Sources and composition of organic matter for bacterial growth in a large European river floodplain system (Danube, Austria)
Introduction
Riverine dissolved and particulate organic matter (DOM and POM) are important components of the global carbon cycle and are the primary drivers of ecosystem functions in freshwater environments (Hedges et al., 2000, Battin et al., 2008). Production and transformation of OM in streams and rivers render the broad range of molecular forms typical for organic material in freshwater systems (Kaplan and Bott, 1989, Kim et al., 2006). Sources and properties of OM are key in controlling microbial processing and carbon cycling in aquatic ecosystems (Kaplan and Bott, 1989). Allochthonous OM from the catchment is often thought to prevail over autochthonous material derived from aquatic primary producers (Ertel et al., 1986, Battin, 1998), whereas carbon released from algae is typically more available for heterotrophic microorganisms (Azam and Cho, 1987, Kaplan and Bott, 1989).
Various environmental factors maintain complex temporal and spatial patterns among autochthonous and allochthonous OM fractions in aquatic systems. Autochthonous, microbially-produced carbon typically shows a pronounced seasonality due to phytoplankton growth driven by light, temperature and nutrient supply (Hein et al., 1999, Kirschner and Velimirov, 1999). Inputs and properties of terrestrially derived OM are determined by land cover of the catchment, riparian vegetation, upland soil profiles (Ertel et al., 1986, Hedges et al., 1986, Eckard et al., 2007) and precipitation and runoff (Buffam et al., 2001, Dalzell et al., 2007). Until recently, it was agreed that most of the terrestrially-derived organic carbon that enters running waters is already largely degraded in upland soils and is transported conservatively along streams and rivers (Ertel et al., 1986, Hedges et al., 1986). Recent studies indicate, however, that river OM is less recalcitrant and has a greater bioavailability than previously thought (Bianchi et al., 2004, Mayorga et al., 2005, Duan and Bianchi, 2006, Hernes et al., 2007, Battin et al., 2008).
In natural river floodplain systems and wet lands, lentic and lotic aquatic habitats cover a broad range of DOM and POM concentration and composition (Aspetsberger et al., 2002, Hein et al., 2003, Schiemer et al., 2006), that are strongly dependent on a dynamic hydrological regime. Hydrology affects OM properties via the connectivity between the main channel, backwaters (e.g. old tributaries and distributaries) and the surrounding watershed, which ultimately control the transfer of terrestrial and aquatic-derived OM in this ecosystem (Hedges et al., 1986, Tockner et al., 1999). Several studies focused on the flux of OM through river floodplain systems (Aspetsberger et al., 2002, Hein et al., 2003) and the importance of hydrology for phytoplankton development and transport (Hein et al., 1999). While characterization of dissolved organic carbon (DOC) and data concerning the terrestrial component exist for the catchments of large rivers (Ertel et al., 1986, Hedges et al., 2000, Onstad et al., 2000, Bianchi et al., 2004, Duan et al., 2007a, Duan et al., 2007b) and marine environments (Moran and Hodson, 1994, Opsahl and Benner, 1997), few studies have examined the sub-habitats in river floodplain systems.
Here we report on a study concerning sources and composition of DOM and POM in the main channel and the backwaters of one of the largest semi-natural floodplains in Europe, the National Park of the river Danube, Austria. The general working hypothesis is that algal productivity is the primary driver of microbial production, while terrestrially-derived OM provides a stable background source of OM for bacterial growth. Our main objectives were to investigate the dynamic changes in OM concentration, sources and properties with respect to hydrological connectivity. The terrigenous component of OM was characterized via its lignin phenol signature. Lignin oxidation products provide a tracer for the source of vascular plant derived OM and for the extent of alteration that has occurred by way of microbial or photochemical degradation (Ertel et al., 1986, Hedges et al., 1986, Opsahl and Benner, 1995) or by leaching/sorption processes (Hernes et al., 2007). C:N ratio, δ13C values of POM and fluorescence index of DOM (McKnight et al., 2001) were applied to characterize chemical properties. We monitored the abundance of algae, bacteria and viruses to estimate the importance of microbially derived material to aquatic OM. Furthermore, bacterial secondary production was measured to detect possible correlations between organic carbon sources and heterotrophic prokaryotic activity. Finally, the hydrodynamics of the main channel of the river Danube were compared with a dynamically connected and semi-natural backwater.
Section snippets
Site description and hydrological conditions
The National Park of the river Danube (Austria) includes the free flowing section of the river downstream of Vienna and its backwaters. At this location, the Danube drains a 104,000 km2 area. The hydrological conditions are governed by an alpine regime, resulting in highly variable flow and highest water level in early summer (Schiemer et al., 1999). The local vegetation of the Donauauen National Park is dominated by deciduous forest (65%), with grassland comprising 15%. The catchment is largely
Characterization of hydrological and hydrochemical conditions
The year 2003 was characterized by exceptional low discharge and water levels below mean water, despite Jan. and Feb. and a short spate in Nov. Full surface water connectivity was hardly established that year (Fig. 1) and water exchange between the main channel and the backwater was largely limited to seepage. Discharge in the backwater ranged from 0.2 to 12.3 m3 s−1 (Table 1). The water temperature was between 3 °C and 22 °C in the main channel and between 4 °C and 27 °C in the backwater (Fig. 1).
Discussion
The extremely hot and dry year of 2003 was likely the cause of the restricted surface water connection between the backwater and the main channel. This is in contrast to the usual high water level found in spring and early summer in the Danube (Schiemer et al., 1999, Tockner et al., 1999). The lack of significant connectivity resulted in apparently unrelated variation in POM properties (POC concentration and δ13C values) in the two water bodies (Fig. 2a and b). At very low water levels the
Acknowledgements
We thank the National Park Authority and the Austrian River Authority for permission to conduct our research in the Danube Alluvial Zone National Park. We thank W. Wanek and the Department of Chemical Ecology and Ecosystem Research for stable isotope and elemental analyses and colleagues from Department of Freshwater Ecology for help in the field and in the lab. We are grateful for valuable suggestions from two anonymous reviewers. The work was supported by grants from the Austrian Science Fund
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