Spatio-temporal diel DOC cycles in a wet, low energy, northern catchment: Highlighting and questioning the sub-daily rhythms of catchment functioning

Sub-daily variations in the rates and dominance of the main controls of stream dissolved organic carbon (DOC) concentration (production, mobility and instream processes) have the potential to create a subtle sub-daily rhythm of DOC variation in streams. We used high-frequency data, covering the spring-summer-autumn period, which included discharge, specific conductivity, pH, groundwater levels, temperature, evapotranspiration and solar radiation to investigate the interplay between factors potentially driving diel DOC cycles in northern catchments. We focused on a peatland dominated 1st order stream (0.65 km²) before investigating the propagation of the signals downstream to a 2nd order stream (3.2 km²), with a lower percentage of peat fringing the stream channel. DOC cycles in the 1st order stream had a median peak time of 14:00 h and temporally varying amplitude, with a median of 0.61 mg l⁻¹. Results supported the hypothesis that diel DOC cycles at the site are driven by hydrological processes, specifically the viscosity-effect theory: viscosity-driven increases in flow from the riparian area in the afternoon flush DOC from the peat to the stream. The temporal variability in the amplitude of the diel DOC cycle was controlled by antecedent temperature. Downstream, the diel DOC signal was weaker, with around 4-fold lower amplitudes and minima in the afternoon. The lower proportion of riparian peat downstream appeared to reduce the influence of terrestrial processes on DOC cycles. In-stream photodegradation and decomposition likely became more dominant as connectivity between DOC sources and stream reduced. The study highlighted that even in climates such as the Scottish Highlands, where energy input is relatively low and precipitation frequent, sub-daily hydrological and biogeochemical rhythms occur. Unravelling the intricacy of such diel cycles is fundamental to fully understanding stream functioning and the global carbon cycle.