Abstract
Streams and rivers transport dissolved organic matter (DOM) from the terrestrial environment to downstream ecosystems. In light of climate and global change it is crucial to understand the temporal dynamics of DOM concentration and composition, and its export fluxes from headwaters to larger downstream ecosystems. We monitored DOM concentration and composition based on a diurnal sampling design for 3 years in an Alpine headwater stream. We found hydrologic variability to control DOM composition and the coupling of DOM dynamics in the streamwater and the hyporheic zone. High‐flow events increased DOM inputs from terrestrial sources (as indicated by the contributions of humic‐ and fulvic‐like fluorescence), while summer baseflow enhanced the autochthonous imprint of DOM. Diurnal and seasonal patterns of DOM composition were likely induced by biological processes linked to temperature and photosynthetic active radiation (PAR). Floods frequently interrupted diurnal and seasonal patterns of DOM, which led to a decoupling of streamwater and hyporheic water DOM composition and delivery of aromatic and humic‐like DOM to the streamwater. Accordingly, DOM export fluxes were largely of terrigenous origin as indicated by optical properties. Our study highlights the relevance of hydrologic and seasonal dynamics for the origin, composition and fluxes of DOM in an Alpine headwater stream.
Highlights
Streams and rivers transport dissolved organic matter (DOM) from the terrestrial environment to downstream ecosystems
This study emphasizes the role of hydrology, seasonal, and diurnal variability for DOM composition and export from an Alpine stream
Our findings on DOM source partitioning with varying discharge and in-stream biological processes expand current knowledge on carbon dynamics in stream ecosystems (e.g., Raymond and Saiers 2010; Inamdar et al 2012; Singh et al 2014)
Summary
Streams and rivers transport dissolved organic matter (DOM) from the terrestrial environment to downstream ecosystems. Headwaters, the smallest and most abundant streams in fluvial networks, are tightly connected with the terrestrial milieu from which they receive dissolved organic matter (DOM) that they transform or export to downstream ecosystems and to the oceans (Battin et al 2008). Near-stream soils often rich in organic carbon can act as a DOM source to streams during elevated precipitation and snowmelt Reduced residence times and dilution effects due to elevated water level may depress in situ heterotrophic metabolism and, increase downstream subsidies of labile DOM during storms (Battin et al 2008). Relatively little is known about the DOM dynamics between storms (Vazquez et al 2012; Wyatt et al 2014)
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