Origin, enzymatic response and fate of dissolved organic matter during flood and non-flood conditions in a river-floodplain system of the Danube (Austria)

Anna Sieczko 0 Peter Peduzzi 0 0 A. Sieczko P. Peduzzi (&) Department of Limnology and Oceanography, University of Vienna , Althanstrasse 14, 1090 Vienna, Austria Spectroscopic techniques and extracellular enzyme activity measurements were combined with assessments of bacterial secondary production (BSP) to elucidate flood-pulse-linked differences in carbon (C) sources and related microbial processes in a riverfloodplain system near Vienna (Austria). Surface connection with the main channel significantly influenced the quantity and quality of dissolved organic matter (DOM) in floodplain backwaters. The highest values of dissolved organic carbon (DOC) and chromophoric DOM (CDOM) were observed during the peak of the flood, when DOC increased from 1.36 to 4.37 mg l-1 and CDOM from 2.94 to 14.32 m-1. The flood introduced DOC which consisted of more allochthonously-derived, aromatic compounds. Bacterial enzymatic activity, as a proxy to track the response to changes in DOM, indicated elevated utilization of imported allochthonous material. Based on the enzyme measurements, new parameters were calculated: metabolic effort and enzymatic indices (EEA 1 and EEA 2). During connection, bacterial glucosidase and protease activity were dominant, whereas during disconnected phases a switch to lignin degradation (phenol oxidase) occurred. The enzymatic activity analysis revealed that flooding mobilized reactive DOM, which then supported bacterial metabolism. No significant differences in overall BSP between the two phases were detected, indicating that heterogeneous sources of C sufficiently support BSP. The study demonstrates that floods are important for delivering DOM, which, despite its allochthonous origin, is reactive and can be effectively utilized by aquatic bacteria in this river-floodplain systems. The presence of active floodplains, characterized by hydrological connectivity with the main channel, creates the opportunity to process allochthonous DOC. This has potential consequences for carbon flux, enhancing C sequestration and mineralization processes in this river-floodplain system. - River-floodplain systems often have a broad range of lotic, lentic and semi-aquatic habitats with high species richness (Ward and Stanford 2006). Such environmental diversity creates opportunities for numerous linkages between the aquatic and terrestrial ecosystem and promotes interactions between abiotic factors and biotic processes. These couplings have received insufficient attention, particularly the linkage between the hydrological cycle and the carbon cycle (Battin et al. 2008). In particular, floodplain backwaters are seldom included in calculations of global carbon flux; they play a significant role in organic matter cycling and carbon mineralization, sequestration and transport (Battin et al. 2008). One of the dominant forms of organic material in aquatic environments, including floodplains, is dissolved organic matter (DOM). DOM is a heterogeneous mixture of molecules, some of which originate from the surrounding area (allochthonous DOM), while others are generated within the system (autochthonous DOM). Allochthonous input of organic material is considered to be the dominant substrate supply for microorganisms in inland waters (Tranvik and Jansson 2002). Most DOM of terrestrial origin is believed to enter streams (Royer and David 2005), tropical rivers (Wiegner et al. 2009) and floodplains (Peduzzi et al. 2008) during seasonal floods and storms. Bacterial utilization of DOM can be highly variable, depending on the lability of the organic matter. In rivers (del Giorgio and Pace 2008) and floodplains (Mladenov et al. 2007), bacterial metabolism may be supported by two different pools: quickly cycling DOM together with more slowly utilized semi-labile DOM. The highly labile pool, typically linked to phytoplankton development, is often rapidly exhausted. In contrast, the semi-labile pool appears to be strongly coupled to terrestrial DOC, which increases after flood events (del Giorgio and Pace 2008). This implies that aquatic food webs are coupled to watershed inputs of organic carbon (Pace et al. 2004). Some lake carbon cycles are reported to be heavily subsidized by organic carbon from the surrounding landscape (Carpenter et al. 2005; Cole et al. 2011). The importance of terrestrial material for bacterial metabolism has been emphasized in boreal headwater streams (Berggren et al. 2009), temperate lakes (Cole et al. 2006; Carpenter et al. 2005; Pace et al. 2004), oligotrophic humic lakes (Daniel et al. 2005) and estuaries (Van den Meersche et al. 2009). Aquatic heterotrophic bacteria may act as links that transfer organic carbon to higher trophic levels, and/or sinks that mineralize DOM, resulting in CO2 production. Numerous studies have coupled bacterial respiration to allochthonous sources, underlining that little of the terrestrial C is passed to higher trophic levels (Cole et al. 2002; Kritzberg et al. 2005; Karlsson et al. 2007). Other evidence, however, suggests that autochthonous organic matter may be significantly respired as well (McCallister and Giorgio 2008). Among the many environmental factors (nutrient availability, temperature, photochemical impacts) (McCallister et al. 2005; Apple et al. 2006) that influence substrate partitioning between biomass generation and respiration, also seasonal disturbance events (floods, storms etc.) clearly affect bacterial processes. Flood pulses deliver substantial amounts of bioavailable DOM that may stimulate bacterial metabolism, for example in tropical rivers (Wiegner et al. 2009), streams (A gren et al. 2008) and lakes (Lennon and Cottingham 2008). These sudden inputs of different carbon sources may increase microbial productivity (Docherty et al. 2006), respiration and rates of biogeochemical cycles (Kritzberg et al. 2005). Hydrological connectivity appears to be a crucial driving force in river-floodplain systems as well, influencing the quantity and quality of DOM and the related microbial processes (Schiemer et al. 2006; Mladenov et al. 2007; Peduzzi et al. 2008). In such highly variable water bodies, DOM bioavailability and the fate of utilized DOM, i.e. its partitioning into bacterial catabolic and anabolic processes, remains largely unresolved. Therefore, the factors determining DOM bioavailability in river-floodplain systems need to be better understood, as does the balance between anabolic and catabolic processes. Losses and changes of organic matter due to microbial degradation have been assessed in several ways. One approach applies spectroscopic techniques to characterize and trace DOM dynamics (McKnight et al. 2001; Weishaar et al. 2003). This method relates the optical properties of DOM to its chemical composition, thus inferring DOM bioavailability. Nonetheless, characterizing DOM based solely on absorbance and fluorescence techniques has drawbacks. These include overlooking the highly reactive and quickly c (...truncated)