The role of slackwater areas for biogeochemical processes in rehabilitated river corridors: examples from the Danube.

Nutrient storage and transformation as well as aquatic production are key ecosystem functions and are related to areas of higher retention in the river corridor. The significance of retention as a main element for understanding ecosystem processes becomes apparent in regulated rivers where both retention and hydrologic connections between the main channel and its slackwater areas and the adjacent riverine landscape have markedly decreased. We give some evidence that in-channel structures and side-arms are important slackwater areas below bankfull level in a braided, anabranched river section, the Danube downstream of Vienna. In that stretch the role of in-channel retention for phytoplankton development and a reopened side-arm and the floodplain for the phosphorus dynamics is discussed. The transformation of phosphate is related to phytoplankton development and increased connectivity below bankfull enhances the transformation capacity of the reopened side-arm. The rate of deposition during floods is still the decisive process in terms of annual total phosphorus budgets and is presented for the years 1997-2002. Up to 1 % of the total annual load transported is retained in the floodplain area. Rehabilitation efforts, like those along the Austrian Danube, which increase the hydrological exchange with slackwater areas, will enhance nutrient processing and the production of autochthonous POM and thereby, also re-establish these ecosystem functions on a larger scale than the local measures.