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.
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
