The essential role of water plants in the Alte Donau

Extensive stands of water plants and clear water are characteristic features of the Alte Donau. A sufficient population of submerged plants is vitally important in order to maintain the good water quality.

In the 1990s it became evident what happens when there is not enough aquatic vegetation. After the water plants disappeared there was a huge deterioration of the water quality in the Alte Donau.

Function

In a shallow body of water, underwater plants (aquatic macrophytes) act as a vital reservoir of nutrients. They compete with the free-floating plankton algae that are responsible for clouding the water. These submerged plants form their own separate microcosm within the lake, providing shelter for numerous aquatic invertebrates and fish such as the pike. They stabilise the lakebed, trapping sediment and reducing turbidity.

Huge proliferation of underwater plants

Although a sharp decline in the submerged vegetation cover was observed in the years 2009 to 2013, there has been a huge proliferation of underwater plants since 2014. Aquatic vegetation growth is influenced by nutrient availability, temperature and light conditions, though the exact cause of the fluctuations has not yet been conclusively explained.

Management of aquatic vegetation

Since 2003, in order to prevent tall underwater plants from spoiling people's enjoyment of boating and swimming, the City of Vienna has been having the plants mown to a height that does not bother swimmers or impede boats.

Mowing of aquatic vegetation in the Alte Donau

New plant species for the Alte Donau

Originally, the Alte Donau supported a range of aquatic vegetation typical of shallow freshwater lakes, notably various species of charophyte (stoneworts). However, since the restoration of the Alte Donau in the 1990s, the aquatic plant population has been largely dominated by a single species, namely spiked water-milfoil (Myriophyllum spicatum).

Monocultures are less resilient to disturbances. For many years now, experts from Municipal Department 45 - Water Management (MA 45) have therefore been attempting to increase plant biodiversity in the Alte Donau and by doing so to stabilise the population of submerged plants. The primary aim is to encourage the proliferation of stoneworts and other low-growing macrophytes, which should simultaneously reduce the amount of mowing that needs to be done.

Experimental underwater planting

Scientists commissioned by Municipal Department MA 45 – Water Management have tested various aquatic plant species that were once native to the Alte Donau. Video recordings made by underwater cameras, for instance, showed how susceptible the different plant species were to grazing pressure from fish. This finally allowed the biologists to identify the most suitable species.

Experimental planting of low-growing aquatic macrophytes in the Alte Donau

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