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By Fiona Dyer
At present, there is limited harvesting of water from Canberra’s lakes and the water levels are constant... Using water from lakes and ponds for watering public spaces will fundamentally change the water level regimes...
There’s been nothing like the decade-long drought across south-east Australia for focussing minds on the use and supply of water in urban environments.
Take Canberra, renowned for its urban green spaces, where few people doubt the future will be drier and the population considerably bigger. As water managers look to secure future water supplies, the notion of using water from urban lakes and ponds rather than clean treated drinking water to maintain public open spaces is starting to look very attractive.
Using lake water to keep our urban spaces green would be to use water that is ‘fit-for-purpose’, while reducing reliance on mains water supplies. The downside is that drawing water from lakes and ponds will fundamentally change the water level regimes and could significantly damage local ecologies.
Ecological communities in Canberra’s lakes are adapted to stable water levels. For the lakes to continue to provide benefit to the community, it is important to know whether changing the water level regime will result in a loss of ecological function.
To understand the risks, scientists from eWater CRC and the Institute for Applied Ecology, with the support of the ACT Government, are developing models that predict ecological responses to changing water levels in Canberra’s lakes and ponds. The models are designed to help answer the following questions:
The models are being developed to trial the use of eWater’s ecological modelling software Eco Modeller for ‘real world’ scenarios.
The study combines information from published studies, historical data, field data, water quality and hydrological models to identify the relationships between water level fluctuations in lakes, aquatic plants and water quality responses.
As a planned City, Canberra’s lakes, ponds and wetlands are an integral feature of the urban landscape. Canberra’s citizens treasure these water bodies as places to enjoy recreational activities like swimming, fishing, and walking, and social gatherings in picnic areas and playgrounds.
In an increasingly urbanised environment, they have the added benefit of being places where people can connect with the natural environment.
Lakes and ponds in urban areas were built to reduce the size of floods in urban creeks by retaining water for slow release downstream. Slowing water also allows time for sediment and pollutants to settle, which improves the quality of water before it joins downstream waterways.
Equally, Canberra’s lakes have an important conservation function. They provide habitat for animals like waterbirds, frogs, turtles and yabbies. Places that sustain habitat for a range of organisms are important because, by supporting a variety of populations, they increase overall biodiversity.
Many of the benefits we receive from our lakes, ponds and wetlands are closely linked to their healthy ecological functioning. For instance contact water sports rely on good water quality, which depends on the state of algal growth and bacteria in the lakes. Algal growth is related to the concentrations of nutrients in the lake. Plants growing in and around the water play an essential role in removing nutrients from the water column.
Recreational use of the areas around the lakes is enhanced by low odours and low numbers of mosquitoes. Aesthetic values come from having a diversity of plants and animals living in and around the lakes.
Solutions for a dry future
At present, there is limited harvesting of water from Canberra’s lakes and the water levels are constant, only dropping when there is not much rain. Using water from lakes and ponds for watering public spaces will fundamentally change the water level regimes.
In order to predict the effect on aquatic plants of water level fluctuations, we first needed to know what plants existed around the City’s lakes and ponds. To this end researchers from the Institute for Applied Ecology mapped the aquatic plants around Lake Ginninderra, Point Hut Pond and Upper Stranger Pond. The surveys showed that while the plants were quite dense, they consisted mainly of Typha orientalis (Cumbingi). There were very few aquatic plant species in these water bodies.
Other studies have shown that constant water levels support an aquatic plant community that is dense, but rarely diverse.
Increasing the water level fluctuations is likely to reduce the density of existing plants, but will support a wider range of species. The problem is that increasing water level fluctuation too much will make it hard for aquatic plants to survive, killing off existing plants and impeding future establishment of plants.
Water Quality Responses
Harvesting water will improve the ability of the lakes and ponds to capture and retain sediment and pollutants. However, harvesting will also repeatedly expose and re-wet the edges of the lakes and ponds, which can cause major changes to the chemistry of the sediments.
When sediments are exposed to air, the extra oxygen results in phosphorus becoming more tightly bound to the sediments and more nitrogen being converted to atmospheric nitrogen (N2) (a process called denitrification). This has benefits for water quality.
When sediments are completely dried out, the bacteria and other living organisms in the sediment die. When these sediments are re-wet, the breakdown of cells from the dead bacteria can result in a pulse of nitrogen and phosphorus into the water column which is available to plants and algae. However, researchers have estimated that the amount of phosphorus that can be released is less than the peak concentrations in stormwater and is unlikely to affect the overall water quality of the lakes and ponds. It may result in local areas of poor water quality in the lakes.
We have hence concluded that harvesting water from urban lakes while still retaining, or even improving, ecological function is possible, with the following qualifications:
Dr Fiona Dyer is an eWater scientist and Research Fellow, Institute of Applied Ecology, Canberra University.
eWater CRC has a range of tool to assist the ecology in its Toolkit, with more on test with partners. These include:
How do urban ponds and wetlands work?
The high proportion of impervious surfaces in urban areas means that a rain storm in an urban catchment produces higher runoff than corresponding rural or natural catchments. This means that there is increased flood risk in urban areas. Lakes and ponds reduce the size of floods by storing stormwater which is released slowly downstream.
Water Quality improvements
A combination of higher flows and impervious flow pathways in urban environments means that the processes that would naturally reduce the concentrations of sediment, nutrients and other pollutants in urban stormwater do not occur. Consequently urban stormwater can be high in litter, organic matter, nutrients, heavy metals, pathogens and other pollutants. Nutrients are of particular concern in urban stormwater because high levels of nutrients can result in nuisance plant growth and high levels of algae in receiving water.
Lakes, ponds and wetlands are commonly used in urban environments to ‘treat’ urban runoff by improving the quality of water. When water enters a lake, pond or wetland it slows down and sediment nutrients and other pollutants settle out. This means that the quality of water that continues downstream is improved.
Once deposited the sediment remains within the pond or wetland and various sediment biogeochemical processes act to cycle the nutrients, metals and other pollutants between the sediment, biota, water column and atmosphere.
Aquatic plants play an important structural and functional role in urban lakes and ponds. They help to slow the water and improve the water quality by using dissolved nutrients from incoming stormwater. Aquatic plants around the margins of lakes and ponds protect the shores from erosion caused by wave action.
Lakes and ponds provide important habitat for animals helping to protect biodiversity in a city landscape. Aquatic plants provide food and shelter for aquatic invertebrates, fish and water birds. The more diverse the aquatic vegetation, the greater is the potential diversity of animals living among it.