The Danube restoration project: functional aspects and planktonic productivity in the floodplain system

Abstract

In Austria, the ‘Danube Restoration Project’ (DRP) was implemented to re-establish the connectivity between the Danube and its floodplain along a free flowing section downstream of Vienna. Before the restoration, the status quo of the ‘Regelsbrunn’ floodplain segment was investigated at different levels of abiotic, biotic and functional properties according to the spatio-temporal effects of hydrological connectivity. The present paper deals with temporal effects on hydrochemistry and on phytoand bacterioplankton in the side channel (parapotamon). A hydrological situation of the present state of short-termed and intensive flood pulses followed by a period of isolation was compared with a period of higher connectivity reflecting the situation after the restoration. Rising water levels in the river, establishing local surface inflows, resulted in periodic nutrient pulses, high particle load and eutrophication in the floodplain. With disconnection after a spate, the nitrate and soluble reactive phosphate (SRP) concentration decreased significantly during 1 month. Phytoplankton correlated negatively with SRP during the first week after a flood pulse, reflecting the dominance of phytoplankton in SRP uptake. Planktonic primary production was stimulated by the nutrient import at higher connectivity and could be controlled by a decrease in retention time, related to the aims of the DRP. The two investigated periods indicated that increasing retention time in the side channel resulted in a shift from primary production towards prevailing bacterial secondary production. Copyright © 1999 John Wiley & Sons, Ltd.