The Fate and Composition of In-Stream Organic Carbon

Abstract

The northern peatlands cover only 3% of the Earth, and store between 20 and 30% of the terrestrial carbon pool. In the UK, 15% of the land is covered in peatland, which is estimated to store 2.3 Pg of carbon. Recently, a trend of increasing DOC concentrations in surface waters has been observed in the northern hemisphere, and the in-stream processing and degradation of DOC to CO2 could represent a major and increasing source of greenhouse gas to the atmosphere.
This thesis measured net DOC loss in unfiltered river water samples across different catchment scales, ranging from 0.005 to 1086 km2, with the peat content varying from 0 to 100%, and over time scales from 30 hours to 10 days. Experiments were carried out monthly for three years, and considered total loss, photo and aphotic degradation and the rate of each process. The composition of DOC and various source materials was analysed.
There was a clear diurnal cycle in the degradation of DOC, with the rates of decline being much higher during the day and lower over night. The initial rates of DOC degradation were higher in source waters than from large downstream sites. Adding nutrients to the water decreased the initial rate, whereas exposing the water to light increased the rate, compared with water kept in the dark. The apparent quantum yield and activation energy of the degradation were calculated.
The initial rate of DOC degradation was found to be related to the oxidation state of the material, with samples that were more reduced being degraded faster.
The total DOC loss was estimated to be 76%, which equates to a loss of up to 14678 Gg CO2eq/yr from UK peat-covered catchments, which is 2.5% of the UK total GHG emissions, or 0.7% of the global CO2 emissions from inland waters.