Spatial changes in instream DOM quality under base flow conditions linked with different DOM sources in small forested catchments

Dissolved organic matter (DOM) plays an important role in aquatic systems controlling metal bioavailability and mobility, and nutrient cycling. The quantity and quality of instream DOM determine its role and behavior in aquatic systems. Headwater streams are particularly sensitive systems to study the changes in DOM dynamics due to their immediate interface with adjoining hillslopes and the high soil-to- water-area ratio. However, the controls, sources and mobilization processes of DOM export in natural forested catchments are still poorly understood. The objective of this study is to combine high temporal resolution spectroscopic DOM analysis with high-resolution mass spectrometry data to characterize the spatial changes in DOM composition along a low-order stream in a forested catchment (Bavarian Forest National Park - Germany) during base flow conditions. Furthermore, the study aims to link the patterns found instream with fingerprints of DOM end member sources. DOM quality was monitored over 1.5 year in three sites along the stream using absorption indices indicators of aromaticity (SUVA₂₅₄) and molecular weight (E₂:E₃) and data aggregating quality parameters and intensity based on formulas derived from discrete samples analyzed by FT-ICR-MS. Additionally, three end members corresponding to DOM in deep ground water, shallow ground water and water in the top layer of the soil were sampled in the catchment.

At base flow conditions, no significant changes in DOC concentration were observed spatially. Yet, absorption indices from DOM exhibited clear spatial patterns, with higher aromatic and lower molecular weight DOM at the lower floodplain compared to upstream. From the FT-ICR-MS data, however, high aggregate quality data showed a small gradient in DOM quality between the upper and lower parts of the catchment, with the relationship between DOC concentration and the relative intensity (RI) of molecular formulas being a better descriptor of the spatial changes. The patterns observed in formulas with an increase or decrease in RI at higher DOC concentrations is indicative of changes in DOM sources between upper and lower parts of the catchment. The characterization of end members could further elucidate the observed changes in RI of formulas. In the studied catchment, formulas with a decrease in RI at higher DOC concentration agree with the formulas most commonly found for DOM from the deep ground water, whereas the formulas being enriched at higher DOC concentration changed along the stream. At the upper part of the catchment, these formulas are the ones most abundantly present at the shallow ground water, whereas at the lower part these formulas are also representative of formulas found in the sample collected at the superficial layer of the soil. The monitoring of DOM amount and quality in the small forested catchments showed that DOM spatial variability is connected with the availability and mobilization of different sources, even during base flow conditions. The use of spectrophotometers allowed us to identify general trends in DOM quality and concentration, while FT-ICR-MS data was crucial to characterize DOM quality and link the findings instream with DOM sources.