High-frequency measurements explain quantity and quality of dissolved organic carbon mobilization in a headwater catchment

Abstract

Abstract. Increasing dissolved organic carbon (DOC) concentrations and exports from headwater catchments impact the quality of downstream waters and pose challenges to water supply. The importance of riparian zones for DOC export from catchments in humid, temperate climates has generally been acknowledged, but the hydrological controls and biogeochemical factors that govern mobilization of DOC from riparian zones remain elusive. A high-frequency dataset (15 min resolution for over 1 year) from a headwater catchment in the Harz Mountains (Germany) was analyzed for dominant patterns in DOC concentration (CDOC) and optical DOC quality parameters SUVA254 and S275−295 (spectral slope between 275 and 295 nm) on event and seasonal scales. Quality parameters and CDOC systematically changed with increasing fractions of high-frequency quick flow (Qhf) and antecedent hydroclimatic conditions, defined by the following metrics: aridity index (AI60) of the preceding 60 d and the quotient of mean temperature (T30) and mean discharge (Q30) of the preceding 30 d, which we refer to as discharge-normalized temperature (DNT30). Selected statistical multiple linear regression models for the complete time series (R2=0.72, 0.64 and 0.65 for CDOC, SUVA254 and S275−295, resp.) captured DOC dynamics based on event (Qhf and baseflow) and seasonal-scale predictors (AI60, DNT30). The relative importance of seasonal-scale predictors allowed for the separation of three hydroclimatic states (warm and dry, cold and wet, and intermediate). The specific DOC quality for each state indicates a shift in the activated source zones and highlights the importance of antecedent conditions and their impact on DOC accumulation and mobilization in the riparian zone. The warm and dry state results in high DOC concentrations during events and low concentrations between events and thus can be seen as mobilization limited, whereas the cold and wet state results in low concentration between and during events due to limited DOC accumulation in the riparian zone. The study demonstrates the considerable value of continuous high-frequency measurements of DOC quality and quantity and its (hydroclimatic) key controlling variables in quantitatively unraveling DOC mobilization in the riparian zone. These variables can be linked to DOC source activation by discharge events and the more seasonal control of DOC production in riparian soils.

Low hydrological connectivity after summer drought inhibits DOC export in a forested headwater catchment

“…For further analysis, the calibrated values (R 2 for HS = 0.98 for all events, R 2 for MG = 0.77 for the events in 2018 and 2019 and R 2 = 0.97 for the event in 2020) were used. As no drift of the DOC concentration could be identified in the measured signal, we decided against a correction for biofouling as done by Werner et al (2019). However, the sensor optics were manually cleaned in the field every 2 weeks using cotton swabs.…”

“…DOC can form complexes with organic pollutants (Hope et al, 1994) and toxic metals such as mercury (Ravichandran, 2004) or lead (Dörr and Münnich, 1991) and, thus, strongly influence drinking water quality. Several hypotheses have been proposed in order to explain the increase in DOC in many surface waters, including a decline in atmospheric sulfur deposition (Evans et al, 2006;Monteith et al, 2007;Ledesma et al, 2016;Hruška et al, 2009), a decline in nitrogen deposition (Musolff et al, 2016), temperature increase (Weyhenmeyer and Karlsson, 2009) and increased precipitation (Hongve et al, 2004). However, Roulet and Moore (2006) stress that it is difficult to isolate a single factor, as there are many different variables influencing DOC production and export, and Clark et al (2010) highlight the difficulties resulting from the fact that drivers of rising DOC concentrations operate on varying temporal and spatial scales.…”

Missing connectivity during summer drought controls DOC mobilization and export in a small, forested catchment

“…For further analysis the calibrated values (R 2 for HS = 0.98, R 2 for MG = 0.77) were used. As no drift of the DOC concentration could be identified in the measured signal, we decided against a correction for biofouling as done by Werner et al (2019). However, the sensor optics were manually cleaned in the field every two weeks using cotton swabs.…”

Hydrology and riparian forests drive carbon and nitrogen supply and DOC:NO3− stoichiometry along a headwater Mediterranean stream

“…Under these circumstances, soil moisture will be high and lateral transmissivity will be relatively larger due to increased hydrological connectivity between the riparian zone and the rest of the catchment. In addition, during large storms preceded by wet conditions, soluble DOC and NO3pools in the riparian soils can be depleted from precedent storm events (Butturini et al, 2003;Zimmer and McGlynn, 2018;Werner et al, 2019), eventually leading to relatively lower increments in stream DOC and NO3concentrations (Fig. 7b).…”

“…CC BY 4.0 License. strongly influencing the supply of C and N to streams (Àvila et al, 1992;Fovet et al, 2015;Zimmer and McGlynn, 2018;Werner et al, 2019;Wen et al, 2020). Resulting stream DOC and NO3concentrations and DOC:NO3stoichiometry might thus significantly differ between base flow and storm flow conditions, as well as among storms depending on antecedent accumulated precipitation, and thereafter influence in-stream DOC and NO3availability for microbial communities.…”