Seasonal pattern of dissolved organic matter (DOM) in watershed sources: influence of hydrologic flow paths and autumn leaf fall
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Seasonal patterns of dissolved organic matter (DOM) were evaluated for multiple watershed sources and stream water during baseflow and stormflow to investigate the influence of hydrologic flow paths and key phenological events. Watershed sources sampled were throughfall, litter leachate, soil water, and deep groundwater. DOM data for a 4-year period (2008–2011) included: DOC concentrations and spectrofluorometric indices such as a254, humification index, protein-like and humic-like DOM. Seasons were defined as—winter (December–February), spring (March–May), summer (June–September) and autumn (October and November). Seasonal differences in DOM were most pronounced for surficial flow paths (e.g., stormflow, litter leachate, throughfall and soil water) but muted or absent for groundwater and baseflow. This was attributed to the loss of DOM by sorption on mineral soil surfaces and/or microbial breakdown. DOM in summer stormflow had higher DOC concentrations and was more humic in character versus DOM in spring and winter runoff. Storm events in early autumn produced a sharp increase in DOC concentrations and % protein-like DOM for stream waters and litter leachate. Elevated DOC concentrations for early spring throughfall were attributed to leaching of organic exudates associated with leaf emergence. Our results underscore that watershed and ecosystem studies need to pay a greater attention to surficial flow paths and runoff sources (including stormflow) for understanding seasonal patterns of DOM. Understanding the influence of phenological episodes such as autumn leaf-fall for DOM is important considering that these transitional events may be especially affected by climate change.
KeywordsOrganic carbon Watersheds Phenology Hydrologic flow paths Seasonality Climate change Storm events
This study was funded through a grant from the National Science Foundation (NSF, Hydrologic Sciences Program, EAR-0809205). We would like to thank the Fair Hill NRMA staff for providing access and security for the study site. We are grateful to the many graduate students who assisted with sampling and watershed instrumentation including Gurbir Dhillon, Sudarshan Dutta, and Rachael Vaicunas. Finally, we thank the editors and reviewers for their very constructive and helpful comments.
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