of jelly. But once the tide is in they become killers, eating any small animal that passes within reach of their deadly stinging tentacles. Filter-feeding worms live more peaceably in lime tubes attached to rocks or seaweed fronds. They trap their food in a crown of fine filaments.

Two species that are now uncommon in accessible rock pools and reefs are the edible abalone pāua (Haliotis australis) and kina, the sea urchin (Evechinus chloroticus). Both have suffered from over-harvesting and poaching.

Bright eyes

A traditional use of pāua shell was as inlay for eyes in Māori carvings – the iridescent colours have a life-like quality. The pāua eyes are called mata-a-ruru (eyes of the owl) and signify that the carved being is all-seeing and all-wise.

Stony beaches

Shores of gravel and small stone are the harshest areas of the coast. Few organisms survive the constant grinding action of stone upon stone as waves surge up the beach. Water drains rapidly from coarse sediments and the tops of stones become too hot and dry for most plants and animals.

Several native lizards favour the upper level of stony beaches for basking, and hunt through tidal drift for small insects and crustaceans. They slither under stones or into crevices if disturbed.

Around mid-shore where the tides reach each day, crabs, snails and limpets occupy shaded undersides of large boulders and feed on encrusting seaweeds and algal slimes growing on the sides and bottoms of the rocks. Beaches composed of large and less mobile stones and boulders support a similar assemblage of animals to that of the rocky shore.


Sand dunes

Dune formation

Dunefields are formed above the beach as dry sand blows inland. Dunes occupy about 1,100 km of the New Zealand coastline. An impressive dunefield stretches 130 kilometres from Paekākāriki to Pātea on the west coast of the North Island, and up to 19 kilometres inland. Wide, gentle-sloping beaches are framed by fore dunes, formed when wind-blown beach sands become trapped by plants and driftwood on the beach. If there is a ready supply of sands on the beach, more dunes may develop in front of the original. The dunes further back may be stabilised by plants, or blown inland if they fail to retain a good plant cover.

Plants of the fore dune

Beyond the high-water mark three plants can colonise the damp sands – pīngao (Desmoschoenus spiralis) and spinifex (Spinifex sericeus), two native sand-binding plants, and the introduced marram grass (Ammophila arenaria). The shape of the dune is influenced by the vegetation: spinifex and pīngao produce dunes with a low, regular profile; clumps of marram produce higher, irregular dunes.

Marram was widely planted on New Zealand dunes in the early 1900s, in an effort to stabilise dunes that were moving inland. It is now the dominant coloniser, often backed by stabilised dunes planted with introduced lupins and pine plantations.

Dune animals

New Zealand’s venomous native spider – the katipō – is a sand-dune specialist. Two species are now recognised: red katipō (Latrodectus katipo), with a red-orange stripe on its back, and black katipō (Latrodectus atritus), a wholly black spider. They inhabit the landward side of fore dunes, where they spread webs in pīngao and marram, or under driftwood. The female can deliver a nasty bite, but by nature is a retiring animal and only attacks if frightened. They are in serious decline throughout New Zealand as dunes become covered by dense marram, lupin, pines or pasture grasses – the spider prefers more open clusters. A similar-looking South African spider (Steatoda capensis), which can inflict a painful bite, has moved into coastal sites and may be competing for resources.

The harmless shiny black sand scarab (Pericoptus truncatus) is one of New Zealand’s largest beetles – 4 centimetres long. Its larvae make numerous little tracks that lead to holes in the dunes.

Middens

Middens – ancient refuse heaps – are scattered along the dunes of the North Island and much of the South Island. They date from pre-European times, and reveal a wealth of environmental and cultural information about coastal conditions 200 to 700 years ago. The oldest shell middens contain bones of forest birds and fur seals. But within 200 years of human settlement, fur seals had disappeared from northern sites and fewer forest birds were eaten at coastal sites.

Advancing dunes

About 450 years ago coastal gardening by early Māori failed in the South Island and southern North Island. This coincided with an advance of coastal dunes inland. One trigger was probably Māori clearance of coastal forest, which removed shelter and exposed the soil to wind erosion. Climate cooling may also have contributed to crop failure.


Sandy beaches – lower shore

Few animals are adapted to life on the surface of beaches and sand dunes, but some shellfish and crustaceans burrow under the sand.

Shellfish

Four native shellfish have fed generations of New Zealanders.

Toheroa (Paphies ventricosa) and two species of tuatua (Paphies donacina, Paphies subtriangulata) favour open exposed beaches regularly pounded by heavy waves. Adult toheroa live just below the high-tide level at depths of 20–30 centimetres. Tuatua prefer deeper water, living at depths of 5–10 centimetres near the low-tide level. They seldom dwell together on the same beaches.

Pipi (Paphies australis) burrow in sandy banks of estuaries and harbour mouths, where there is some fresh water.

Toheroa were once found in their millions on the heavy surf beaches of the North Island’s west coast, and on a couple of Southland beaches. They were over-harvested in the 20th century and since the 1960s have been protected from commercial gathering.

Crustaceans

Crabs, sandhoppers, slaters, lice and shrimps are all found on New Zealand beaches. Swimmers may be familiar with the nasty nip of the paddle crab (pāpaka, Ovalipes catharus). This swimming crab lives on sandy sea beds and is commercially harvested. Its paddle-shaped hind legs are used for digging backwards into the sand as well as for swimming. It forages by night and is a significant predator of tuatua and other shellfish. Cannibalism is quite common and the young are especially vulnerable when moulting – they shed their old shell-like covering as they grow.

Paddle crab romance

The paddle crab’s love-life is remarkable. A male needs to mate with a recently moulted female and will grab a pre-moult female and carry her around underneath him for a number of days until she moults. He will then mate with her, generally for 12–36 hours (but up to 4 days), before he releases her.

Less familiar are the mantis shrimp (Squilla armata) and ghost shrimp (Callianassa filholia). They dig tunnels in tidal sands and leave little mounds of sand and faeces at the entrance.

Kick over some dried seaweed and thousands of little sandhoppers (Talorchestia quoyana) will leap out. These creatures are decomposers – garbage eaters – and at night, away from the blazing sun and predatory gulls, they munch their way through dead bodies and seaweed cast up on the beach. During the day, if there is no food to shelter under, they burrow into the sand.

Also performing clean-up duties is the sea slater (Scyphax ornatus). It looks like the common slater, and makes burrows on the upper beach, moving down to the tidal zone at night to scavenge for food.

The drift line

One of the pleasures of the beach is checking out the flotsam along the shore. Driftwood – smooth bleached remains of forest trees – dominates river mouth beaches on the west coast, and provides habitat for small animals. Pumice, a floating volcanic rock, is common on North Island beaches. It is brought down by rivers carrying deposits from the Taupō eruption of 232 AD.

After heavy storms it is possible to find shell remains of deep-water shellfish and Janthina, the floating violet snail. Crab shells and claws from moulting animals are also common wash-ups.

Much of the drift material is human-produced rubbish. Plastic products such as polypropylene ropes and strapping bands, containers, fishing line and packing materials comprise about 70% of New Zealand’s beach litter. Rubbish that has been afloat for some time usually carries barnacles, seaweeds or shellfish.


Shore birds

Most shore birds gather in summer around the large harbours and estuaries of northern New Zealand. Mudflats and soft sands are exposed at low tide and provide rich feeding grounds for long-legged wading birds such as godwits, oystercatchers, stilts and herons. A few are year-round residents, but many are migratory visitors – from the Arctic or up from the South Island.

There is little bird life on exposed surf and gravel beaches, apart from the ubiquitous gulls, ever on the scrounge. Rocky coasts are also poor bird-spotting areas, although red-billed gulls will nest on rocky headlands and gannets colonise the clifftops.

Breeding on the coast

Shore birds breed in spring or early summer and nest on the ground above the high-tide mark. For many wading birds and terns, a nest is simply a shallow scrape in the ground. Gulls arrange sticks, grass and seaweed into a substantial construction. Shore-bird eggs and hatchlings are well camouflaged – brown, grey or sandy yellow, splotched with darker markings.

Before humans and their camp-follower mammals arrived on New Zealand shores, coastal birds faced relatively few threats. Unseasonal storm waves could sweep away nests, sands could bury eggs and chicks, predatory black-backed gulls might consume nestlings, but overall, shore birds thrived.

Since the late 19th century some shore-bird populations have seriously declined. By 2004:

  • shore plovers had disappeared from mainland New Zealand and were found naturally only on the Chatham Islands.
  • New Zealand fairy terns were reduced to 36, and bred only at three sites in Northland.
  • New Zealand dotterels were down to 1,500 birds; 200 or so formed a Stewart Island population, the rest were confined to northern coasts.

Present-day threats

Stoats, rats, cats and hedgehogs are a major threat to nesting shore birds because they sniff out defenceless chicks and eat eggs. Until 1940 it was legal to shoot shore birds; Europeans took thousands for sport and food. In summer hordes of holidaymakers descend on the coast, frightening adult birds off their nests. Off-road vehicles hurtle along the high-tide line, crushing camouflaged eggs and flightless chicks. Since the 1970s the coast has become prime real estate, and breeding habitats near beachfront subdivisions have all but disappeared.


External links and sources

More suggestions and sources


How to cite this page: Maggy Wassilieff, 'Coastal shoreline', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/coastal-shoreline/print (accessed 15 October 2019)

Story by Maggy Wassilieff, published 12 Jun 2006