Continuous measurement of spectrophotometric absorbance in peatland streamwater in northern England: implications for understanding fluvial carbon fluxes

Bog systems tend to have a flashy hydrological regime with low baseflows and rapid and high storm peaks. Water derived from peatlands often contains significant amounts of organic humic and fulvic materials which form the largest fraction of the dissolved organic carbon component of the fluvial carbon flux. However, most estimates of dissolved organic carbon flux from peatlands are based on sampling that is infrequent and which may miss the periods of high flux during storm events. In order to better characterize the behaviour and fluxes of fluvial carbon it is necessary to operate more frequent sampling. This paper presents data from a continuously operating field-based spectrophotometer simultaneously measuring absorbance across 200-730 nm at 2.5 nm intervals in runoff from an upland peatland stream. It is shown that absorbance at different wavelengths that have previously been used to characterize dissolved organic carbon varies rapidly during storm events. The probe is shown to even detect changes in absorbance characteristics in response to rainfall events before the stream discharge starts to rise. The high-resolution behaviour of absorbance characteristics during storm events is different depending on the wavelength studied. Thus, the choice of wavelength used as a proxy for dissolved organic carbon needs careful attention and it may be that automated spectrophotometric methods which provide rich time-series data from across the spectrum can tell us more about fluxes, processes and sources of aquatic carbon in peatland systems in the future than traditional practices have hitherto allowed. Copyright (C) 2011 John Wiley & Sons, Ltd.