Limnological concepts as the basis to understanding river networks: Perspectives for the Danube
Hein, Thomas; Preiner, Stefan; Tockner, Klement
The role of environmental regulation of ecosystems in the understanding of ecosystem processes is especially important in river networks because they are open systems whose physical structure changes drastically over several spatial and temporal scales. Here we review some of the most stimulating concepts in fluvial ecology, starting with the River Continuum Concept up to recent concepts for river systems. We focus on ecosystem processes and, therefore, take an overview from large to finer scale concepts. The main focus is on biogeochemical cycling at different scales and the links between hydrology, geomorphology, and ecology with examples of complex river corridors such as the Danube River in Austria. The environmental template of complex river corridors is characterized by a shifting habitat mosaic of aquatic, amphibious and terrestrial habitats, ecotones and gradients encompassing biotic distribution and biogeochemical cycles. Most river corridors have, however, been drastically altered within the last century, creating deficits that have led to increasing rehabilitation and restoration activities over the last decade. An integrated approach including landscape dynamics as well as key ecosystem processes is required. The Danube Restoration Project near Vienna demonstrates how basic criteria for a successful restoration can be fulfilled. The fundamental aim of the project was to re-initiate key physical processes typical for braided rivers, namely hydrologic connectivity. To quantify ecosystem processes, an ecohydrological approach based on a detailed hydrological model was developed. Model outputs with a high predictive power are the metrics connectivity, giving the average duration of upstream surface connection, and "water age", a surrogate for residence time adapted to the multi-input system. Based on recent concepts and future management requirements, research ideas including experimental and empirical work in fluvial ecology under the pre-requisites of integrating different disciplines and using a landscape approach are presented.