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Wetlands

by  Peter Johnson

There is a serenity to wetlands – a still lagoon, the lonely call of a bittern, rushes stretching to the horizon. They may be less dramatic than some New Zealand landscapes, but they are home to many land birds and rare species, and they are vital for sustaining healthy waterways. Six are now listed as having international importance.


What are wetlands?

Wetlands occur where there is poor drainage or where water accumulates.

Freshwater wetlands merge with lakes and rivers, and with brackish or saline wetlands near the coast.

The Resource Management Act 1991 defines wetland as ‘permanently or intermittently wet areas, shallow water, and land water margins that support a natural ecosystem of plants and animals that are adapted to wet conditions.’

Diverse types and sites

New Zealand’s high rainfall encourages wet soils, and because of its varied landforms there are many types of wetland. The nature of the wetland depends on factors such as substrate (the surface on which it lies, such as silt or peat), the degree of wetness, and the fertility (or nutrient status) of the soil or water. Examples include bogs, swamps, fens and marshes. Vegetation ranges from cushion plants and turf to rushlands and forests.

It is not always easy to classify a wetland – typically one type will merge into another. Often there is a mixture of swamp and pools, with patches of bog and marsh.

A wetland site usually has a close relationship with the adjacent or surrounding dry land. The boundary is not always easy to define, because wetlands visible on the land surface are linked to an unseen amount of groundwater within a larger water catchment.

Some wetlands dry out in summer. Others change over time as they fill with sediment or become eroded.

Life in wetlands

All forms of life need water, but wetland plants are adapted to cope with an oversupply of wetness and its consequences – such as nutrient shortages and the need for an oxygen supply to underwater parts.

Each plant, insect, bird, and fish of the wetlands lives in the places that match its needs, tolerances and ability to compete with other species. For instance, raupō (bulrush) prefers fertile lowland swamps, while the sundews, which gain nitrogen not from the soil but by trapping insects, live in less fertile sites such as bogs.

Māori use

The vast mosaics of lowland swamps and wet forests were important for early Māori. These places contained plentiful food – fish, birds, pollen and roots – and plants such as flax for weaving and thatching, and moss for bedding. The waterways within and between the wetlands provided canoe routes to these resources.

Loss of wetlands

Before human settlement (from around 1250–1300 AD), freshwater wetlands covered about 670,000 hectares of New Zealand. By the 21st century this had been reduced to 89,000 hectares – an estimated loss of 87%. Fertile lowland swamps have been lost in greater numbers than those that were infertile or at high altitudes.

There is regional variation: Southland retains some 35% of its original wetlands, whereas the Bay of Plenty has less than 1%.

Ecological and social significance

Wetlands are ecologically important because:

  • they contain a large proportion of New Zealand’s native plants, including rare and endangered species
  • they provide habitat for native fish and invertebrates
  • They are home to a quarter of New Zealand’s land birds, and provide vital feeding and resting areas for the many migrating birds that visit each year.
  • they help maintain water flows, lessening the impact of floods, and maintain water quality by acting as sediment traps
  • their plants also improve water quality by adding oxygen and taking up dissolved nutrients.

For people, they provide:

  • recreational and educational opportunities – nature study, fishing, hunting, birdwatching
  • income for those harvesting sustainable sphagnum peat and eels
  • essential materials (flax, rushes) for Māori crafts.

New Zealand’s Wetlands of International Importance

Based on their ecology, biodiversity and hydrology, wetland sites are designated as Wetlands of International Importance under the Ramsar Convention on Wetlands. In 2007 New Zealand had six such sites, covering almost 40,200 hectares:

  • Waituna Lagoon, Southland
  • Farewell Spit, Nelson
  • Whangamarino, Waikato
  • Kopuatai Peat Dome, Waikato
  • Firth of Thames, Waikato
  • Manawatū River estuary.

Another 67 New Zealand sites qualify for this status.


Types of wetland

Swamps

A swamp receives a relatively rich supply of nutrients, and often sediment, through surface runoff and groundwater from nearby land. Swamps usually lie on a mixture of mineral and peat substrates. Channels of water are common, and the water table is often above the ground in places.

Swamps usually occur in basins, and on valley floors, deltas and plains. Common plants are sedge, rush, reed, flax, tall herbs or scrub, often intermingled, and also forest.

The largest example is the Whangamarino swamp – some 7,000 hectares, alongside the Waikato River.

Bogs

A bog receives its water supply only from precipitation (rain, mist, snow). It does not have groundwater or any nutrients from adjacent or underlying mineral soils. Bogs are therefore very low in nutrients. They lie on peat, which is acid, and they are poorly aerated and drained. The water table is generally close to or just above the ground surface.

Bogs mostly occur on level ground or gentle slopes. Their varied vegetation includes mosses, lichens, cushion plants, sedges, grasses, rushes, ferns, shrubs or trees.

A fine example is New Zealand’s largest existing wetland – the 8,800-hectare Kopuatai Peat Dome on the Hauraki Plains. Another example is the peat bogs of the southern tablelands on Chatham Island.

Peat

Peat is made of the partly decomposed remains of plants and animals. It forms in wet environments such as bogs, fens and marshes – acidic, oxygen-poor sites that prevent full decay. Gardeners dig it into soil to increase moisture retention.

Fens

A fen is a wetland classed somewhere between bog and swamp. Fens are mainly peaty, but because they receive groundwater and nutrients from adjacent mineral soils, they have moderate fertility and low to moderate acidity. The water table is usually close to or just below the surface, and relatively constant.

Fens often lie on slight slopes. Plants that favour them are sedges, ferns, tall herbs, scrub and tussock grasses. Red tussock fens are widespread in New Zealand’s high-country basins.

Marshes

A marsh has mainly mineral soil and reasonable drainage. It is fed by groundwater or surface water, and has a fluctuating water level. Marshes are often flooded by standing or slowly moving water. They have medium to high fertility, and are slightly acid to neutral.

Marshes occur mainly on gentle slopes, especially on valley margins and floors, and beside rivers and lakes. Vegetation is most often rushland, grassland, sedgeland or herbfield.

Ephemeral wetlands

Ephemeral wetlands are found in dips or hollows that lack a surface outlet, in areas with great variation in seasonal rainfall and evaporation. Ephemeral (temporary) ponds may appear in winter and spring, drying out completely in the summer months or in dry years. They form from groundwater, and the underlying soil is usually mineral, and pH neutral. Turf and sward grow on the margins, and sometimes there is rushland and scrub.

Ephemeral wetlands occur in a range of landscapes:

  • coastal sand dunes (Tangimoana, Manawatū coast, with rushland of oioi – Apodasmia similis)
  • hollows lined with volcanic tephra (Rangitīkei, central North Island, with turf and sedgeland)
  • kettle-hole depressions on old glacial moraine (Glenmore, Lake Tekapo, with zones of turf revealed when ponds dry in summer).

Pakihi and gumlands

Pakihi are flat or gently sloping areas with poor drainage. They have leached, mineral soils that lie on an impervious base. The best known are on the West Coast and the gumlands of Northland, but they also occur on very infertile surfaces in Nelson–Marlborough, Otago and Southland. Heath-like shrubs grow alongside sedges, rushes and ferns.


Wetland forests and shrubs

Forest communities

Swamp forests are dominated by kahikatea, New Zealand’s tallest tree. In wet conditions it produces long, spreading roots, and the trunk is usually buttressed for support.

Other common plants of the wetland forests include:

  • Pukatea and swamp maire. These trees often grow in association with kahikatea, in the North Island and the top of the South Island. Both produce pneumatophores (breathing roots) that rise above the ground.
  • Cabbage trees. Before human settlement, these grew at the edge of swamp forests and on river plains. They formed a narrow band between rush and sedgelands, growing near open water and forest on more elevated land. As people cleared the forests and lit fires, cabbage trees thrived – they are ‘pioneer’ plants, regenerating well in open conditions.
  • Silver pine and yellow-silver pine. These small native conifers tend to dominate forests of infertile bogs. They often grade into shrublands containing bog pine, pink pine, celery pine and mānuka.
  • Willow. Eleven species and hybrids of willow (non-natives) were introduced to stabilise river banks. They have spread into wetlands alongside rivers and lakes. Grey, pussy and crack willow have achieved weed status and infest fertile swamplands.

Shrub communities

Mānuka is a common shrub of wetlands. Its roots tolerate submersion for long periods.

Small-leaved divaricating shrubs are often associated with wetlands, especially on valley floors. Mingimingi (Coprosma propinqua) is particularly common, but a number of coprosma species may be present, along with Olearia virgata, swamp broom (Carmichaelia arborea), and swamp māhoe (Melicytus micranthus).

The heath family (Ericaceae) is well represented on peat bogs and infertile wetlands. Included here are snowberry (Gaultheria depressa), inaka (Dracophyllum longifolium, D. lessonianum and the Chatham Island D. scoparium), and Epacris pauciflora.


Reeds, rushes, sedges and low growers

The dominant plants of wetlands are monocotyledons. This big group of flowering plants includes grasses, reeds, rushes, sedges, New Zealand flax and cabbage trees. Typically they have long, narrow leaves with parallel veins, and flower parts in groups of three. Those that grow to the size of a tree do not form wood or bark.

Reeds

Raupō

Growing throughout New Zealand in fertile wetlands, raupō or bulrush (Typha orientalis) is a tall reed that forms dense leafy stands up to 3 metres high in still water. Raupō has thick, starchy rhizomes that expand through soft silt or form a mat in watery places. The tall, strap-like leaves are spongy, with air spaces that help aerate the underwater parts. All foliage dies down each winter. The flowers are held in a brown spike.

Māori baked cakes called pungapunga from the abundant pollen gathered from the cylindrical flower stem. Raupō seeds are borne on fluffy masses which are dispersed by the wind.

New Zealand flax

Like raupō, New Zealand flax (harakeke, Phormium tenax) also forms dense, leafy stands up to 3 metres tall in lowland swamps throughout New Zealand. Red, tubular flowers are held on tall (5–6-metre) spikes. Unlike raupō, it does not die down in winter months. Nor does it tolerate as much immersion, growing on slightly higher ground if the two occur together at a site.

Rushes

Rushes are typical wetland plants. Some are leafy and grass-like, while others, known to Māori as wīwī, are leafless and consist of clumps of stems.

New Zealand has 15 native and 32 introduced species of Juncus – the most common type of rush. They range from the 2-metre-tall J. pallidus of coastal swamps to tiny 2-centimetre J. pusillu s, found in wetland turfs.

The wire rush Empodisma minus and cane rush Sporodanthus ferrugineus belong to the Restoniaceae family. They are significant peat-formers. Wire rush consists of thin, many-branched stems. Its hairy roots form a dense carpet that acts like a sponge – it can hold up to 15 times its weight in water.

Most bogs that formed with cane rush have been drained for conversion to pasture. The species is now confined to a few sites in the Waikato region, although there are large bogs with the related rush S. traversii on the Chatham Islands.

Sedges

New Zealand has many sedges – 170 native and 43 introduced species. Common wetland species include various cutty grasses (Gahnia, Cyperus and Carex species), bastard or hook grasses (Uncinia), the comb sedge (Oreobolus pectinatus), and various spike sedges (Eleocharis).

One of the most widespread in swamps is Carex secta, a large tussock-like sedge that grows above 1–2-metre trunks.

Cushion fields

Cushion fields of low-growing shrubs and herbaceous plants form in alpine and subantarctic bogs. The dominant plants have such closely set branches and leaves that they form little mounds resembling cushions. Commonly seen are comb sedge, and species of Donatia, Phyllacne, Gaimardia and Centrolepis.

Turf communities

Turf communities are made up of plants that are prostrate (growing flat along the ground) and tiny, often less than 3 centimetres tall. They grow close together, forming a dense carpet at the margin of wetlands. These are typically species-rich communities – 416 plants (one-fifth of the native flowering species) have been found in wetland turfs. Of these, 62 are listed among New Zealand’s threatened or most uncommon plants.

Mosslands

At least nine species of sphagnum moss are known from New Zealand wetlands. Sphagnum cristatum is found in infertile swamps throughout the country, from sea level up to about 2,000 metres. It forms dense, spongy clumps made of thousands of leafy stems, about 15–20 centimetres long. The entire mass can absorb up to 25 times its weight in water. Other common wetland mosses include Drepanocladus species, Polytrichum commune and Campylopus introflexus.


Wetland wildlife

Many birds, fish and invertebrates depend on wetlands for their survival.

Birds

Wetlands usually contain a variety of habitats that provide food, shelter and breeding sites for many native birds.

Pūkeko (swamp hens), white-faced herons, little shags and some waterfowl (dabchicks, grey teal, paradise ducks, scaup and New Zealand shovelers) are fairly common. Banded rails, Australasian bitterns, spotless and marsh crakes and fernbirds are secretive, seldom seen as they hide among raupō and rushes.

Introduced game birds (black swans and mallards) are common on lakes and wetlands and are hunted in the winter months, between May and July.

Species in decline

Grey duck numbers have declined throughout the country as wetland habitats disappear. The once common brown teal is seriously endangered, with remnant populations surviving only in Fiordland (the far south) and Northland (the far north).

Fish

Longfin and shortfin eels are found throughout New Zealand’s wetlands. The shortfin eel is most common in lowland and coastal sites. Longfin eels travel further inland and may be found at higher altitude.

New Zealand’s five species of mudfish (Neochanna) are specialised for life in wetlands and pools that dry up in summer. During droughts they burrow into the mud, and survive curled up in the damp sediment. They remain there for weeks until the rains arrive and their pools fill with water again.

Common whitebait (Galaxias) species of wetlands include īnanga and the giant kōkopu. The rare gollum galaxias inhabits swamps and shallow lakes of Stewart Island and Southland, and in the Catlins and the Nevis River in Otago.

Perch are the most widespread introduced species. Tench and rudd are common in wetlands of the Waikato and northern North Island.

Frogs

The three introduced frog species are associated with lowland wetlands, in particular at their tadpole stage. Green and whistling frogs are common throughout the country. The green and golden bellfrog is restricted to the northern half of the North Island.

Insects

Insects abound in and around wetlands and provide food for the fish and birds there. In summer swarms of gnats are commonly seen flying above wetlands, and dragonflies and damsonflies sun themselves on rushes. Many species of moth – such as flax loopers, wirerush loopers, cabbage tree moths and wetland oranges – flit about day and night.

Snails

Freshwater snails are common, and include the ubiquitous dark snail (Potamopyrgus antipodarum), the left-handed water snail (Glytophysa variabilis) and Lymnaea tomentosa, the host for liver-fluke, which affects grazing stock.


Hononga, rauemi nō waho

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How to cite this page: Peter Johnson, 'Wetlands', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/mi/wetlands/print (accessed 18 June 2019)

Story by Peter Johnson, published 24 Sep 2007