Seasonal dynamics of diurnal fluctuations of in-stream dissolved organic carbon concentrations and quality metrics in a forested headwater catchment

Diurnal fluctuations of hydrological processes, such as discharge and groundwater level, and of in-stream concentrations of various solutes have been observed in many catchments. The timing of minima and maxima of hydrometric and hydrochemical parameters during the 24 hour cycle can be used to elucidate the baseflow dynamics of catchment hydrological processes and of the mobilization of solutes into streams. In general, diurnal fluctuations of discharge and in-stream solute concentrations have been related to effects of evapotranspiration, temperature-controlled viscosity changes of water, freeze-thaw cycles and a temperature-dependent increase of biological activity during the day. The aim of this study was to better understand the seasonal and topography-driven release of dissolved organic carbon (DOC) during inter-event periods in a small forested headwater catchment within the Bavarian Forest National Park (Germany). We analyzed DOC concentrations and DOC absorbance metrics (as an indicator for DOC quality characteristics) at three topographically different positions of the headwater stream in high frequency by means of in-situ UV-Vis spectrometry over the period of two years. Our data show distinct seasonal differences in the amplitude of diurnal fluctuations of discharge as well as DOC concentrations that are accompanied by clear differences in DOC absorbance characteristics. The timing of diurnal minima and maxima of discharge and DOC concentrations changes over the seasons and along the stream. We present a comprehensive analysis of diurnal fluctuations of discharge, DOC concentrations and DOC quality metrics as influenced by season and topographical position and relate this to findings from other research studies. Disentangling the patterns and dynamics of diurnal variations of hydrological and biogeochemical variables is crucial for fully understanding catchment functioning and the export of carbon from terrestrial catchments as one component of the global carbon budget, particularly because extended drought (i.e., baseflow) periods are forecast to occur more often as a consequence of climate change.