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

Abstract

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.

Mimicking floodplain reconnection and disconnection using <sup>15</sup>N mesocosm incubations

“…A similar trend in higher denitrification rates has also been documented for wetlands connected to the Po River (Racchetti et al, 2010). Higher denitrification rates (94 mg N m −2 h −1 , ca.…”

“…In this study, the restored site experienced an increase in denitrification rates compared to the disconnected site, suggesting that increasing the continuous and long-lasting surface water connection periods will increase the overall denitrification rate as well as its efficiency (Kjellin et al, 2007;Klocker et al, 2009;Racchetti et al, 2010). Yet, as shown in Experiment 1, increasing nitrate concentration does not lead to higher denitrification rates in short time.…”

“…Nitrate was kept in constant abundance in the experiment, but NO − 2 increased throughout the incubation period, a result of NO − 3 reduction. DOM originating from the Danube River has been shown to be a mixture of terrestrially and microbially derived sources, depending on the discharge and season (Besemer et al, 2009;Peduzzi et al, 2008;Preiner et al, 2008). Previous studies have suggested that OC is primarily derived from terrestrial sources (dominated by protein-like signatures) during average flow conditions (Hein et al, 2004).…”

Spatial variability in floodplain sedimentation: the use of generalized linear mixed-effects models

“…Focussing on organic carbon (TOC) and nitrogen (TN), deposition during overbank floods is an important ecosystem function which provides important benefits as water quality enhancement or mitigation of greenhouse effect (Johnston, 1991;Day et al, 2004;Verhoeven et al, 2006, IPCC, 2007. At the reach scale, TOC and TN exchange between the main channel and its adjacent floodplain plays a key role in the ecological functioning (Junk, 1999;Robertson et al, 1999;Tockner et al, 1999;Tockner at al., 2000;Thoms, 2003;Knosche, 2006;Preiner et al, 2008). Previous research has shown how human-induced changes at the basin and reach scale have decreased the potential of riverine floodplains to act as sediment-associated nutrient sinks (Noe and Hupp, 2005;Owens et al, 2005;Pierce and King, 2008;Cabezas et al, 2009;Cabezas and Comin, 2010).…”

Effects of Timber Harvesting with Best Management Practices on Ecosystem Metabolism of a Low Gradient Stream on the United States Gulf Coastal Plain

“…Reaeration is strongly influenced by turbulent mixing [37], and because of the low velocities in a stream with a relatively moderate cross sectional area (approximately 3.5 m in width by 0.5 m in depth), we assumed the reaeration coefficient to be zero, i.e., disregarding stream reaeration caused by water movement. We acknowledge that neglecting the coefficient may result in lower estimation of metabolic rates, as some studies have shown that CO 2 exchange occurs between the atmosphere and still water in lakes [38] and river backwater [39]. The primary purpose of our study is to investigate potential harvest effect on stream metabolism, rather than give a precise quantification of metabolic rates.…”