Quantifiable success of the LIFE+ Alte Donau project

An accompanying monitoring process was carried out to review the success of the measures implemented under the LIFE+ project.


The lilypad whiteface dragonfly - an endangered dragonfly species of Europe-wide significance - has been discovered at the Alte Donau.

The experts tasked with the monitoring analysed the development of indicator organisms such as fish, invertebrates, lakeshore plants and submerged aquatic vegetation.

Other criteria evaluated were the water quality and the visual landscape of the Alte Donau. Progress made with regard to biodiversity was also documented, with a special focus on protected and endangered fauna and flora.

Expansion of near-natural zones

Thanks to the setting aside of unmown refuge zones in the lake, the partial re-naturalisation of reinforced banks, rejuvenation of reed beds, and above all by allowing the lakeshore to develop naturally without interference, the share of near-natural stretches of lakeshore rose substantially during the project period. This has significantly reduced the negative impact of the growing user numbers and more intensive use of the lake on the flora and fauna in these zones.

Approximately 1,500 metres of lakeshore saw a significant structural improvement. As a consequence of the project measures, the extent of intact reed beds - i.e. zones supporting a variety of lakeshore plant species - rose from around 6,000 metres to some 8,000 metres. Willow saplings were planted and neophytes, i.e. non-native species, removed on around 300 square metres of land close to the lakeshore.

Re-naturalisation of lakeshore and creation of protected zones

Measures benefit fish

With over 20 different species, the Alte Donau supports a highly diverse fish population. Restructured shorelines and unmown refuge zones in the water foster the development of young fish. In quantitative terms, there is always a time lag before positive impacts on the fish population are felt; however, initial results indicate that both the restructured stretches of lakeshore and the refuge zones are well accepted by fish.

New dragonfly species

Richly structured habitats also promote invertebrate biodiversity. The longer-term development of the dragonfly fauna on the Alte Donau is very pleasing, with a survey during the project period documenting 27 dragonfly species. A third of the species occurring on the Alte Donau today are actually categorised as endangered on the Red List of Austrian Dragonflies. Likewise especially gratifying is the fact that a species identified in the EU Habitats Directive as being of European importance - the lilypad whiteface - has been recorded on the Alte Donau for the first time.

Living space for three beaver families

The measures implemented under the LIFE+ project are designed to maintain a stable beaver population on the Alte Donau and ensure the peaceful coexistence of humans and beavers. The monitoring results show that it has been possible to maintain a constant population of three beaver families, numbering a total of eleven to fourteen animals. Although it is intensively used, the Alte Donau continues to provide a suitable habitat for beavers. Despite the wide-ranging tree protection measures installed by the City of Vienna, the beavers have an adequate food supply, even in winter.

Evaluation and management of underwater vegetation

The team of experts constantly monitored and documented the submerged aquatic vegetation, focusing their attention on plant biodiversity and the development of the low-growing macrophytes. The proportion of low-growing aquatic macrophytes increased again for the first time in the course of the LIFE+ project.

Mowing of the submerged aquatic vegetation poses a major challenge in the management of the Alte Donau. Due to the proliferation of the underwater plants in the past few years, innovative and more efficient measures are required to prevent them from interfering with people's recreational enjoyment of the Alte Donau. Within the framework of the LIFE+ project, experts addressed the challenge of optimising the City of Vienna's mowing schedule to deal with the proliferating vegetation.

Numerous improvements and adjustments were implemented as part of the project. The on-board GPS systems and digital vegetation maps developed and tested as part of the project have made the mowing operations much more efficient. The project also yielded valuable findings with regard to optimum mowing depth, mowing priority and time sequence.

Mowing of aquatic vegetation in the Alte Donau

Biological filter bed: impact on water quality and water balance

The impact of the new biological filter bed in the Water Park on the water quality and water balance of the Alte Donau was also evaluated. The filter bed allows fresh, filtered water from the Neue Donau to be fed into the Alte Donau.

Tests performed in the inflow and outflow zones of the filter bed showed that the passage through the biological filter reduces the phosphate content of the infed water by an average of 25 percent. The chlorophyll content of the water is reduced by a hefty 75 percent, and the suspended solid content by 50 percent.

In its first one-and-a-half years of operation, the filter bed fed in a volume of fresh water roughly equivalent to the entire upper section of the Alte Donau. The new filter bed is thus an effective water management tool that is able to ensure a continual inflow of impeccable quality water to refresh the water in the Alte Donau.

Biological filter bed in the Alte Donau Water Park

Logo of the LIFE+ Programme of the European Union: The word "Life" in yellow, surrounded by yellow starsn

The LIFE+ Alte Donau project was funded under the LIFE+ Programme of the European Union as part of the "Environment Policy and Governance" strand.

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City of Vienna | Water Management
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