Key Ideas about Wetlands

24K image of saltwater marsh

Back in 1840, when the first colonists arrived from England, New Zealand was rich in wetlands as the country has an abundance of storms, earthquakes and ice, all of which contribute to wetland formation.

Over millions of years, mires and bogs formed as rivers overflowed their banks during storms and water collected in adjacent lowlands.

Glaciers and volcanoes formed lakes and surrounding wetlands.

Rushing rivers created swamps and broad tidal flats as they deposited eroded soil on flatlands.

Where rivers met the sea, they formed estuaries and lagoons as they interacted with the sand dunes and tides along coastal flatlands.

Wetlands were not highly thought of by our forefathers. Swamps, Mires, Quagmires, Bogs were unwholesome places, associated with disease, decay and treacherous footing. They were impossible to travel through and useless in every way. Farmers, with government subsidies, drained and filled more than 90 percent of New Zealand's wetlands between 1840 and 1940.

The amount of wetland filling varies from one part of New Zealand to another. Southland still has 37% of its original wetland area. south Canterbury retains 25%, Waikato 15% and the Bay of Plenty less than 1%.

New Zealand's largest wetlands were at top Firth of Thames where Captain Cook recorded a giant forest of 200ft tall Kahikatea trees (Dacrycarpus dacrydiodes). Only small remnants of these grand forests survive - for example at the Arahaki Lagoon in central North Island and the Riccarton Kahikatea Reserve in the middle of Christchurch. In Manawatu, the Pukepuke Lagoon is all that remains of a large area of wetlands. The Hikurangi Wetlands in Northland were all but totally destroyed.

In all, wetlands occupy 311,300 hectares, which is less than 2% of New Zealand's land area.

We now know that wetlands play a vital role in our natural systems. They act as a huge sponge, storing water during heavy rainfall and thus reducing the loss of soil and river bank erosion and flooding downstream. During droughts, they slowly release the water again, regulating the flow of water and nutrients from the land to rivers and the ocean. The thick abundance of special rushes and other aquatic plants trap sediment and nutrients, thus protecting the water quality in rivers and coastal areas.

Wetlands are an important stage in the evolution of the land. Rivers and lakes in flatlands gradually fill in from eroded soil, cutting new channels through the wetlands over hundreds and thousands of years. Much of the fertile agricultural areas were created from river wetlands. Without wetlands this normal process is stopped and the sediment washes out to sea, harming coastal life and robbing the land of vital nutrients.

Wetlands are buffer zones and, because they are difficult for many animals to move through, they are refuges for birds and aquatic wildlife. They are not wastelands, but a biological treasure offering a wide variety of habitats for plants and animals.

Today, biologists understand the true value of wetlands. The RAMSAR Convention is an international treaty to protect wetlands set up in 1971. 93 countries, including New Zealand belong to the convention. More than 800 sites have been nominated as wetlands of international importance requiring protection. Unfortunately, the loss of wetlands has become so extensive the problem is no longer how to prevent further loss, but how to restore and rebuild wetlands.

Wetlands can be rebuilt or even constructed in places where they can help reduce water pollution. Excess nutrients from agricultural activities are classed as the most significant water quality problem of New Zealand rivers. Agricultural holding ponds have been established to attempt to reduce this problem and they do help. But their effluent is still unacceptably high in phosphates and nitrates. During times of heavy rains, they may release large amounts of nutrients and sediments into nearby rivers. By constructing wetlands between agricultural holding ponds and rivers, the excess water and nutrients can be absorbed and held by wetland plants.

Artificial wetlands have been designed to treat sewage effluent and effluent from agricultural ponds. These go through a series of tanks of algae and then into a wetland with a carefully designed association of plants. By the time the effluent trickles out of the wetland it is completely cleansed into water of exceptionally high quality.

Scientists at the National Institute of Water and Atmospheric Research discovered that natural wetlands produce tannins which chemically combine with phosphates and precipitate out of solution. The best systems seem to include a constructed wetland directly in the path of the pond effluent and a "feeder" input from a natural wetland that introduces water rich in tannin to the last part of the constructed wetland. This protects the natural wetland from contamination and prevents the constructed wetland from becoming "saturated" with phosphates.

Resources to use

From the Mountains to the Sea by Nic Bishop. 1994. Reed. The text and photos are superb. Bishop offers real insight into New Zealand wetlands.

Wetland Plants in New Zealand by Peter Johnson and Pat Brooke. 1989. DSIR Publishing.

A Directory of Wetlands in New Zealand by Pam Cromarty and D.A. Scott. New Zealand Department of Conservation. 1996.

Wetlands down on the farm. by Chris Tanner. NIWAR Water and Atmosphere 1(2) 1993

Rivers Wetlands River Surveys

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