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One of the consequences of human impacts on floodplains is a change in sedimentation leading to enhanced floodplain aggradation. Thus, accumulated sediments rich in nutrients might interfere with floodplain restoration. In this study we investigated the phosphorus release behavior of sediments from shallow backwaters of an isolated floodplain of the Danube River situated east of the city of Vienna with the aim to understand the effects of changes in dry/wet cycles on established floodplain sediments. In the light of restoration plans aiming at increased surface water exchange with the river main channel, the response of sediments to frequent alternations between desiccation and inundation periods is a key issue as changes of sediment properties are expected to affect phosphorus release. In order to determine the effect of changing hydrological conditions on internal phosphorus loading, we exposed sediments to different dry/wet treatments in a laboratory experiment. Total phosphorus (TP) release from sediments into the water column increased with increasing duration of dry periods prior to re-wetting. Partial correlation analysis showed significant positive correlations between ΔTP and ΔNH 4 + as well as between ΔTP and ΔFe 3 + concentrations (Δ refers to the difference between the final and initial concentration during the wetting period), indicating that enhanced mineralization rates leading to a concomitant release of NH 4 + and TP and the reduction of iron hydroxides leading to a concomitant release of Fe 3 + and TP are the mechanisms responsible for the rise in TP. Repeated drying and wetting resulted in elevated phosphorus release. This effect was more pronounced when drying periods led to an 80% reduction in water content, indicating that the degree of drying is a major determinant controlling phosphorus release upon re-wetting. The reconnection of isolated floodplains will favor fluctuating hydrologic conditions and is therefore expected to initially lead to high rates of phosphorus release from sediments.

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 )1 in 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.

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Dissolved and particulate organic matter (DOM and POM) distribution, lignin phenol signatures, bulk elemental compositions, fluorescence indices and microbial plankton (algae, bacteria, viruses) in a temperate river floodplain system were monitored from January to November 2003. We aimed to elucidate the sources and compositions of allochthonous and autochthonous organic matter (OM) in the main channel and a representative backwater in relation to the hydrological regime. Additionally, bacterial secondary production was measured to evaluate the impact of organic carbon source on heterotrophic prokaryotic productivity. OM properties in the backwater tended to diverge from those in the main channel during phases without surface water connectivity; this was likely enhanced due to the exceptionally low river discharge in 2003. The terrestrial OM in this river floodplain system was largely derived from angiosperm leaves and grasses, as indicated by the lignin phenol composition. The lignin signatures exhibited significant seasonal changes, comparable to the seasonality of plankton-derived material. Microbially-derived material contributed significantly to POM and DOM, especially during periods of low discharge. High rates of bacterial secondary production (up to 135 μg C L −1 d −1 ) followed algal blooms and suggested that autochthonous OM significantly supported heterotrophic microbial productivity.

Summary We investigated the effects of temporary surface water connections with the main river on the water chemistry of large and small waterbodies in a Danube floodplain by comparing hydrochemical data before, during and after water exchange with the Danube during two large floods in 2009 and 2013. In addition, we studied the effects of an increased frequency of flood pulses on the water chemistry of the floodplain via long‐term monitoring data. We used conservative (i.e. biologically non‐reactive) parameters (alkalinity, Ca, Cl, K, Mg, Na, SO4, conductivity) to identify hydrochemical changes due to the mixing with river water. Changes in non‐conservative parameters (PO4‐P, NO3‐N, NO2‐N, NH4‐N, total phosphorus, dissolved organic carbon, dissolved oxygen, chlorophyll‐a), which deviated from pure mixing effects, were ascribed to alterations of biogeochemical processes induced by the imported river water. In large floodplain waterbodies, both conservative and non‐conservative parameters changed towards Danube water concentrations during the connection with the main river and returned to pre‐connection levels after the water exchange ended. Only chlorophyll‐a concentrations were almost twice as high in the post‐connection phase than before the connection. In small floodplain waterbodies, conservative and non‐conservative parameters showed similar changes during the connection with the main river. However, the concentrations of PO4‐P, total phosphorus and chlorophyll‐a did not return to pre‐connection levels, but increased further in the post‐connection phase, exceeding even Danube water concentrations. We attribute these changes to phosphorus releases from the sediments and enhanced primary and bacterial production induced by the river water. In years with an increased flooding frequency, both conservative and non‐conservative parameters approached Danube water concentrations in large floodplain waterbodies due to the increased surface water exchange. In small floodplain waterbodies, conservative parameters remained unaffected by the increased connection with the river, while nitrate, ammonium, phosphate, total phosphorus and chlorophyll‐a concentrations were considerably higher in wet years than in dry years. We conclude that the temporary input of oxygen‐ and nutrient‐rich river water to small, mainly isolated backwaters can induce changes in benthic biogeochemical processes, which may provoke an internal eutrophication of these waterbodies.

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