Limited Diel Variability of Greenhouse Gases, Carbon and ~60 Major and Trace Solutes in Humic Northern Peatland Headwaters

Small peatland waters are important interfaces for carbon transfer to the atmosphere and downstream networks, yet the extent to which daytime sampling represents a complete diel cycle remains uncertain. We conducted continuous 51-h campaigns in August in five headwater water bodies of a permafrost-free ombrotrophic bog and a discontinuous-permafrost peatland in northeastern Europe. Hourly physical measurements and 3-hourly analyses of dissolved organic and inorganic carbon, CO₂ and CH₄, nutrients, and approximately 60 major and trace solutes were obtained under stable anticyclonic conditions. The operational night intervals lasted approximately 4–7 h but were dominated by twilight rather than full astronomical darkness. Water temperature and dissolved oxygen showed the clearest diel structure, whereas most dissolved constituents displayed small amplitudes and no consistent day–night phase. Across the available major- and trace-element/site combinations, 93% of mean day/night ratios were between 0.8 and 1.2. Dissolved CO₂ and CH₄ often increased at night, whereas calculated diffusive fluxes were commonly similar or higher during daytime. Lower nighttime wind-dependent gas-transfer velocities may attenuate the modeled flux response to these concentration increases, particularly in the permafrost-free bog. Exploratory tests did not detect day–night differences for most variables; however, the small number of nighttime observations precludes interpreting non-significance as equivalence. These results document limited diel amplitude under stable late-summer baseflow conditions in highly humic, acidic peatland waters. They do not constrain ebullitive CH₄ release and should not be extrapolated to other seasons, weather regimes, or clearer waters without additional high-frequency observations. Within this restricted domain, daytime measurements can characterize mean hydrochemistry, whereas greenhouse-gas flux estimates additionally require representative gas-transfer coefficients.