Metabolism and Soil Water Viscosity Control Diel Patterns of Nitrate and DOC in a Low Order Temperate Stream

We investigated diel dissolved organic carbon (DOC) and nitrate (NO3−) dynamics in White Clay Creek, a third‐order stream in Pennsylvania, USA. High‐frequency DOC and NO3− concentrations were modeled from absorbance spectra collected with a deployable spectrophotometer. Periodicities of 24 hr in solute concentrations and streamflow were frequent throughout the year. To test potential drivers of diel oscillations, we focused on five periods with at least 5 days of clear diel changes that were not drastically influenced by high‐flow events. Maxima and minima of the diel variation in solute concentrations, and significant correlations between their daily amplitude and gross primary production (GPP) indicate that stream metabolism was the major driver diel oscillations in April (r = 0.86 and −0.87 for GPP vs. DOC and NO3−, respectively) and May (r = 0.86 and −0.90 for GPP vs. DOC and NO3−, respectively). In summer, drivers of daily oscillations of DOC and NO3− were unclear but likely influenced by stream metabolism. In winter, we observed no correlation between GPP and amplitude of daily concentration change, instead streamflow was the dominant driver via the viscosity effect (r = −0.92 and 0.89 for streamflow vs. DOC and NO3− respectively). As the hydraulic conductivity of the riparian soil increased throughout the daytime, the contribution of DOC‐rich and NO3−‐poor soil water to the stream relative to groundwater also increased. Results from this work highlight that watershed processes influence stream solutes on widely variable time scales, ranging from hours to centuries; and that underlying controls depend highly on seasonality. Plain Language Summary Dissolved organic carbon (DOC) and nitrate are key components of natural waters. Nitrate is an essential nutrient and DOC a major source of energy for aquatic microorganisms. Current field sensor technologies allow us to examine how these substances change temporally in streams and rivers at much greater resolution than manually‐collected samples. Using field sensors we measured DOC and nitrate concentrations in White Clay Creek at high frequency (5‐min intervals) during 2018. Our focus was to investigate how these solutes behaved at baseflow conditions, when the influence of storm events is minimal. We found that both DOC and nitrate concentrations showed diel (24‐hr periods) oscillations with patterns that varied across seasons. In the spring, the diel changes were largely the result of the metabolic activ