DOM quality in a peatland and forest headwater stream: seasonal and event characteristics

Other literature type English OPEN
Broder, Tanja ; Knorr, Klaus-Holger ; Biester, Harald (2016)

Peatlands and peaty riparian zones are major sources of DOM, but are poorly understood in terms of export dynamics and controls thereof. The 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 discharge from a bog and a forested peaty riparian zone, particularly considering drought and storm flow events. DOM quality was monitored using spectrofluorometric indices for aromaticity (SUVA<sub>254</sub>), apparent molecular size (S<sub>R</sub>) and precursor organic material (FI), as well as PARAFAC modeling of excitation emission matrices (EEMs). <br><br> Indices for DOM quality exhibited major changes due to different hydrologic conditions, but patterns were also dependent on season. The forested site with mineral, peaty soils generally exhibited higher variability in concentrations and quality compared to the outflow of an ombrotrophic bog. During snowmelt and spring events surface-near protein-like DOM pools were exported. During drought a microbial DOM fraction originating from groundwater and deep peat layers was exported, characterized by high FI, high S<sub>R</sub> and increasing humic-like fluorophores C1% or C4%. During discharge events this deep sourced DOM pool was diluted by humic-rich surface-near DOM pools, which were more aromatic, also of microbial origin with high FI and S<sub>R</sub>, and an increase in humic-like fluorophores C2% and C3% at the forested site. The FI suggested export of DOM of strong microbial alteration particularly under discharge events with dry preconditions. This might be due to accelerated microbial activity in the peaty riparian zone of the forested site. Our study demonstrated that DOM export dynamics are not only a passive mixing of different hydrological sources, but assessing DOM quality can greatly improve our understanding of DOM sources and their response under different hydrological preconditions.
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