The Lobau UNESCO Biosphere Reserve Case Study encompasses the Danube River floodplain located in a sub-continental climate, within the limits of the City of Vienna (Austria) with 1.6 million inhabitants (2000). Aquatic habitats addressed by the Case Study include floodplains, wetlands and riparian corridors, rich in flow sensitive biota. Threats are flood impacts due to river canalisation, and metamorphosis of water bodies through wetlands to dry land due to siltation. The major goal is to sustain and enhance biodiversity and aquatic plants ecosystem services, and major actions concern ecohydrological planning, legal protection and administrative balance of stakeholders.

"Schwarzes Loch" floodplain lake in the Lobau Biosphere Reserve (C Strausz, 2006).

Background

Vienna is located in one of the most prosperous regions of Europe. This situation forms the background of all socio-economic and environmental aspects related to the city and aquatic habitats. Over the last decades large public housing complexes and individual housing expanded into the areas to the North and North-East of the Danube River.

The Lobau is part of the historical floodplain area of the Danube River. Today floodplain lakes, wet ditches and riparian wetlands still exist. Their ecological value is enhanced by the fact that they form a last refuge for aquatic organisms having lost their former habitats in the adjacent landscape, which is dominated by urban development and agricultural areas.

The catchment area of the Danube at Vienna is c. 100,000 km², with river discharge (mean 1,900 m³.s^-1) ranging from min 650 m³.s^-1 to max 15,000 m³.s^-1 (calculated for the 1,000-yr return period). The water quality is at, or close to, the natural conditions expected for a river of this size; e.g., phosphorus concentrations and the composition of the invertebrate fauna in the river sediment classify its condition as oligo- to mesosaprobic, which corresponds to the carbon load under natural conditions. Of course, this is the result of rigorous sewage treatment throughout the catchment, which includes tertiary treatment.

The Lobau floodplain ensemble includes still waters and riparian forests remaining after river regulation in the late 19^th century. The Lobau is separated from the river by a levee, but passage of seepage water is uninterrupted and floods flow in an out through a small opening in the levee at the downstream end of the area. Prevailing still water conditions and 130 years of siltation trigged natural succession processes, which resulted in enhanced development of reeds and other wetland vegetation and considerably reduced cross-section and shape of the water bodies. Abundant macrophytes and the strong groundwater influx from the river keep the water quality in the floodplain lakes at a high level.

Despite regulation and intensive development of the city, the Lower Lobau area was declared a UNESCO Biosphere Reserve (1977), Natural Sanctuary under Viennese law (1978) and incorporated into the Danube Floodplain National Park under the Austrian Federal Law (1996). It is also a protected area according to the RAMSAR Convention, and under the NATURA 2000 Directive of the European Communities.

Key aquatic habitat issues in urban water management

The current key issues include: finalising flood protection for Vienna, reducing sedimentation in the Lobau water bodies during the recession of floods, re-establishing higher hydrological dynamics in the wetland in an attempt to restore at least some of the former character of the natural floodplain system, but saving at the same time existing biodiversity. Conflicts exist over the future role of drinking water wells in the Lobau. In the Danube reach downstream of Vienna, the river bed incision (c. 1.5 m in the Vienna reach between 1875 and 1990) and the corresponding sinking of the groundwater level were caused by straightening the once meandering river course: these issues need ecosystem friendly solutions.

The 21 km long artificial water course, called the New Danube, is located in parallel to the main river channel. It diverts floods, but its two impoundments are centres for leisure activities in combination with the 'Danube Island'. Even the maximum calculated discharge of the Danube in Vienna (c. 15,000 m³s^-1) should safely pass the city today. In the New Danube abundant aquatic plants regulate the nutrient regime and compete with microscopic algae; thus, preventing possible toxic algal blooms (Janauer, Wychera, 2000).

Objectives of the Case Study

The 'UNESCO Biosphere Lobau' was created to save the ecological values of a wetland and floodplain waters ensemble of international importance, indicated by its high biodiversity and a set of still water species of plants and animals, which have found a last refuge in this part of the Danube River Corridor. The special combination of moderate to intermediate flood pulses and a still effective groundwater connection with the main river channel dominate the hydrological regime in the Lobau area.

Final steps in flood control needed for the river reach in Vienna, and at the same time saving the high quality of potable water reserves in the groundwater system of the Lobau wetlands, must be merged with the intrinsic ecological values of the area.

Planning the ecological future of the Lobau and respecting the life in its water bodies is not a single-purpose activity as in the past. Aiming at an ecological situation that reflects the conditions prior to the river regulation calls for "water flowing through" the Lobau. This certainly is a situation not in favour of many of the still water organisms surviving there today.

The prime task of the 'Demonstration Site Project Lobau' is therefore the intensive collaboration among all stakeholders and searching for solutions, which:

1. will fulfil the need to sustain existing biodiversity as well as possible,
2. try to find new refuges for the established still water species of flora and fauna, and
3. not to interfere too much with the need for flood protection and the wish to re-establish more dynamic flow conditions in the main channel system of the floodplain.

Stakeholders

The institutions listed below are involved in the future development of the Lobau Biosphere Reserve: The Councillor for the Environment, Vienna, several municipal administration units (MA 45 - Water Engineering, MA 22 - Environmental Protection, MA 31 - Waterworks, MA 49 - Forestry), the Danube National Park Administration, the Department of Freshwater Ecology at the University of Vienna, the Austrian Academy of Sciences, the Austrian UNESCO Commission, and the Federal Province of Lower Austria. Stations monitoring water quality are operated by the Federal Office for Water Management, the MA 45 - Water Engineering, and the Austrian Hydro Power, Inc (AHP). Education and training is conducted by the Municipal Office, MA 22 - Environmental Protection, the Danube National Park Administration, the Austrian UNESCO commission, the Federal Agency for Water Management, and the Department of Freshwater Ecology.

See References