Wetlands and water health
BIRD: linking the biodiversity community
Healthy water systems depend on wetlands. Wetlands act as filter beds (the kidneys) for river systems, ground water and landscapes. Without wetlands to help filter the various sediments and nutrients collected from the landscape, the standard of our water quality would decrease, resulting in disruption to ecological and hydrological processes and lower biodiversity. This in turn leads to landscape health issues, such as changing weather patterns, altered flood regimes and lowered ground water recharge.
Wetlands are highly productive systems. They cycle vast amounts of nutrient within the landscape in the form of living organisms. These nutrients move through food chains via the fauna of surrounding terrestrial areas in an expanding ripple effect.
Natural processes such as runoff and erosion deposit nutrients into wetland ecosystems where they accumulate to support diverse and complex living communities. These nutrients and sediments then cycle throughout the wetland system, being absorbed into aquatic vegetation and becoming available to other organisms. Birds and animals then carry these nutrients back out into the terrestrial landscape in the form of food for predators, and as droppings. As Tim Flannery pointed out in his history of the Australian continent: fire saw the fat of the land slowly flushed onto the floodplains and into the estuaries, where today it supports swamp and mangrove. It is no accident that such areas supported the greatest density of Aboriginal occupation at the time of European settlement. [1]
Removing wetland ecosystems breaks the sediment and nutrient cycle. Excessive amounts of nutrients are either flushed out to sea where they impact on marine environments, or become concentrated in rivers and lakes leading to a general decline in water quality and outbreaks of toxic blue-green algae. It has been calculated that 1 hectare (2.5 acres) of tidal wetland can do the job of US$123,000 worth of state of the art waste-water treatment, and many communities and companies are now recreating wetlands to cleanse their waste.[2]
Although wetlands are highly effective water purifiers, they can only function within reasonable limits. Because wetland plants, like all plants, need sunlight to survive, excessive amounts of suspended sediment prevent growth. Land clearing, over-grazing, and the conversion of rivers and floodplains into channels has resulted in increased turbidity in many aquatic systems. Fine sediments are suspended in water making the water opaque and preventing sunlight from entering deep into the water. Submerged and young emergent aquatic vegetation is dependent on good light penetration into the water so that they can effectively photosynthesise, and thus produce the organic compounds (including sugars) necessary for rapid growth. Many wetland plants die back annually to underground parts, and thus effective photosynthesise is necessary to maintain populations of aquatic plants.
Aquatic plants (particularly those that are finely divided) are effective water purifiers, aiding the settling out and entrapment of fine particles. A healthy wetland plant community is able to cleanse water. Excessive turbidity disrupts this process.
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[edit] Wetlands as carbon sinks
Nutrients washing into wetlands promote substantial annual growth of wetland vegetation. Some nutrients, such as phosphorus and nitrogen, are recycled from year to year, while others, particularly carbon, are deposited through decaying vegetation into the beds of wetlands, progressively turning to peat and eventually forming coal.
In addition to the organic processes which deposit carbon as peat, wetlands also receive significant amounts of carbon in the form of charcoal from fires occurring within the landscape. This carbon is laid down in the wetlands (and river ponds and pools) as part of the sediment deposition process. Here the fresh carbon acts like an activated carbon filter, extracting nutrients and impurities such as heavy metals, biogenic salts and other compounds from the water.
These processes make functioning wetlands vital for providing an effective method of removing carbon from the atmosphere for the long term. Burning of drained wetlands releases substantial quantities of carbon dioxide (CO2) and carbon monoxide (CO) back into the atmosphere.
- Peat-forming wetlands, coral reefs and some marine plankton are today the only things that perform the vital function of keeping the gases of the atmosphere in balance. They are the true "lungs" of the Earth. The world’s peatlands alone cover more than 2.3 million square kilometres with 330 billion tonnes of organic matter. If burned or drained and so opened up to slow oxidation, they would release 500 billion tonnes of carbon dioxide, almost double the amount present in the atmospheric greenhouse today.[3]
[edit] Flood mitigation
Floodplains and wetlands are the greatest flood mitigators. They slow water flow, and even out flood peaks. Wetlands act like giant sponges absorbing water during wet times and slowly releasing it throughout the course of the year to aquifers, streams and the atmosphere.
- In the Charles River of Massachusetts in the United States preservation of 38 square kilometres of mainstream wetlands provides natural valley storage of flood waters. It is estimated that had 40 per cent of these wetlands been reclaimed, the increased flood damage would have cost US $3 million each year. And had they been filled completely, the added flood damage would have been over US $17 million per year.[4]
Drainage of floodplains, wetlands and rivers through the cutting of channels allows water to rapidly exit the landscape to the sea, rather than being delayed in the landscape. This reduces freshwater within the soils and recharge to aquifers. Drained landscapes have reduced soil moisture content during the summer period. This reduces the quality and quantity of vegetative growth, including grass growth available for stock grazing.
[edit] Ground water and salinity
A fresh water resource in aquifers must be replenished by fresh water recharge. Wetlands provide a significant portion of fresh water recharge points for aquifers. All wetlands seep to various degrees, with the water filtering through their beds (both through vegetation and soil) into ground water and aquifers.
Freshwater recharge dilutes saline ground water. This increases the range of plants able to survive by using the groundwater, as the water is available for (not toxic to) a wider range of plant species. Loss of wetlands decreases the amount and the quality of water entering aquifers, which in turn leads to a decrease in ground water resource and a reduction in the quality of ground water. The salinisation of fresh water wetlands could lead to saline recharge entering freshwater aquifers.
Converesly, high salt content groundwater can make freshwater creeks and wetlands saline, or increase the salt levels within naturally saline areas. Removal of wetlands on the massive scale that has occurred in Victoria (roughly 1,944 km2)[5] has had a significant impact on soil, ground water, and surface water salinity levels.
[edit] Wetlands and weather
Rainfall requires moist air. Wetlands (particularly large ephemeral wetlands) evaporate large amounts of water, providing moisture to build weather systems (particularly early summer thunderstorms across south-eastern Australia), thus increasing rainfall.
Landscape alteration through land clearing, removal of wetlands, channelling and drainage to remove water from the landscape as rapidly as possible, and the alteration of flood plains reduces the distribution of water within the landscape. By limiting both the quantity of surface water on the landscape and the duration of its presence, evaporation rates are reduced, leading to changes in the distribution and behaviour of weather patterns.
Changing weather patterns affects land use, which contributes to changes in the distribution and composition of vegetation, the nature and volume of water within the landscape — whether it is permanent or ephemeral, whether it sits within the landscape (lakes and wetlands), or whether it flows in creeks and rivers on a permanent or seasonal basis (rivers and floodplains) — and the condition (fresh or saline) and level of the ground water table and of water stored in aquifers.
- A canopy of broad-leaved "dry" rainforest species, such as survives in tiny fire refuges across the north of Australia today, could, if they were more widespread, enhance rainfall by up to 60 per cent and push rainfall much further south. This is because the plants and the soils they protect retard the runoff of water. Through the leaves in their dense canopy they release vast amounts of the trapped water as moisture into the atmosphere. During the wet season, the winds blow in from the coast. As a result, the moisture transpired by the plants is formed into clouds and blown southwards to fall again as rain.[6]
[edit] Managing wetlands for the future
The aim of wetland management is to maintain the balance of vegetation and fauna so as to sustain functioning wetland ecosystems. To achieve this there are three main management strategies which need to be applied:
- Manage the threats to wetland ecosystems. These include drainage, flooding, grazing, alterations to salinity and nutrient levels, and invasive weeds.
- Replace losses of wetland ecosystems, including individual plant and animal species, as well as ecological communities.
- Net gain. The current rate of decline in wetlands is endangering large numbers of wetland species. In the Wimmera Catchment alone there are 58 Victorian rare or threatened species associated with wetlands. To protect these species and stabilise their populations, as well as wetland ecological communities as a whole, there needs to be no further loss of wetlands resulting from threats such as salinity, grazing pressures, drainage, and development.
If these threats to wetland ecosystems are not stopped there will continue to be significant losses and decline of wetland species
To stabilise wetland species, we need to restore wetlands, create new wetlands to replace historic losses, and to eliminate threats. This is necessary to achieve a significant net gain on current wetland habitat (ie. regain a reasonable percentage of lost wetlands). This will help to stabilise threatened wetland species and provide for the long term sustainability of our wetland EVCs.
The window of opportunity to retain Australian wetlands biodiversity is closing. Within 20 years, Victorian wetland ecosystems are facing a catastrophic collapse. Inaction will guarantee this. Best results will only be achieved with immediate action aimed at obtaining net gain of wetland communities (ecological vegetation classes). The longer we wait the harder and more expensive it will become.
[edit] See also
[edit] References
- Tim Flannery, The Future Eaters: An Ecological History of the Australian Lands and People, Reed, 1994, p. 233.
- David Bellamy, Introduction, in Patrick Dugan (Ed.) Wetlands In Danger: A World Conservation Atlas: Oxford University Press, 1993.
- ibid.
- Mitchell Beazley, in Dugan (op cit), p. 23.
- Department of Natural Resources and Environment, Wetlands: the heart of the South-west Wimmera, DNRE, 2000.
- Tim Flannery, op cit, p. 234.
[edit] Links
- Home Page of the Victorian Wetlands Network
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