Page 2 – Types of wetland
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.
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 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.
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.
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 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.