The significance of side‐arm connectivity for carbon dynamics of the River Danube, Austria

Summary 1. Side‐arms connected to the main stem of the river are key areas for biogeochemical cycling in fluvial landscapes, exhibiting high rates of carbon processing. 2. This work focused on quantifying autochthonous and allochthonous carbon pools and, thereby, on comparing transport and transformation processes in a restored side‐arm system of the River Danube (Regelsbrunn). We established a carbon budget and quantified carbon processing from March to September 2003. In addition, data from previous studies during 1997 to 1999 were assessed. 3. Gross primary production (GPP) and community respiration were estimated by diel oxygen time curves and an oxygen mass balance. Plankton primary production was determined to estimate its contribution to GPP under different hydrological conditions. 4. Based on the degree of connectivity, three hydrological phases were differentiated. Most of the organic matter, dominated by allochthonous carbon, was transported in the main channel and through the side‐arm during floods, while at intermediate and low flows (and thus connectivity), transformation processes became more important and autochthonous carbon dominated the carbon pool. The side‐arm system functioned as a sink for particulate matter [total suspended solids and particulate organic carbon (POC)] and a source of dissolved organic carbon (DOC) and chlorophyll‐a. 5. Autochthonous primary production of 4.2 t C day−1in the side‐arm was equivalent to about 20% of the allochthonous inputs of 20 t C day−1(POC and DOC) entering the area at mean flow (1% of the discharge of the main channel). Pelagic photosynthesis was generally high at mean flow (1.3–3.8 g C m−2 day−1), and contributed up to 90% of system productivity. During long stagnant periods at low discharge, the side‐arm was controlled by biological processes and a shift from planktonic to benthic activity occurred (benthic primary production of 0.4–14 g C m−2 day−1). 6. The transformation of the organic matter that passes through the side‐arm under different hydrological conditions, points to the importance of these subsystems in contributing autochthonous carbon to the food web of the main channel.