Diurnal variations in the carbon chemistry of two acidic peatland streams in north-east Scotland

1. Two acidic peatland upland streams in north-east Scotland draining catchments of 1.3 and 41.4 km(2) were sampled each season for 2 years to investigate diurnal variations in dissolved and gaseous forms of carbon. Stream metabolism, alkalinity, discharge, pH, air and water temperatures were measured to aid data interpretation.

2. Free CO2 showed marked diurnal variation with lowest concentrations during the period from late morning to early afternoon and highest during the hours of darkness. Although alkalinity and pH also showed some diurnal fluctuations, in comparison with other more productive alkaline systems, variation was small. Dissolved organic carbon (DOC) showed no significant diurnal pattern. However, significant changes in stream discharge influenced DOC concentrations, as well as over-riding diurnal patterns of free CO2, alkalinity and pH.

3. The highest diurnal ratios (maximum concentration/minimum concentration) in CO2, gross primary productivity (GPP) and community respiration (CR) occurred in spring and summer and the lowest in autumn and winter. Variation in biotic in-stream processes caused changes in CO2 concentrations and temperature affected both the solubility Of CO2 and changes in up-stream CO2 inputs. There was no significant difference in diurnal fluctuations between the two orders of stream studied.

4. The mean GPP (as CO2) was 0.81 g CO2 m(-2) day(-1) and mean CR 2.67 g CO2 m(-2) day(-1). The mean primary production/respiration (P/R) ratio was 0.26 +/- 0.09 and 0.33 +/- 0.15 in the first and second order streams, respectively. These values are low compared with published data because these heterotrophic headwater streams are dominated by benthic respiration and upstream allochthonous inputs with little autotrophic metabolism, particularly during the colder autumn and winter months.

5. The results have implications for the calculation of dissolved inorganic carbon (DIC) fluxes in streamwater. Samples taken during daylight hours tend to have lower concentrations of free CO2 and HCO3- than samples taken during darkness. During spring, concentrations of free CO2 were measured up to 2.4 (annual mean 1.8) times higher at night than during the day at a similar discharge. It is suggested that fluxes based on daytime measurements alone will under-estimate the annual flux of these determinands in streamwater by as much as 40%.