Wetlands: Kidneys of the landscape
By Bala Tikkisetty
Intensive farming practices can result in significant amounts of contaminants, notably nitrogen, phosphorus, faecal matter and sediment, getting into our waterways.
With World Wetlands Day occurring on February 2, it is a good time to reflect on the impact of contaminants like these on our waterways, and the role of wetlands in helping to protect them.
Natural wetlands have been called the ‘kidneys of the landscape’ because of their ability to store, assimilate and transform contaminants lost from the land before they reach waterways. Like a giant kidney, wetlands help to dilute and filter material that could otherwise harm our lakes, rivers and other waterways.
Sadly, large areas of wetlands have disappeared with the development of farmland and they now occupy only about two per cent of New Zealand’s total land area. It is estimated that about 90 per cent of New Zealand’s wetlands have been drained – one of the largest wetland losses anywhere in the world.
Wetlands once covered large areas of the Waikato, but they are now some of our rarest and most at-risk ecosystems.
Wetland is a generic term for the wet margins of lakes, ponds, rivers, streams, estuaries, lagoons, bogs and swamps. They contain a diverse range of plants and animals and are home to many rare and threatened species, so conserving and restoring wetland habitats is worthwhile for many reasons.
Rainfall patterns, soil water status, groundwater levels, soil properties, drainage system design and land management practices can all affect the contaminant loads generated in farm drainage.
Nitrogen and phosphorous enter waterways through leaching and surface run off. Wetland vegetation uses these nutrients for growth. Wetlands remove up to 90 per cent of nitrates from ground water through a process called de-nitrification. Microbes living in wetlands absorb and break down nitrogen improving water quality.
Wetlands also play an important role in sediment management and reducing erosion. The plants trap sediment suspended in water, improving water quality, and in riparian areas their roots hold riverbank soil together.
Wetlands also help to regulate the flow of water from land, soaking up excess floodwater and then slowly releasing it to maintain summer flows or recharge ground water.
Providing habitat for many different plants and animal life, including rare or threatened species, is another role for wetlands. These areas are also essential breeding areas for whitebait species and game fish, as well as providing a rich source of insects for fish, birds and amphibians.
Fishers, shooters, naturalists and other water-based recreationists also make extensive use of wetlands. Their importance to Maori as mahinga kai (food gathering areas) and as a source of plants for medicines and dyes, is well recognised.
On an international level, healthy peat wetlands are important in helping to combat global warming, as they soak up excess carbon.
Waikato Regional Council provides free advice to landowners on managing wetlands, including information on fencing, planting of suitable riparian margins and weed control.
Fencing keeps stock out, stops pugging of wetland margins and enrichment from animal wastes. Appropriate planting around the edges of the wetland reduces pollution from surrounding farmland, provides cover for wildlife, reduces bank erosion and reduces the temperature of water through shading.
Of late, constructed wetlands, as trialled by NIWA, have been recognised as an effective technology for treatment of tile drainage waters.
Wetlands are worth caring for – failing to maintain their health is a risk too great to ignore.