Abstract
Abstract. Peatlands and peaty riparian zones are major sources of dissolved organic matter (DOM), but are poorly understood in terms of export dynamics and controls thereof. Thereby quality of DOM affects function and behavior of DOM in aquatic ecosystems, but DOM quality can also help to track DOM sources and their export dynamics under specific hydrologic preconditions. The objective of this study was to elucidate controls on temporal variability in DOM concentration and quality in stream water draining a bog and a forested peaty riparian zone, particularly considering drought and storm flow events. DOM quality was monitored using spectrofluorometric indices for aromaticity (SUVA254), apparent molecular size (SR) and precursor organic material (FI), as well as PARAFAC modeling of excitation emission matrices (EEMs). Indices for DOM quality exhibited major changes due to different hydrologic conditions, but patterns were also dependent on season. Stream water at the forested site with mineral, peaty soils generally exhibited higher variability in DOM concentrations and quality compared to the outflow of an ombrotrophic bog, where DOM was less susceptible to changes in hydrologic conditions. During snowmelt and spring events, near-surface protein-like DOM pools were exported. A microbial DOM fraction originating from groundwater and deep peat layers was increasing during drought, while a strongly microbially altered DOM fraction was also exported by discharge events with dry preconditions at the forested site. This might be due to accelerated microbial activity in the peaty riparian zone of the forested site under these preconditions. Our study demonstrated that DOM export dynamics are not only a passive mixing of different hydrological sources, but monitoring studies have to consider that DOM quality depends on hydrologic preconditions and season. Moreover, the forested peaty riparian zone generated the most variability in headwater DOM quantity and quality, as could be tracked by the used spectrofluorometric indices.
Highlights
Dissolved organic matter (DOM) is ubiquitous in soils and aqueous ecosystems
Variability in stream water dissolved organic matter (DOM) quantity and quality was primarily generated at the forested site with peaty riparian zones and not at the bog site
Thereby, changes in headwater DOM quality were mainly induced by hydrologic conditions, which points out the importance of high-resolution studies and consideration of high-discharge events, which generate the highest DOC concentrations, but export different DOM pools with different chemical properties and fate in aquatic systems
Summary
Dissolved organic matter (DOM) is ubiquitous in soils and aqueous ecosystems It plays a fundamental role in surface water chemistry, e.g., in metal bioavailability and mobility (Tipping et al, 2002), nutrient cycling (Jansson et al, 2012), pH buffering and ionic balance (Hruška et al, 2003). It affects light penetration (Karlsson et al, 2009), the aquatic food web structure (Jansson et al, 2007), is an energy source for microbial metabolism (Cole et al, 2007; Amon and Benner, 1996) and is part of the carbon cycle (Cole et al, 2007). DOM concentrations and characteristics can vary largely over time due to seasonal changes in production, consumption and transport of DOM
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