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

In Austria, the 'Danube Restoration Project' (DRP) was implemented to re-es tablish the connectivity between the Danube and its floodplain along a free flowing section downstream of Vienna. Before the restoration, the status q uo of the 'Regelsbrunn' floodplain segment was investigated at different le vels of abiotic, biotic and functional properties according to the spatio-t emporal effects of hydrological connectivity. The present paper deals with temporal effects on hydrochemistry and on phyt o- and bacterioplankton in the side channel (parapotamon). A hydrological s ituation of the present state of short-termed and intensive flood pulses fo llowed by a period of isolation was compared with a period of higher connec tivity reflecting the situation after the restoration. Rising water levels in the river, establishing local surface inflows, resul ted in periodic nutrient pulses, high particle load and eutrophication in t he floodplain. With disconnection after a spate, the nitrate and soluble re active 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. Pla nktonic primary production was stimulated by the nutrient import at higher connectivity and could be controlled by a decrease in retention time, relat ed to the aims of the DRP. The two investigated periods indicated that incr easing retention time in the side channel resulted in a shift from primary production towards prevailing bacterial secondary production. Copyright (C) 1999 John Wiley & Sons, Ltd.