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

Abstract

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 largely form the dissolved organic carbon fraction of the fluvial carbon flux. However, most estimates of dissolved organic 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 characterise 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 to 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 characterise 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.

Biotic and Abiotic Factors Interact to Regulate Northern Peatland Carbon Cycling

“…Consequently, CO 2 fluxes and [DOC] showed strong seasonality: the lowest CO 2 fluxes were observed during the winter months, when low temperatures suppressed photosynthesis and decomposition processes, and the highest fluxes occurred during summer, when biological processes were promoted by warmer temperatures (Clark and others 2005;Bonnett and others 2006). Pore water [DOC] did not peak until autumn, attributable to the relatively high temperatures in summer and autumn that promoted decomposition and mobilization into the dissolved phase in autumn when the water table rose (Tables 4, 5) (Grayson and Holden 2012). CH 4 also peaked in autumn, which coincided with relatively high water tables and temperature (including at depth) (Tables 4, 5).…”

“…In northern latitudes, there are strong seasonal controls over GHG exchange and [DOC]: warmer temperatures generally enhance C cycling in summer (Ward and others 2007;Clark and others 2008) while seasonally lower water tables decrease photosynthesis rates and net CH 4 emissions, and increase respiration and DOC production (Blodau and others 2004). However, increases in DOC production may not be apparent in pore and stream water DOC samples until autumn when the water table commonly rises causing an 'autumn flush' of DOC produced over the summer (Grayson and Holden 2012).…”

How runoff components affect the export of DOC and nitrate: a long-term and high-frequency analysis

“…Since then, several studies (Aubert and Breuer, 2016;Avagyan et al, 2014;Grayson and Holden, 2012;Huebsch et al, 2015;Jeong et al, 2012;Krause et al, 2015;Strohmeier et al, 2013) made use of the technological progress in biogeochemical monitoring and increased the frequency and duration of measurements. High-frequency monitoring indeed has clear benefits.…”

“…Jeong et al (2012) compared the response of DOC and particulate organic carbon (POC) in a forested headwater catchment in South Korea under various hydrologic conditions and found that POC was largely exported at high flow, while DOC is mainly exported at low flow. Grayson and Holden (2012) investigated the behavior of DOC in a bog system in northern Englandfocusing on the absorbance at different wavelengths to characterize the specific composition of DOC. Another study on 20 high-frequency DOC behavior was done by Strohmeier et al (2013) in a small forested catchment in Germany.…”

Improved automation of dissolved organic carbon sampling for organic-rich surface waters

“…Grayson and Holden (2012) previously explored the use of an in-situ UV-Vis spectrophotometer to measure absorbance across the UV-Vis spectrum within a UK peatland stream. However, their study was focussed on the response of absorbance to flow conditions and did not include a DOC calibration.…”