Abstract
AbstractWe combined high‐frequency dissolved organic matter fluorescence (FDOM) data with stable isotope observations to identify the sources and ages of runoff that cause temporal variability in dissolved organic carbon (DOC) within a peat‐dominated Scottish catchment. FDOM was strongly correlated (r2 ∼ 0.8) with DOC, allowing inference of a 15 min time series. We captured 34 events over a range of hydrological conditions. Along with marked seasonality, different event responses were observed during summer depending on dry or wet antecedent conditions. The majority of events exhibited anticlockwise hysteresis as a result of the expansion of the riparian saturation zone, mobilizing previously unconnected DOC sources. Water ages from the main runoff sources were extracted from a tracer‐aided hydrological model. Particularly useful were ages of overland flow, which were negatively correlated with DOC concentration. Overland flow age, which ranged between 0.2 and 360 days, reflected antecedent conditions, with younger water generally mobilizing the highest DOC concentrations in summer events. During small events with dry antecedent conditions, DOC response was proportionally higher due to the displacement and mixing of small volumes of previously unconnected highly concentrated riparian soil waters by new precipitation. During large events with wet antecedent conditions, the riparian saturation zone expands to organic layers on the hillslopes causing peaks in DOC. However, these peaks were limited by dilution and supply. This study highlights the utility of linking high‐frequency DOC measurements with other tracers, allowing the effects of hydrologic connectivity and antecedent conditions on delivery of DOC to streams to be assessed.
Highlights
Dissolved organic carbon (DOC) is an important component of surface water quality in northern catchments, integrating hydrological, and biogeochemical processes that connect landscapes and riverscapes
The highest instantaneous discharge of 0.24 mm 15 min21 occurred in August 2014 in response to the highest single precipitation event, when 32.2 mm of rain fell in 1 day (Table 1)
We examined the link between DOC dynamics and tracers in a peat-dominated catchment in the Scottish Highlands with the overall goal of identifying the causes of temporal variation in DOC concentration
Summary
Dissolved organic carbon (DOC) is an important component of surface water quality in northern catchments, integrating hydrological, and biogeochemical processes that connect landscapes and riverscapes. In many northern temperate regions, DOC concentrations have been increasing over recent decades [Dawson et al, 2009; Monteith et al, 2007; Worrall and Burt, 2004] Such increases have potentially significant impacts on the carbon budget [Meybeck, 1993; Hope et al, 1997], ecosystem productivity [Tank et al, 2010], and the cost of treating drinking water [Ledesma et al, 2012]. Numerous interactive processes influence the production and transport of DOC in the landscape, which result in temporal variability in DOC concentration in streams spanning subhourly to annual (i.e., seasonal) time scales [Ågren et al, 2014; Ko€hler et al, 2008; Laudon et al, 2013]. The nature and connectivity of dominant flow paths within a catchment exert a strong influence on this temporal variability in stream water DOC, the connectivity between organic-rich soils and stream channels [Aitkenhead et al, 1999; Casper et al, 2003; Laudon et al, 2011; Chaplot and Ribolzi, 2014; Dick et al, 2015; Inamdar et al, 2011]
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