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.
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.