Abstract
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 river-floodplain 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.
Highlights
River-floodplain systems often have a broad range of lotic, lentic and semi-aquatic habitats with high species richness (Ward and Stanford 2006)
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)
Our results confirm that the flood introduced into the backwaters dissolved organic matter (DOM) that consisted of more allochthonously derived, aromatic compounds, as has been found in other rivers (Gueguen et al 2006) and wetlands (Hunsinger et al 2010)
Summary
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, the linkage between the hydrological cycle and the carbon cycle (Battin et al 2008). One of the dominant forms of organic material in aquatic environments, including floodplains, is dissolved organic matter (DOM). Allochthonous input of organic material is considered to be the dominant
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