Alternative stable states in floodplain ecosystems

Floodplain and ox-bow lakes form excellent examples of alternative stable states. Clear water, macrophyte dominated stages can alternate with turbid conditions characterised by high algal concentrations. Stable states can switch from one to the other domination through alterations of natural factors such as changes in water level, reduction in through-flow etc. Forward switches are often associated with anthropogenic pressure. In such cases, restoration to the original, macrophyte dominated stage is difficult and needs careful planning to establish sustainability. A case study from a shallow, urban, seepage lake, the ‘Alte Donau’ within the city proper of Vienna is presented. Results on switches between stable states, causes, consequences and restoration measures will be detailed. The analysis is augmented with data from floodplain lakes east of Vienna which have different levels of connectivity to the main river.