Sub-daily rates of degradation of fluvial carbon from a peat headwater stream

In-stream processing of allochthonous dissolved organic carbon (DOC) and particulate organic carbon (POC) in peat-sourced headwaters has been shown to be a significant part of the terrestrial carbon cycle, through photo- and bio-degradation, with both DOC and POC converted to carbon dioxide (CO2). This study reports a series of 70-h, in situ experiments investigating rates of degradation in unfiltered surface water from a headwater stream in the River Tees, North Pennines, UK. Half the samples were exposed to the normal day/night cycle (ambient); half were continuously dark. The study found that the DOC concentration of samples in the ambient treatment declined by 64 % over the 70 h, compared with 6 % decline for the samples kept in the dark. For POC, the loss in the ambient treatment was 13 %. The average initial rate of loss of DOC in the ambient treatment during the first day of the experiment was 3.36 mg C/l/h, and the average rate of photo-induced loss over the whole 70 h was 1.25 mg C/l/h. Scaling up these losses, the estimate of total organic carbon loss from UK rivers to the atmosphere is 9.4 Tg CO2/year which would be 0.94 % of the global estimate of CO2 emissions from streams and rivers from the 2013 IPCC report. Initial rate kinetics in the light were as high as 3rd order, but the study showed that no single rate law could describe the whole diurnal degradation cycle and that separate rate laws were required for night and day processes. The comparison of dark and ambient treatment processes showed no evidence of photo-stimulated bacterial degradation.