Hydrological connectivity and flood pulses as the central aspects for the integrity of a river-floodplain system

Preparations are currently under way to establish a river restoration concept for the free-flowing section of the Austrian Danube downstream of Vienna. This should serve as a basis for a National Park management. Studies have been initiated to assess the effects of hydrological connectivity, flood pulses and successional processes on backwaters with regard to their hydrochemistry and suspended solid load. Water level fluctuations in the ground- and surface waters of the floodplain are dynamic because of the porous aquifer structure and several inflow areas. In a lateral transect, increasing distance from the Danube correlates with decreasing connectivity. Different influences in a longitudinal transect are due to the positions of the inflow areas. At low water level, the Danube and its backwaters have no surface connection. Internal processes determine the conditions within the backwaters, which are typically characterized by low concentrations of inorganic nutrients and particles versus the Danube. At higher water levels, local surface inflow and seepage water lead to periodic nutrient pulses and eutrophication. The input of inorganic nutrients and particles, as well as the flushing of phytoplankton during floods, establish an initial setpoint in the backwaters. The significance of flood pulses for the dynamics of biological processes is emphasized by short-term changes, e.g. in the relationship between nutrients, inorganic particles and chlorophyll. The response in different water bodies depends on the grade of connectivity to the Danube.