Loss, value and protection of wetlands

By Dave Hansford

A rich natural asset

Wetlands are one of our richest natural assets - or were. We're only just beginning to understand the many reasons why we should have left them alone and the huge range of services they have to offer.

Wetlands, Chatham Islands

Dairy farms built on wetlands

Highway 27, north out of Morrinsville, is 5th gear country. Arrow-straight and flat to the horizon, it'll whisk you to the bottom of the Bombay Hills in just over an hour, depending on how many milk tankers you have to pass.

On any evening, convoys of them cart millions of litres to processing plants across the Hauraki Plains and Bay of Plenty, but in a sense, what's really sloshing around in those stainless steel tanks is bequeathed energy; a legacy from a million hectares of mangrove, peat bog, fenland, flax swamp, low manuka and towering kahikatea. Nutrients fixed and cycled by a billion microbes, bacteria, fungi, plants and animals that once thronged in the greatest network of wetlands in the country.

These dairy farms, and the towns that serve them, are all built on the dehydrated corpse of the Hauraki swamp, a vast morass that once stretched from the Waikato plains to west, to the Kaimai Range in the east.

Cows in Kakaho Stream, Pauatahanui
Inlet, Wellington Region

Historic swamp draining

But the European settlers were unmoved by the sheer energy and productivity of these wetlands. They saw them instead as a blight; a barrier to development. They resolved to drain the Hauraki swamp.

In 1908, the Government passed an act authorising drainage on a scale never before dreamed of; a thousand kilometres of stopbanks actually stopped the sea coming in, and enslaved the Piako and Waihou rivers. A decade of labour left the plain cross-hatched with the deep, unswerving drains you can still see today.

What was once 65,000 hectares of kahikatea, raupo, harakeke, swamp orchid, is now fescue, fence and farm.

Further loss of wetlands

Over the hills to the west, the same thing happened to the once vast Whangamarino swamp, and to the Rangitaiki Swamp to the south. The Wairarapa was left high and dry. Wetlands everywhere were drained, burnt and ploughed under with mind-boggling rigour. A 2008 Landcare Research and Department of Conservation study calculated that wetlands once covered 2.4 million hectares - nearly 10% - of the country; less than 250,000 hectares remain. The loss was greatest in the North Island, where fewer than 5% of wetlands survive. In the South Island, wetlands slightly fared better; 16% remain.

In all, 90% of our wet places have vanished, and only now are we starting to understand the magnitude of that loss, because it turns out that wetlands do a vast tasklist of chores for us.

Whangamarino Wetland, Waikato

Value of wetlands

Protect land from flood damage

Low-lying, and flat, they readily store, and slowly release, surface water, rain, snowmelt and flood waters. Wetland plants trap water, slowing the flow of floods, then gently releasing them over surrounding plains, softening impacts on land and towns. Whangamarino alone, even at a third of its original extent, has still prevented millions of dollars in flood damage.

In 1998, a "100-year" flood burst from the Waikato River. The Whangamarino wetland, found a 2007 report, absorbed that deluge, sparing 73 square kilometres of surrounding farmland from inundation, and its owners a $3.8 million (in 1998 dollars) cleanup bill.

Maintain water flows

In places like Canterbury, where water is in high demand, wetlands play a critical role in recharging groundwater aquifers, and maintaining stream flows during dry spells.

Treat and clean water

Wetlands have another unique talent; they take dirty water and clean it for us, completely free of charge. Beds of reeds, rushes and raupo trap fine sediment and pollutants in water and hold them fast, and do it better than sophisticated - and very expensive - mechanical water treatment plants.

Around the globe, cities are waking up to this service; New York City found it far cheaper to spend US$1.5b to protect the Catskills-Delaware water catchment than build a filtration plant for US$6b, then spend US$300m a year running it. In downtown Wellington, a planted wetland now cleans stormwater before it enters the harbour. Another restored wetland is helping clean up Lake Okaro near Rotorua.

Mugwort

Prevent topsoil washing into sea

They also prevent valuable topsoil being washed into the sea; the services provided by the wetlands of the Charles River Basin in the US state of Massachusetts have been valued at more than USD$95 million per year, and many southern US states are restoring coastal wetlands as a buffer against storm surges from hurricanes.

Make carbon sinks

In today's warming world, wetlands play another important part. While they only soak up greenhouse gases at modest rates, thick beds of peat make ideal carbon sinks. Whereas trees are felled, or eventually topple from senescence, peat bogs can go undisturbed for centuries, providing a stable, secure store well into the future. Wetlands also regulate local temperatures and rainfall, lending some stability to agriculture and winemaking.

Supply nutrients

Into the bargain, they trap, store, process and recycle nutrients in soil and water. Microbes in the muddy sediments break down organic compounds, releasing nitrogen, sulphur, and phosphorous. While wetland plants absorb the phosphorous, much of the sulphur and nitrogen is eventually released into the atmosphere, to be taken up by plants and nitrogen-fixing bacteria in surrounding soils.

Provide medicine and food

Maori always knew wetlands as larders, and a store of materials to fashion into mats, ropes, walls, clothes. Healers knew them as dispensaries of medicines, tinctures and supplements, and it's highly likely that wetland plants hold still more, undiscovered, medicinal and genetic value.

Offer recreation

Elsewhere, wetlands offer huge recreational value; a study of Whangamarino found it was worth $13.5m a year in amenity values to gamebird hunters (Fish and Game sells some 30,000 gamebird licenses in the Waikato every year), fishers, birdwatchers and other users.

Brown teal

Support rich wildlife

Wetlands are above all one of the most biologically-rich environments we have. They are a powerhouse of productivity, generating thousands of tonnes of new plant biomass every year. Their waters are a soup of nutrients, supporting millions of small organisms that, in turn, nourish vast food webs. Around 30% of our native birds are wetland species, including secretive and mysterious birds such as the Australasian bittern, marsh and spotted crakes, and the banded rail.

Below the waters range some of our most interesting, yet threatened, native fish. The mudfish can survive without water for two months if it must, going into a sort of hibernation, so long as it has shade and vegetation to keep itself moist. When it rains again, and water flows back into the wetland, they resume their lives.

But drainage has been devastating for our native wildlife, and it still continues. Mudfish are now found only at a few scattered sites around the country. The bittern is being forced into ever smaller refuges and numbers are dropping. Some believe even the long-fin eel - an iconic Kiwi childhood memory -is threatened with extinction.

Children from South End School helping
to restore Shield's Wetland, part of
Conservation Week, Wairarapa, 2009

Protecting wetlands

But Hugh Robertson, a Department of Conservation wetland ecologist, says that the fortunes of our wetlands may finally be turning a corner. Robertson provides scientific advice to Arawai Kākāriki an initiative launched in 2007, which puts an extra $2.2m of funding into protecting three of our most important wetlands; Awarua, near Invercargill, Ashburton Lakes (O Tu Wharekai), in the mid-Canterbury high country, and Whangamarino.

"People are starting to see that the clearing and drainage of wetlands has an endpoint, and they want to protect what's left. Increasingly, we realise the importance of these systems to fisheries, to tourism, to maintaining water quality. The Rotorua lakes are a very good example; there's a multi-million dollar project to clean them up, because the area is so important to the local economy".

Robertson says we would have saved ourselves a fortune, had we just left more wetlands behind; "Christchurch is built on a swamp, Invercargill is built on a swamp. There are a lot of places in New Zealand where, if we had moved to the side a little, we could have kept all the services that those swamps provided."

The intensification of farming, especially dairying, is putting pressure on some of the few wetland systems that remain. At Awarua, 70% of the surrounding catchment used to be sheep and beef farm; now 70% of the catchment is supporting dairy herds 400-strong. All that effluent has to go somewhere, and for now, much of it is running into the wetland.

Says Robertson, "Maintaining agricultural productivity is vital but we have to do this more intelligently; by prescribing buffer zones around wetlands, and managing where water flows. And we need to identify and set limits and thresholds for catchments."

Ecologists have a term for when those thresholds are breached, when a wetland is irrevocably harmed; they call it "flipping." You might recognise a flipped system by a thick scum of bright green algae on its surface, says Robertson. "Years of effluent and fertiliser runoff can pollute water bodies with such heavy nutrient loads that they're continually being released into the system. That perpetuates the decline in water quality that characterise a flipped system.

"Once you've gone past the wetland's ability to sustain that, it's very difficult to get it back. We need to be very careful in the future because it's already too late for some wetlands."

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